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It is lucky for us, and for contemporaryornithology, that many chemical elementscome in more than one form. You might not

think so at this point but, hopefully, no matterhow boring you find sub-particulate chemistry,by the end of this article you will have been per-suaded that this is indeed the case! Althoughgeologists have studied stable-isotope ratios inmaterials for almost 50 years and their use,therefore, is not new, their application to thestudy of birds has taken off only in the last10–20 years, as access to the instrumentationrequired for their analysis (mass spectrometers)

has improved and analytical procedures havebecome cheaper and more efficient. As ourunderstanding of the processes affecting thedistribution and abundance of stable isotopesin nature increases, so does the number andrange of potential applications. It thereforeseems timely for a brief review of this rapidlyevolving field of ornithology, to see how widelythe techniques have been applied to date and toconsider the potential for future applications.

The term ‘stable-isotope ratios’ soundscomplex, but the concept is in fact relativelysimple. In nature, each chemical element tends

112 © British Birds 101 • March 2008 • 112–130

The use of stable-isotope ratios

in ornithologyTony (A. D.) Fox and Stuart Bearhop

ABSTRACT The use of mass spectrometry to analyse the stable-isotope ratiosof bird tissues has become an important new tool for research ornithologists in the last 20 years. Because stable isotopes vary geographically and according

to specific biological processes in the environment, they provide a uniqueforensic means of understanding more about avian biology and ecology than we can learn using conventional techniques alone.The stable-isotope ratios

present in different tissues of birds reflect the ratios in the environment at thetime those tissues were constructed. However, because of the rapid turnoverof some tissues compared with others, an individual bird will bear within its

body constituents a record of its present and past exposure to differentisotopic environments. Because the stable-isotope ratios of specific elementsvary geographically (e.g. hydrogen along oceanic to continental gradients) and

between habitats (e.g. nitrogen and carbon in marine versus terrestrialecosystems), they offer a unique means of studying the ways migratory birdsmove between different parts of the planet and of understanding the habitatsthey exploit.This review looks at some of the innovative ways in which the

technique has been used in a variety of recent studies of birds.The technique isnot restricted to breaking new ground in high science; in terms of

understanding migration strategies, the importance of breeding, migratory andwintering habitat and the feeding ecology of birds, the study of stable-isotoperatios is becoming ever more important in supporting conservation actions.

to occur in one or more forms, called isotopes.These isotopes differ in the number of neutronspresent in each atom; the more neutrons, theheavier the atom and the greater the atomicmass. Stable isotopes persist over prolongedperiods of time – unlike radioactive ones, whichare unstable and decay rapidly so that theirratios in the environment change more quicklyover relatively short time periods. The impor-tant point here is that the ratios of many stableisotopes of common elements in the environ-ment reflect well-defined natural processeswhich often differ in time and space. Forexample, hydrogen is an abundant element thathas two stable isotopes, the normal ‘light’ form(conventionally denoted as 1H, because it hasone proton and no neutrons in its atom, sothere is just one subatomic particle in thenucleus) and the ‘heavy’ form (also known asdeuterium, and denoted 2H because it has twoparticles in its nucleus, one proton and oneneutron). Deuterium is relatively rare in nature– sea water contains one deuterium atom toapproximately 6,500 light atoms of hydrogen.Nevertheless, despite the fact that deuterium isalways much the rarer member of the pair, theactual ratio of these two stable isotopes variessubstantially around the globe, for instance

with ambient temperature. It is this type ofvariation in stable-isotope ratios that offerssuch a remarkable range of possibilities, manyof which are discussed below, which we can useto improve our understanding of the naturalworld around us.

The atomic mass of the hydrogen respon-sible for forming water makes a difference to itsphysical properties. ‘Heavy water’ is formedfrom oxygen combined with the isotope deu-terium (atomic mass 2) and is denser than‘light’ (ordinary) water, which is formed fromoxygen and hydrogen (atomic mass 1). Heavywater tends to be precipitated out of cloudsbefore light water, so as moist air travels inland,the amount of heavy water is increasinglydepleted. This means that highly continentalareas (such as the middle of the Russian Arctic)receive much lower levels of deuterium in pre-cipitation compared with Europe’s westernseaboard, which experiences a much higher rel-ative proportion of heavy water in precipita-tion. A similar process happens with increasingaltitude, meaning that in mountainous areas,we can also expect deuterium to be less abun-dant in precipitation than in comparablelowland areas.

All this gives us good reason to believe that

113British Birds 101 • March 2008 • 112–130

The use of stable-isotope ratios in ornithology

Box 1. Measuring stable-isotope ratios using mass spectrometryStable-isotope ratios are measured using a mass spectrometer. This equipment measures the ratioof the electric charge of a particle in relation to its mass. This is achieved thanks to Newton’ssecond law of motion, namely that the acceleration of a particle is inversely related to its mass. Themass spectrometer vaporises the sample and converts the constituent parts into ions, charged par-ticles that are then accelerated from a chamber and concentrated into a narrow beam which is sub-jected to a strong magnetic field. Because each isotope atom has a unique combination of mass andcharge when ionised, the magnetic field bends the stream of particles to a different degreedependent on these two characteristics. For instance, light hydrogen will be deflected less by themagnetic field than heavy hydrogen, so the stream of concentrated ions comprising a mixture ofthese two isotopes will separate in the chamber and arrive at two different points in the detectionpart of the apparatus, dependent on their mass. Detectors can be positioned in the detectorchamber, according to the degree to which the ion stream of a specific element of specific mass andcharge would be expected to be deflected. To determine the relative abundance of one isotope toanother, the machine measures the relative charge arriving at the different positions expected foreach isotope. For hydrogen and many common elements, this means collecting the electric currentfrom two streams originating from the two different isotopic forms, but for some elements withmore isotopes, more detectors are required (see below). Although the theory is straightforward, inreality the differences in mass between isotopes of the same element are typically incredibly smalland the less abundant isotopes in a sample usually very rare, so modern mass spectrometers haveto be extremely sensitive to obtain accurate measurements. The wider application of stable-isotoperesearch in ornithology in recent years has undoubtedly been due to both the increased availabilityof such machines in universities and research establishments throughout the world and the tech-nological developments allowing rapid throughput of large numbers of samples.

the stable-isotope ratiosof hydrogen in rainwater(and hence in ground-water) show some pre-dictable geographicalpatterns – but how doesthis affect birds? All bio-logical activity relies onwater, and living organ-isms not only take upwater from their envir-onment but are totallydependent on it formaintaining all of theircellular activity. Hence,the hydrogen stable-isotope ratio of thebodies of all living thingswill reflect that of theenvironment in whichthat organism grew up.For example, BullfinchesPyrrhula pyrrhula inBritain (subspeciespileata) and Denmark(subspecies coccinea)have hydrogen stable iso-topes in their feathersthat are characteristic ofthe oceanic ground-waters of westernEurope, whereas those ofthe large nominate sub-species pyrrhula, origi-nating from northernScandinavia and contin-ental Russia, show moredepleted levels of heavyhydrogen in theirfeathers (see fig. 1 andNewton et al. 2006). Thestable-isotope ratio ofoxygen shows similarpatterns, so the ratio of18O/16O (the two com-monly occurring naturalisotopes of oxygen)shows very strong gradi-ents in groundwater thatcorrelate with tempera-ture and oceanic–conti-nental and upland–lowland gradients similarto those of hydrogen.

114 British Birds 101 • March 2008 • 112–130


66. Female Northern Bullfinch Pyrrhula pyrrhula pyrrhula, Sweden, November 2004.

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Fig. 1. Mean (± standard errors) hydrogen stable-isotope determination ratios of feather samples taken from Bullfinches Pyrrhula pyrrhula caught in Nimtofte,

eastern Denmark. Birds were either non-breeding migrants from Scandinavia andRussia (subspecies pyrrhula, shown by diamond symbols) or resident, locally bred

birds (subspecies coccinea, shown by triangle symbols); filled symbols indicatefemales, open symbols indicate males, for both samples.The data are those

presented in Newton et al. (2006). Given the hydrogen stable-isotope gradients in groundwater, the predicted range of values in feathers grown in Denmark

and Scandinavia/Russia are shown as shaded areas for the two regions as indicated, based on contour maps in Hobson et al. (2004a) (see also fig. 3).

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These geographical patterns of isotope ratiosin rainfall and groundwater (and hence byinference in bird tissues) have been establishedfor many years on a continental scale, both inNorth America (Hobson & Wassenaar 1997)and in Europe (Hobson et al. 2004b), and,recently, on a global scale (Bowen et al. 2005).As with all living organisms, when birds absorbchemicals from water and their food, theirmetabolism subtly changes the stable-isotoperatios so that, when converted into their owntissues, they are slightly different from those inthe environment from which they wereobtained. Thanks to field and laboratorystudies, our knowledge of how the stable-isotope ratios of particular elements in bird dietchange when they become incorporated intodifferent avian tissues (so-called diet-tissuefractionation or enrichment rates; see Hobson& Clark 1992a,b, Mitzutani et al. 1992) is nowincreasing. What this means is that birds areeffectively carrying around a record of wherethey have been and what they have been eatingin the stable-isotope ratios of their tissues. And,if we know something about how these ratiosvary across different areas and different foodtypes, then, based on these tissue isotope ratios,we can make inferences about where birds havebeen and what they have been feeding on.

Migratory patterns traced by feather stable-isotope ratiosNow things start to get interesting. The situa-tion is relatively simple if a bird is sedentary.Under these circumstances, the stable hydrogen(and oxygen) isotopes of the various organs andtissues of the bird (feathers, claws, blood, bones,fat, muscle, etc.) will all reflect the compositionof the local precipitation in the areas where thatbird was raised and lived its life. But what if thatbird migrates from, say, the Russian Arctic(extreme continental environment) to westernEurope (moist oceanic environment)? How willthe stable isotopes in various bodily organsdiffer? To answer this question, it is importantto realise that different body parts are subject todifferent rates of renewal, another process thatwill affect the stable-isotope composition or‘signature’ of different tissues. In birds, theclaws grow slowly and continuously, whilefeathers tend to be grown during relativelyshort and discrete periods. An important pointabout these two tissues is that they are inert, inthe sense that they are metabolically inactive.

Hence, the stable-isotope ratio of a feather willreflect the highly specific isotopic environmentin which it was grown, and this informationwill remain ‘locked away’ in that feather until itis shed at the next moult. Some birds regularlyreplace different feather tracts at different stagesof the annual cycle; and many do so in quitedifferent areas along their migratory corridor.This means that the stable-isotope ratios ofadjacent feather tracts on a bird may provideinformation on habitat selection and diet frommultiple discrete periods in the past. Stable-isotope-ratio-analysis therefore provides a powerful tool than can (potentially) reveal thesecrets of where a particular bird has beenduring a migratory episode. This is extremelyuseful in cases where, for example, it is knownthat certain feathers were grown in the natalareas while others were replaced on the win-tering grounds – because if the isotopic envi-ronments of these two places are radicallydifferent, then, even with less than a milligramof feather material (see Box 1 and Wassenaar &Hobson 2006), it becomes possible to distin-guish these differences in fragments of feathers.

We recently used this technique to show thata first-winter Baikal Teal Anas formosa fromsoutheast Denmark in November 2005 wasalmost certainly a wild bird (fig. 2 and Fox et al.2007). It had been shot mistakenly as a EurasianTeal A. crecca and submitted as part of theDanish national scheme (organised by theNational Environmental Research Institute) tosample the age and sex ratios of huntablespecies. In this case, it was clear that the wingfeathers and some of the old, notched and worntail feathers would have been grown on the sitewhere the bird was hatched and reared, whereasadjacent new, unworn tail feathers had beengrown in the previous few weeks. We were alsoable to find body feathers on the neck thatshowed signs of peripheral wear, in contrastwith others that were clearly of more recentorigin. We reasoned that if the bird was gen-uinely wild, the old feathers grown on theSiberian breeding grounds would show signs ofbeing grown in a highly continental isotopicenvironment, namely with highly negativehydrogen and oxygen stable-isotope ratios (seefig. 3). In contrast, the newly grown feathersshould reflect stable-isotope ratios more char-acteristic of the oceanic environment of westernEurope. On the other hand, if the bird had beenraised in captivity in Europe, it would be highly



unlikely that old and new feathers would showsuch a pattern, and more likely that both wouldreflect a western/oceanic hydrogen stable-isotope ratio, even accounting for possible vari-ation caused by commercial feed provided in acaptive environment. The mass spectrometeranalysis confirmed that the old feathers showedhighly depleted stable-isotope ratios ofhydrogen and oxygen, quite unlike those of thenew feathers, which matched those of MallardA. platyrhynchos feathers from west Europeanbirds (fig. 2). The feathers grown beforefledging matched the ratios expected based onthe Eurasian isotopic contour maps of Bowen etal. (2005), strongly suggesting that the bird wasindeed a genuine vagrant that had been raisedin continental Siberia and migrated toDenmark in its first autumn.

In this case, we were fortunate to be able totest a well-founded hypothesis, comparing theresults from mass spectrometry against our pre-dictions to infer the route taken by this indi-vidual on autumn migration. The use ofstable-isotope analysis is not an automaticmeans of proving vagrancy in all cases of rarebirds, however, and a number of criteria need to



be met before a potential case becomes suitablefor application of the technique:

1. It must be possible to establish the bird’s ageand the different feather tracts (and the stateof these) that could potentially be used inany analysis.

2. Moult patterns and timing in the speciesconcerned should be well described, so thatthe time of growth is known for specificfeather tracts. This is necessary to pinpointwhich feathers were grown when (and hencewhere) in the annual cycle.

3. It is important to be able to infer the poten-tial route and timing of the migration thathas taken the bird to the point of capturesince the last moult of specific feathers.

4. There must be sufficient isotopic contrastbetween the environments in which the different feathers were grown, based on goodavailable geographical knowledge (such as the maps of Bowen et al. 2006), because thetechnique is not suitable for pinpointinggeographical origins with a high degree ofaccuracy.

Although the tech-nique is not particularlyexpensive, it is still notwidely available andthus it can be difficult toget such analyses per-formed; moreover, it islikely to be of muchmore use in identifyingthe provenance of first-winter birds, in whichany old feathers cer-tainly reflect the isotopicpatterns of natal areas.Nevertheless, it is a pow-erful technique that, inthe example above,enabled confirmationthat Baikal Teal canarrive in western Europeas an apparently genuinevagrant, whereas mostEuropean countries(including Britain) stilltreat all occurrences asbeing of presumedcaptive origin. It maytherefore be potentially

Fig. 2. Plot of stable-isotope ratios of feathers from a Baikal Teal Anas formosashot in southeast Denmark in November 2005 (open symbols). Feather samples

come from: (1) worn neck feather; (2) secondaries; (3) old unmoulted tail feather;(4) primary upperwing-covert; and (5) secondary underwing-covert (all grown onthe natal area).These can be compared with: (6) new body feather; and (7) newlygrown tail feather adjacent to (3) (6 and 7 grown in the wintering area).The mean

of samples from Mallards Anas platyrhynchos obtained in summer from westernEurope is shown as a solid square ± standard deviations (from Hobson et al.2004b).The approximate range of values expected for feathers regrown in western Europe and the breeding range of Baikal Teal (after Kear 2005) are

shown as shaded blocks for the two elements concerned, based on the maps of Hobson et al. (2004a) and Bowen et al. (2005).

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valuable to examine museum skins of speciesthat are of similarly uncertain vagrancy status(e.g. Ruddy Shelduck Tadorna ferruginea), incases where the above criteria are met, so thatan objective assessment of the status of theindividuals involved can be made.

Using stable-isotope ratios of other organicelements for ornithological researchThe potential of the technique is by no meansrestricted to proving genuine vagrancy. There isan enormous range of possibilities to help us tounderstand linkages between different geo-graphical areas used by migratory birds at dif-ferent stages of their annual cycle. Althoughringing recoveries provide much informationand insight into migration patterns, in non-hunted species (especially passerines) recovery

rates are typically minuscule; as a result, ourknowledge is highly restricted and fragmented,and often biased according to the way birdswith rings are recovered and reported byhumans. Ringing recoveries of quarry speciesmay often explain more about the distributionof hunting effort, for example, than that of thebirds themselves! Analysis of stable-isotoperatios in avian tissues cannot offer pinpointaccuracy to the same degree as place of ringingand recovery, but it can provide excellent infor-mation on regional movements of individuals,allowing the identification of different migra-tory strategies and population structures withinflyways.

It is not just hydrogen and oxygen that showgeographical variation in isotopic ratio. Forexample, the stable-isotope ratios of carbon



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Fig. 3. Mean deuterium stable-isotope ratio ‘contours’ in January precipitation. As water-saturated air massesleave the oceans and travel inland, they deposit precipitation in the form of rain and snow. Because air masses

lose heavy isotopes (in this case 2H – deuterium – but also 18O) faster than the light isotopes, the heavy isotope is depleted as the air mass travels inland.As a result, the ratio of the heavy to light isotopes in precipitation

falling from the air mass decreases with time, as more and more water is lost.This precipitation influences thedeuterium stable-isotope ratios in groundwater, as well as all living things feeding in the immediate vicinity.This phenomenon explains the gradients in deuterium stable-isotope ratios shown here, which run from

relatively high heavy-to-light ratios (reds and yellows) to more deuterium-depleted ratios (greens and blues),and are aligned roughly north–south across North America and east–west across Eurasia.This map helps us tointerpret the results presented in figs. 1 & 2; in particular, the marked difference in deuterium stable-isotope

ratios between western Europe and continental Russia enables us to test whether the Danish Baikal Teal Anasformosa (see text and fig. 2) might well be a genuine Siberian vagrant.This map is derived from c. 40 years of

data analysed from the Global Network for Isotopes in Precipitation database, administered by the InternationalAtomic Energy Association and World Meteorological Organization; map generated by Gabriel Bowen and

collaborators at Purdue University, Utah, USA; see Bowen et al. (2005) and www.waterisotopes.org

(13C/12C) and nitrogen (15N/14N) in zoo-plankton show a strong east–west gradient, withmore enriched ratios of both elements in theBering and Chukchi Seas relative to the arcticwaters of eastern North America. Zooplanktonsupport the entire food chain up to the level oftop predators, such as King Eider Somateriaspectabilis, and zooplankton carbon andnitrogen isotope ratios are transferred up thefood chain in a predictable manner. King Eidersnesting in the central Canadian Arctic winter inboth the Atlantic and the Pacific Oceans, andthe very limited ringing recoveries availablesuggest that approximately half go in eitherdirection. However, the situation is complicatedby the fact that there is much higher huntingpressure in Greenland than elsewhere in theArctic, so many more birds are recovered to theeast than to the west. Mehl et al. (2004) usedcarbon and nitrogen stable isotopes in headfeathers (which are known to be grown on thewintering grounds; Mehl et al. 2005) to showthat, in contrast to the ring-recovery data,between two-thirds and three-quarters of birdsin their sample had wintered in the west(Pacific) while only between a third and aquarter had wintered in west Greenland and the

northwest Atlantic.Stable-isotope analysis thus offers the poten-

tial of establishing a chemical link betweenbreeding locations, migration stopover sites andwintering locations of individual birds. Forexample, by sampling the hydrogen stable-isotope ratios in the feathers of Wilson’s War-blers Wilsonia pusilla from museum skins(known to have been collected at specificbreeding sites), it was possible to show that thetechnique was a good measure of breeding lati-tude. On this basis, monitoring of featherssampled from autumn migrants in New Mexicoshowed that birds breeding farthest northmigrated earliest in autumn (Kelly et al. 2002).The same authors gathered feather materialfrom birds throughout the wintering areas,from Mexico down through Central Americaand were also able to demonstrate that birdsbreeding farthest north in North America win-tered farthest south in winter. A classic exampleof leapfrog migration, this was completelyunsuspected in this species and providesanother example of a biologically significantresult from stable-isotope analysis that was notapparent from limited ring-recovery data. A dif-ferent migration strategy, that of ‘chain migra-

tion’, was discovered in astudy of Sharp-shinnedHawks Accipiter striatus inNorth America (Smith et al.2003). Hydrogen stable-isotope ratios were used toshow that birds from lowerlatitudes passed throughearlier and wintered farthersouth than those fromhigher latitudes.

Stable-isotope ratios canalso be valuable in identi-fying previously unknownpopulations with distinctranges. Using hydrogenstable-isotope ratios,Hobson et al. (2001) tried tolink populations of Bick-nell’s Thrush Catharus bick-nelli breeding in northeastNorth America with knownwintering grounds in theDominican Republic. Thisthey achieved very success-fully but the study also iden-tified a subpopulation of



67. First-winter male American Redstart Setophaga ruticilla, Connecticut, USA,September 2007. For many migratory species, the effort of arriving early onthe breeding grounds is rewarded by having the best breeding territories tochoose from. For American Redstarts wintering in Jamaica, stable-isotopework has linked winter habitat quality with arrival times; birds wintering in

good habitats are in better condition for the return journey to NorthAmerica, and leave earlier. Marra et al. (1998) showed that stable-isotopesignatures of invertebrate prey differed between good and poor winterhabitats, which in turn could be detected in the muscle tissue of birds

arriving on the breeding grounds in spring (see pp. 122–123).

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wintering birds with more depleted stable-isotope ratios than those measured in knownbreeding areas. This was unexpected and sug-gested that populations existed at higher eleva-tions or latitudes than those previously known,prompting a search for the ‘missing’ thrushpopulations in southeastern parts of the borealforests of Quebec. Using point counts and songplayback in likely areas, two previouslyunknown nesting areas were eventually located.Analysis of feathers from these new populationsproduced values which corresponded withthose of the more depleted ratio types found inthe winter quarters (Hobson et al. 2004a), con-firming the value of such forensic studies inextending our knowledge of avian distributionand dispersal patterns.

Turnover rates of stable isotopes in differentavian tissues and organsIn contrast to inert feathers and claws, the bodyorgans are maintained in an aqueous solutionand supplied with energy and nutrients, derivedpartly from the bird’s own body stores, but pre-dominantly from the environment in which thebird finds itself at a given point in time. Conse-quently, the stable-isotope ratios of bloodrespond more rapidly to thesurrounding isotopic envir-onment than those ‘lockedup’ in feathers and claws. Infact, studies have shownthat, even within the bloodconstituents, blood cells andplasma have different stable-isotope turnover rates:plasma generally replacesitself every 3–5 days whilstavian blood cells may have ahalf-life of up to four weeks(Hobson & Clark 1993;Hobson 2005). Hence, if theliquid and cellular fractionsof a blood sample are sepa-rated, it is possible tocompare isotopic dietarycomposition over the pastfew days (using plasma)with that of the past fewweeks (based on the sepa-rated blood cells). Specificorgans show other patternsover time, because they maybe constructed and broken

down at specific stages in the annual cycle whenthe bird is exposed to different environmentswith different patterns of isotopes. For example,breast muscles, typically built up during prepa-ration for migration, will be formed usingprotein derived from food consumed at pre-migratory staging areas where such bodychanges take place. These staging areas mayhave very different stable-isotope signaturescompared with both the wintering and thebreeding grounds. Like the feathers, musclesmay retain a record of a bird’s movement forparticular periods of the life cycle. However, incontrast to feathers, muscle is replaced con-stantly and the original signal is graduallyquenched over time. Other tissues, includingbone collagen, may integrate stable-isotoperatios over longer periods of time, giving evenmore opportunities for their use in tracing thechemical history of a single individual.

‘Capital’ versus ‘income’ breedersSuch isotopic knowledge about specific organsis also important when it comes to under-standing how females invest nutrients in theireggs, not least in deciding between the ‘capital’(using body stores as the raw material for eggs)



68. Several stable-isotope studies have been carried out on Red Knots Calidris canutus.This individual was ringed and colour-marked in May 2001 inDelaware Bay, where Red Knots of the race C. c. rufa stage and refuel on theeggs of horseshoe crabs (Merostomata).This bird was photographed here atFort Myers, on Florida’s Gulf Coast, in August 2003, where Red Knots werethought possibly to be of the race C. c. roselaari, breeding on Wrangle Islandand northwest Alaska, and which are thought to use the east Pacific flyway.However, stable-isotope studies now confirm that birds wintering in the

Gulf of Mexico are of the race rufa.

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versus ‘income’ (deriving nutrition for egg for-mation from diet) strategy (see Drent et al.2006). This is particularly important for highArctic breeders, such as geese, which have littletime on the nesting grounds to accumulate thereserves of protein and fat required to formeggs. How important are the stores of nutrientsand energy that these birds carry with themfrom the spring staging areas and even the win-tering grounds, compared with those they canglean from the snow-covered tundra on theirarrival? In situations where the isotopic signa-tures of wintering, spring staging and breedingareas differ (which, fortunately for us, is oftenthe case), it is possible to gain some insights.For example, based on the stable-isotope ratiosof carbon (13C/12C) and nitrogen (15N/14N) inthe adults and different parts of the eggs, it hasbeen found that high Arctic Greater SnowGeese Anser caerulescens atlanticus derive morethan two-thirds of the resources invested intheir eggs from the tundra breeding areas, andthat less than one-third is brought with them asbody stores from elsewhere (Gauthier et al.2003). In contrast, many high Arctic wadersapparently do not mix nutrients stored in theirbody tissues from staging areas with thosederived at the nesting grounds (Klaassen et al.2001). In this case, Arctic invertebrates derivedfrom tundra ecosystems have distinctly differentstable-isotope ratios from those of the estuarieswhere the birds winter and stage in spring.Analysis of the eggs and down of hatchlingsfrom ten different Arctic-breeding waders,including long-distance migrants such as RedKnot Calidris canutus, showed isotopic ratioscharacteristic of Arctic invertebrates rather thanthose of marine estuaries – showing that theseArctic waders are ‘income’ not ‘capital’ breedersas previously thought.

You are what you eat – describing dietthrough the use of stable-isotopemeasurements Carbon and nitrogen are just as interestingwhen it comes to inferring what birds havebeen eating, as opposed to where they havebeen or where nutrient stores came from.Plants tend to contain less 13C than the atmos-phere because the processes involved in carbondioxide uptake tend to discriminate against 13C– the reason being that 13C is heavier than 12C(and is consequently slower to defuse intoleaves and cells) and forms slightly stronger

chemical bonds than the lighter isotope.Almost all plants on the planet fall into twocategories based on their exclusive use of one ofthe two contrasting ways of assimilating carbondioxide into their cell metabolism. The vastmajority (about 95% of known species) beginthis process by forming pairs of three-carbon-atom molecules and are called C3 plants. Theremaining 5% form four-carbon-atom mole-cules and are known as C4 plants; such plantstend to be adapted to warmer or more aridenvironments. Despite being in the minority,however, the C4 plants include some importantcrop species, notably some cereals and maize,which fractionate the two carbon isotopes dif-ferently from C3 plants. Plants can also differin their stable-nitrogen-isotope signatures as aconsequence of variation in a number ofprocesses, with nitrogen fixation and soilchemistry being important drivers; and it isworth remembering that the latter is heavilyinfluenced by agricultural activities such as fer-tiliser applications. All this means that there aredifferent stable-isotope ratios of different ele-ments in plant tissue, even when they grow inclose proximity, which in turn means that wecan differentiate the diets of, for example,Lesser Snow Geese A. c. caerulescens that feedon natural marsh vegetation, on farmed rice oron maize. The geese exploit three food sourceswith distinctively different carbon and nitrogenstable isotopes and if individuals specialise onany one food type, it shows up in their bodytissues. By analysing stable-isotope ratios inindividual geese, Alisauskas & Hobson (1993)showed that, when faced with a choice ofwinter feeding areas, individuals appeared tospecialise on particular foods, a mechanismthat appeared to define feeding subpopulationson the basis of feeding ecology and habitatchoice.

The forensic knowledge locked in thefeathers of an individual bird enablesresearchers to look back at precisely what thatindividual has been feeding on. As the LesserSnow Goose example shows, this can highlightdifferences in feeding ecology, diet and habitatuse between individuals. In that case, the sameinformation could have been derived by col-lecting droppings from marked individuals, butthat would be a laborious and in reality difficultproject. For groups such as diving seabirds,however, it is generally impossible to getdetailed information about what individuals



feed on and how they obtain their food. Usingstable-isotope ratios in both feathers and blood,Bearhop et al. (2006) were able to show sex dif-ferences in the diets of Gentoo PenguinPygoscelis papua, Kerguelen Shag Phalacrocoraxverrucosus and South Georgian Shag P. atricepsgeorgianus, which, in the case of the last species,persisted over long time periods. These differ-ences probably relate to differences in body size,the larger males being able to dive deeper thanfemales. More intriguingly, there were strongrelationships between feather and blood isotoperatios in the two shags, suggesting that individ-uals are highly specialised in terms of diets andthat specialisation is maintained over longperiods (because the feathers were grown in thenon-breeding season and the blood wassampled during nesting). In other words, birdshave individual ‘tastes’ and differ in what theyeat, which is probably dependent upon thefoods that a particular individual is adept atcatching. Similar age- and sex-specific differ-ences in diet have also been shown amongSouthern Giant Petrels Macronectes giganteususing similar techniques (Forero et al. 2005).Another study of south Atlantic diving birdsused stable isotopes in feathers to show that

four different species of petrel from KerguelenIsland dispersed over a much wider range ofhabitats (coastal to oceanic waters from Antarc-tica to the tropics) compared with the samefour species on South Georgia, which winteredmainly locally around the archipelago (Cherel etal. 2006). These studies demonstrate the poten-tial of stable-isotope analysis of feather tissuefor locating the moulting areas of seabirds thatundergo complex migration patterns every year,especially in helping to investigate foragingecology during the poorly known non-breedingperiod (Cherel et al. 2000).

Of course, if you eat junk food, it too willshow up in your body! For this reason, stable-isotope ratios of scavengers (such as crows(Corvidae) and gulls (Laridae)) that userubbish tips and other sources of human wastemay be highly variable and extremely exotic,because they tend to eat anthropogenic materialimported from around the world, all reflectingthe varied isotopic environments in which theyoriginated (Hobson et al. 2004b). On the otherhand, these remarkable mixes of stable-isotopesignatures can be useful in showing how impor-tant birds are in the nutrient cycles of urbanlandscapes. A study in Japan has shown that up



69. Gentoo Penguins Pygoscelis papua, Sea Lion Island, Falklands, November 2007. For some groups of birds, suchas diving seabirds, detailed information on diet is almost impossible to collect.The stable-isotope ratios of both

feathers and blood of Gentoo Penguins revealed significant differences in diet between males and females(Bearhop et al. 2006).These differences probably relate to body size, with the larger males being able to dive

deeper than females.

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to 53% of the phosphorus and 27% of the totalnitrogen input to evergreen forest fragments inurban landscapes came from the droppings ofthe large roosting aggregations of Jungle CrowsCorvus macrorhynchos (Fujita & Koike 2007).Using stable-isotope analysis, they could showthat the crows played an important role for thewoodland by importing nutrients in their faeceswhich were derived from rubbish (from fish,livestock, and/or C4 plants such as corn) withhigh 13C and 15N ratios, gleaned from residen-tial and business areas, which would not nor-mally appear in such an ecosystem.

There are many other pitfalls and shortcom-ings when considering the use of stable isotopesin the tissues of birds to determine their prey,which confirms the need always to study bothpredator and prey to understand the processesinvolved. For example, freshwater birds mayfeed on fish that spend most of their life in thesea but which migrate up rivers temporarily –for example to spawn – and thus will have dif-ferent signatures from birds feeding on local,non-migratory fish. However, an understandingof such processes will invariably enable the useof such anomalies to be used to advantage infurthering our understanding of avian diets.

Understanding how factors operatingthroughout the annual cycle affect birdabundance (‘carry over’ effects)We have already seen that the study of stableisotopes can be a powerful approach in under-standing some of the more complex patterns inavian ecology. In particular, as in the ‘capital’versus ‘income’ breeder debate, it helps us toconnect different phases in the annual cycle andunderstand better how factors operating at onepoint in the cycle affect an individual at othertimes: ‘carry over’ effects.

It is well known that, for migratory birds,early arrival on the breeding grounds in goodphysical condition is an important prerequisitefor successful reproduction. American RedstartsSetophaga ruticilla arrive at nesting areas over aperiod of about a month, with later arrivalsoften in poor condition. These late arrivals notonly have diminished chances of finding a goodterritory but their poor condition is likely toaffect survival too. Nothing was known aboutthe causes of these differences until a stable-isotope study of their Jamaican winter quartersrevealed that winter habitat quality determinedthe birds’ physical condition and departure datefrom the wintering grounds, and in turn their



70. Southern Giant Petrel Macronectes giganteus, Ushuaia, Argentina, October 2006. Individual birds may be morespecialised in their diet than we realise – to some extent, birds have particular ‘tastes’ and differ in what they eat– and this probably depends on the foods that a particular individual is adept at catching. Stable-isotope studies

have confirmed age- and sex-specific differences in diet among Southern Giant Petrels (Forero et al. 2005).

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condition and arrival time in nesting areas(Marra et al. 1998). Birds wintering in BlackMangrove Avicennia germinans forest (the bestquality habitat) began their spring migrationearlier and in better condition than those win-tering in secondary-growth scrub (a lowerquality habitat). The two habitats were charac-terised by different amounts of C3 and C4plants so that isotope signatures of invertebrateprey differed between mangrove and scrub; andthis in turn could be detected in muscle tissueof birds arriving on the breeding grounds inspring. Similarly, studies of stable-isotope ratiosin the claws of Black-throated Blue WarblersDendroica caerulescens in the Bahamas duringspring migration showed that birds wintering inbetter quality habitats were in better condition(with greater fat stores and larger pectoralmuscles) than those in suboptimal scrub habi-tats (Bearhop et al. 2004). Again, good condi-tion during spring migration is likely totranslate into earlier arrival and/or better con-dition on return to the breeding grounds, whichin turn has consequences for reproduction.These two studies show that eventsin tropical wintering areas affect thecondition of migratory songbirdsduring migration, their reproductivesuccess on the breeding groundsand, potentially, survival. They alsoprovide some evidence that winterhabitats may be limiting in suchspecies, forcing less fit individuals toexploit suboptimal habitats, inwhich they are less successful thanfitter birds in better habitats. Bothstudies are examples of research thatnot only revealed a great deal aboutthe ecology of bird populations, butprovided vital information tosupport effective conservation, inthis case, the importance of naturalforest as wintering habitat for NorthAmerican passerine migrants.

Another fascinating study thatconcerns ‘habitat matching’ betweensummer and winter involves Ice-landic-nesting Black-tailed GodwitsLimosa limosa islandica. Gunnarssonet al. (2005) used stable isotopes todetermine the habitat quality usedby marked individuals which couldbe followed to the winteringgrounds. The analysis of 13C isotope

ratios in feathers grown in late winter showedthat those godwits which used estuarine sites inEurope at that time (the best overwinteringhabitats) tended to breed on the most produc-tive sites in Iceland; in other words, individualsthat occupy higher quality breeding sites alsoused better quality wintering sites. Since adultgodwits are highly philopatric (returning to thesame site year after year), the initial choice ofwinter habitats by juveniles (which are notaccompanied by their parents from thebreeding areas) may be crucial to the future sur-vival, timing of migration and reproductiveoutput of those individuals.

Using stable isotopes to unravel migrationroutes and identify winter quartersThe technique can also be extended to delineatemigratory divides and provide indication of thepotential wintering areas of some long-distancemigrants. Willow Warblers Phylloscopus trochilusare particularly amenable to such studies since,unlike many other species, they undergo twocomplete flight-feather moults each year. Thus,



71. Icelandic Black-tailed Godwit Limosa limosa islandica, Iceland,June 2006. Stable-isotope studies have shown that individuals nesting in high-quality breeding sites are those which occupy

better quality wintering habitats (Gunnarsson et al. 2005).

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feathers collected early in the breeding seasoncontain information on winter habitat selection.In Sweden there are two subspecies of WillowWarbler, with only marginally overlappingbreeding ranges: P. t. trochilus breeds mostly atlatitudes below 61°N whereas P. t. acredula tendsto breed at latitudes above 63°N. Chamberlain etal. (2000) measured the 13C and 15N isotopes offeathers from the two subspecies, which theyfound to be isotopically distinct. They showedthat the average 13C and 15N ratios in wingfeathers (grown on the African wintering quar-ters) of acredula were significantly higher thanthose in wing feathers of trochilus. This con-firmed the sparse ringing data, which hintedthat the two subspecies occupy geographically(and isotopically) distinct wintering grounds inAfrica or used different habitats (exploiting dif-ferent diets) in the same area. A study of dif-ferent breeding populations of Barn SwallowsHirundo rustica showed a similar pattern.Stable-isotope ratios in the feathers of birdsbreeding in Switzerland were significantly dif-ferent from those in the feathers of birds thathad bred in England (Evans et al. 2003). The 13Csignatures of Swiss birds were significantly lowerthan those of English birds, but 15N signaturesdid not differ between the two populations.Here, the authors concluded that Swiss birdsprobably feed on prey in winter that are more



reliant on C3 vegetation,from woodlands, than theprey of English birds, whichare more reliant on C4 vege-tation, from grasslands. Incontrast, a study of a singlepopulation of Barn Swallowsbreeding in Denmarkshowed a very clear bimodaldistribution of both carbonand nitrogen stable-isotoperatios that strongly suggestedtwo discrete and differentwintering areas for birdsexploiting the same nestingarea (Møller & Hobson2004).

Atkinson et al. (2005)used 13C and 15N isotopes inflight feathers of Red Knotsto identify at least three dif-ferent discrete winteringareas used by birds caught onspring migration in Delaware

Bay, northeast USA, en route to their high Arcticbreeding areas. The data suggested that around58% probably wintered in Bahia Lomas (inChile), c. 30% in Florida, c. 6% in Rio Grande(Argentina), while the remaining 8% were notclassifiable. This sort of approach has been usedon a much wider spatial scale to determine thewintering localities of a range of wader species,based on flight-feather stable-isotope chemistryof birds captured on their breeding grounds(Farmer et al. 2004). In this case, a combinationof different isotopes present in feathers grown inthe winter quarters could be used with somesuccess to identify where breeding birds win-tered, even discriminating birds from two closelyspaced wintering sites in Tierra del Fuego.

Using sequences of feather regrowth tounderstand avian biologyIn the case of the Baikal Teal (see study men-tioned above), and indeed virtually all wildfowl,the flight feathers are all grown simultaneously,and all will reflect the isotopic characteristics ofthe moult site. However, most other birds moultflight and body feathers sequentially, whichmeans that adjacent feathers on a bird willreflect the particular chemical environmentsprevailing at the time of construction. Hence,Thompson & Furness (1995) were able to showthat the diet of Fulmars Fulmaris glacialis

72. Willow Warbler Phylloscopus trochilus, Estonia, May 2005.We have littleinformation on the wintering ranges of the two subspecies of Willow Warbler

that breed in Europe, P. t. trochilus and P. t. acredula. Stable-isotope analysis of wing feathers grown on the wintering grounds suggests that the two

subspecies remain quite separate in their African winter quarters, separatedeither geographically or by habitat (Chamberlain et al. 2000).

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changed during the period of sequentialprimary moult. A Fulmar’s innermost primariesare regrown at the end of the chick-rearingperiod, while the outer feathers are progres-sively replaced well into winter. Isotopic ratiosshowed that the birds were consuming preyfrom increasingly lower in the marine foodchain as the primary moult progressed. In long-distance migrants, these differences provideclues about where specific feathers were grown,and help us to understand the often highlycomplex moult strategies adopted by somebirds, such as Savi’s Warbler Locustella luscin-ioides, which may replace feathers twice in theannual cycle (Neto et al. 2006). Such knowledgeof moult patterns is also essential for sup-porting effective conservation. Until recently,the sub-Saharan African wintering quarters ofthe Globally Threatened Aquatic Warbler Acro-cephalus paludicola were completely unknown.Such a lack of information about the winteringgrounds hampers conserva-tion efforts, so Pain et al.(2004) looked at stable-isotope ratios in the flightfeathers of Aquatic War-blers, which were known tobe grown on the winteringgrounds. Although theresults were not sufficient topinpoint the winter quartersaccurately, they did find sig-nificant differences in 13Cratios, indicating that geo-graphic segregation of pop-ulations also occurred in thewinter quarters. Even moreinterestingly, they foundthat winter isotope signa-tures were correlated withbreeding latitude and longi-tude, suggesting strong linksbetween the breeding andwintering grounds and pos-sibly some evidence ofleapfrog migration. Subse-quently, the discovery of asignificant wintering popu-lation of Aquatic Warblersin northwest Senegal,perhaps holding up to athird of the world popula-tion of the species, owedmuch to the role of stable-

isotope analysis in signposting likely regions ofWest Africa. The technique also confirms theimportance of stopover sites, since studies ofthe stable-isotope ratios in the feathers ofmigrating passerines in sub-Saharan Africashow close similarity from year to year(Yohannes et al. 2007). This has enormous con-servation implications, because it shows thatthese migrating birds feed on the same preyevery season, emphasising that their site fidelityand habitat selection makes them highly vul-nerable to habitat change and destruction instaging areas.

Stable isotopes can reveal facets of theevolutionary processAs well as revealing much about migration pat-terns, stable isotopes can also help to explainthe evolution of the migratory process. Bearhopet al. (2005) used habitat-specific stable-isotopesignatures to study Blackcaps Sylvia atricapilla.



73. Fulmar Fulmaris glacialis,Westray Firth, Orkney,April 2004.The majority ofbirds moult their feathers sequentially and, for larger birds, the stable-isotopecomposition of particular feathers may show how diet changes as the moult

progresses.Thompson & Furness (1995) mapped the changing diet of Fulmars,in which moult begins at the end of the breeding season and is often not

completed until late winter.

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During the last 50 years, Blackcaps have beenincreasingly wintering in Britain and northernEurope, and studies have shown that this newmigratory behaviour has a genetic basis.Bearhop et al. (2005) showed that birds win-tering in new areas (Britain & Ireland) could bedistinguished from those using traditional win-tering areas in Iberia and North Africa, basedon stable-isotope signatures in their claws.Moreover, the study showed that these twogroups of birds mated assortatively with respectto wintering area – in other words, they matedwith birds from the same wintering areas farmore often than would be expected by chance.They found evidence that this was probablybecause birds wintering farther north weremore likely to arrive back at their breeding areasbefore those wintering farther south. Thismechanism effectively keeps the two types sepa-rate, even though they overlap in their breedingranges and habitat. Furthermore, birds win-tering farther north also produced largerclutches and fledged more young. This clearlyshows that birds adopting the ‘new’ winterstrategy were far more successful at producingyoung than those retaining the ‘traditional’



strategy. This is striking evidence for the mech-anism behind the rapid increase in the numbersof Blackcaps coming to British and Irish birdfeeders in winter. These findings are not justimportant for our knowledge about Blackcaps,they also describe a fundamentally importantprocess in the evolution of migratory divides,new migration routes, and wintering quarters.In particular, the results show that the timing ofbreeding is a way in which subpopulations ofbirds may become genetically isolated, eventhough they overlap in breeding range andhabitat.

Marine versus terrestrial stable-isotope ratiosAnother facet of using carbon and nitrogenstable-isotope ratios is that 13C and 15N tend tobe much more abundant in marine ecosystemscompared with freshwater or terrestrial systems,and their ratios in bird tissues can be used,therefore, to demonstrate to what degree indi-vidual birds forage in the two habitat types.This has great advantages for comparinglonger-term differences in foraging strategiesbetween both individuals and species. Forexample, Bearhop et al. (1999) used isotopic

74. Female Blackcap Sylvia atricapilla, Grutness, Shetland, October 2004. Stable-isotope work has added to ourknowledge about the Blackcaps which are now wintering – very successfully – in Britain & Ireland. Birds winteringfarther north are likely to arrive back on their breeding grounds earlier than those wintering in Iberia; this helps

to keep the two groups separate and, since early breeders are generally more successful, bodes well for thecontinuing rise in winter sightings at British and Irish bird tables.

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profiles of feathers grown at different times inthe annual cycle to investigate variability in theamount of marine protein in the diet of GreatCormorants Phalacrocorax carbo in England.Coastal-breeding birds were sampled at inlandsites in winter. Stable-isotope analysis of theflight feathers, which are renewed after thebreeding season, showed that most birds hadbeen feeding exclusively on freshwater fish priorto being sampled. Coastal breeders had thusbecome freshwater specialists at inland sitesduring the non-breeding season – and do notcommute between inland waters and the sea tofeed in the winter. By using stable-isotope ratiosin blood samples, it has also been possible tochart the shift in diet of wintering Brent GeeseBranta bernicla, from sea grasses such as Zosterain early winter to the point where they arefeeding almost exclusively on terrestrial grassesin spring (Inger et al. 2006). A similar approachhas been used to estimate the relative propor-tion of marine and terrestrial protein in thediets of gulls feeding on rubbish tips (Hobson1987).

Even within the marine environment, it ispossible to use 13C and 15N ratios in bird tissuesto see at which trophic levels birds are feeding,

because both elements show higher values athigher levels in the food chain. Stable carbonisotopes in the Pacific also reflect an inshoreversus offshore gradient in prey that can help toidentify where seabirds are feeding (Hobson etal. 1994). It is often important to know whatseabirds are feeding on (zooplankton, crus-taceans, small pelagic fish, etc.), but studies ofdiet based on stomach contents, direct observa-tions or prey remains collected at breedingcolonies give only a snapshot of diet over timeand are often biased towards items that areresistant to digestion (Votier et al. 2001).Several studies have now shown that stable-isotope analyses confirm the trophic relation-ships of seabirds suggested by the results ofconventional methods, and many have helpedto explain how different species can co-exist byfeeding on different prey (e.g. Hobson et al.1994, Dahl et al. 2003, Forero et al. 2004).

Applications using stable isotopes of otherelementsYet other elements present in geological sub-strates characterise local isotopic ratios whichinfluence those in the food chain and which canthen be detected in a given organism. Some ele-



75. Great Cormorant Phalacrocorax carbo, Littleport, Cambridgeshire, November 2006. Bearhop et al. (1999)analysed the feathers of Cormorants shot under licence at inland waters in England to provide support for the

idea that coastal-breeding birds switch their diet from predominantly marine prey in the breeding season tobecome freshwater specialists at inland sites in winter.

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ments, such as sulphur, strontium and lead (alloccur in four different stable isotopic forms),can be used because they show geographicaldifferences linked to geological patterns or topollution by humans. Sulphur can be usefulwhere predators exploit a mixture of terrestrialand marine prey, because sulphur 34S ratiostend to be higher in marine systems than in ter-restrial ones (Lott et al. 2003). Strontium hasbeen shown to be useful in North America, forexample, where high 87Sr ratios are typical ofgeologically old crystalline rocks of theAppalachian Mountains, in contrast to lime-stone bedrock elsewhere. Chamberlain et al.(1997) caught Black-throated Blue Warblers ontheir Caribbean wintering grounds and identi-fied birds from different breeding areas basedon strontium isotope ratios in tissues. Leadfrom earlier industrial sources (such as petroladditives or residues from mining or smelting)provides another potential source of informa-tion about spatial patterns. Pain et al. (2007)used lead isotope analysis to show that the mostlikely source of lead responsible for poisoningRed Kites Milvus milvus in England was ammu-nition used to kill animals on which the kiteswere scavenging.

Some concluding thoughtsThis review has barely scratched the surface ofthe literature available on the use of stable-isotope analyses in current ornithology, butthen the ornithological world has hardly begunto scratch the surface of the possibilities thistechnique opens up. The availability of stable-isotope-ratio measurement offers such a vastrange of forensic possibilities for the study ofbirds that there is no doubt that its future appli-cation will further extend our knowledge in thecoming years.

One particular field that offers excitingprospects is that of ornithological archaeology.In fact, one of the first ornithological applica-tions of the technique was an investigation ofthe foraging habits of the Great Auk Pinguinusimpennis based on isotopic signatures of bonecollagen (Hobson & Montevecchi 1991). Stable-isotope analysis has also been applied to theageing of seabird colonies (based on 13C and15N influence in soils from marine habitats;Hawke 2004), as well as to examining changesin diet over extended timescales (e.g. Fulmars inthe North Atlantic over a period of c. 90 years,where birds shifted from feeding on offal asso-ciated with whaling activities at the turn of the

twentieth century to prey oflower trophic status inrecent times; Thompson etal. 1995). In this sense, col-lections of museum speci-mens around the worldassume an even greaterimportance when consid-ering the many secrets thatmight be revealed by featheranalysis. One such majorproject is already underway,analysing feathers ofSlender-billed CurlewNumenius tenuirostris speci-mens, to try to establishformer breeding and win-tering areas more accuratelyand in turn better under-stand its present status andpotential for recovery.

In a rapidly changingworld, it is all the moreimportant that we under-stand changes in habitat useand migratory behaviourthat are occurring among



76. First-summer male Black-throated Blue Warbler Dendroica caerulescens,Connecticut, USA, May 2006.Although hydrogen, oxygen, nitrogen and carbon

have been the most widely used elements in stable-isotope work, otherelements may be significant too. Based on strontium isotope ratios in

tissues, Chamberlain et al. (1997) identified Black-throated Blue Warblers from different breeding areas among the birds they caught on the

wintering grounds in the Caribbean.

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birds in relation to major human developmentsand climate change. It is also increasinglyimportant that we understand the importanceof breeding, wintering and staging habitats inthe annual cycle to particular bird populations.As shown in this article, stable-isotope studiescan play a major role in providing new infor-mation, which can then play a direct role ineffective conservation. It is crucial that researcheffort continues to support stable-isotoperesearch, now and into the future.

Acknowledgments

We thank the many people who have enlightened usabout the use of stable isotopes, most importantly KeithHobson, who has been the major pioneer in this field andwho has been gracious in his help, advice and support inusing the technique. Thanks also go to the many authorsand co-workers that have been responsible for applyingthe technique to the many different fields of ornithology.Grateful thanks also to Gabriel Bowen for supplying theglobal map of deuterium isotope ratios.

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Dr Tony (A. D.) Fox, Department of Wildlife Ecology and Biodiversity, National Environmental ResearchInstitute, University of Aarhus, Kalø, Grenåvej 14, DK-8410 Rønde, DenmarkDr Stuart Bearhop, Centre for Ecology & Conservation, School of Biosciences, University of Exeter,Cornwall Campus, Penryn, Cornwall TR10 9EZ

Finally released from his duties as BBRC secretaryduring the interregnum between Mike Rogers andNigel Hudson, Pete Fraser is once more turning hisattention to the scarce migrants report. A report cov-ering the years 2004–2006 is now in preparation. Theproduction of this report depends in no small partupon the goodwill of the county and regionalrecorders who provide the raw data. In order to makethe report as complete as possible, data for the rele-

vant species, for the years 2004–2006 inclusive, wouldbe gratefully received; these can be e-mailed (prefer-ably as soon as possible) to [email protected], copies of all county or regional reportsmay be sent to: Pete Fraser, 2 The Parade, Truro,Cornwall TR1 1QE.

Information concerning records for the report for2004–2006 can be found at www.scarce-migrants.org.uk

Report on scarce migrant birds in Britain

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