Short-Term Response of Forest Birds to Experimental...

Short-Term Response of Forest Birds to Experimental Clearcut Edges (Respuestas a Corto Plazode las Aves de Bosque a Bordes Creados Experimentalmente por Tala Rasa)Author(s): Marc-André Villard, Fiona K. A. Schmiegelow and M. Kurtis TrzcinskiReviewed work(s):Source: The Auk, Vol. 124, No. 3 (Jul., 2007), pp. 828-840Published by: University of California Press on behalf of the American Ornithologists' UnionStable URL: http://www.jstor.org/stable/25150340 .

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^^ The Auk 124(3):828-840, 2007

[ jJk ? The American Ornithologists' Union, 2007.

^TL Printed in USA.

SHORT-TERM RESPONSE OF FOREST BIRDS TO EXPERIMENTAL CLEARCUT EDGES

Marc-Andre Villard,1'4 Fiona K. A. Schmiegelow,2 and M. Kurtis Trzcinski3

^Canada Research Chair in Landscape Conservation, Departement de biologie, Universite de Moncton, Moncton,

New Brunswick E1A 3E9, Canada;

2Department of Renewable Resources, University of Alberta, Edmonton, Alberta T6G 2H1, Canada; and

department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

Abstract. ?Numerous studies have addressed the potential consequences of

increasing the density of edges through human activities, but most have docu

mented responses to existing edges. Here, we monitored the response of seven

forest bird species to experimentally created edges around five plots (10 ha, n = 3;

25 ha, n = 2) in the boreal mixed-wood forest of Alberta, Canada. We also mapped

bird detections in six control plots (10 ha, n = 5; 25 ha, n =

1). The focal species were Least Flycatcher (Empidonax minimus), Red-eyed Vireo (Vireo olivaceus),

Yellow-rumped Warbler (Dendroica coronata), Black-throated Green Warbler (D.

virens), Ovenbird (Seiurus aurocapilla), Mourning Warbler (Oporornis Philadelphia), and White-throated Sparrow (Zonotrichia albicollis). In the two breeding

seasons

following experimental clearcutting, we

quantified birds' responses to edges in

the absence of substantial edge-induced changes in vegetation by comparing the

distribution of detections between treatment and control plots. We predicted that

forest-edge specialists would be attracted to edges, forest-interior specialists would

avoid them, and interior-edge generalists would show a neutral response. None of

these predictions was consistently supported among plots and years, except in the

Mourning Warbler. However, none of the significant responses was the opposite of predictions. We also predicted that postharvest colonization of treated plots

would mainly involve forest-edge specialists, whereas most local extinctions

would involve forest-interior specialists. Only the colonization of 10-ha fragments followed the predicted pattern. The relatively neutral response to forest edges we

observed suggests that, in general, boreal forest birds do not respond to the edge

itself or to proximate cues of edge proximity. Rather, significant responses may be

delayed until edge-to-interior gradients in vegetation are established. Received 30

March 2005, accepted 14 July 2006.

Key words: edge effects, field experiment, forest landscape, forest management,

habitat fragmentation, spot mapping, vegetation gradients.

Respuestas a Corto Plazo de las Aves de Bosque a Bordes Creados Experimentalmente por Tala Rasa

Resumen.?Muchos estudios han abordado las potenciales consecuencias del

aumento de la densidad de bordes generados por las actividades humanas.

Sin embargo, la mayoria de estos han documentado las respuestas a bordes

previamente existentes. En este estudio, monitoreamos la respuesta de siete

especies de bosque a bordes creados experimentalmente alrededor de cinco

parcelas (10 ha, n = 3; 25 ha, n =

2) en el bosque boreal mixto de Alberta, Canada.

4E-mail: [email protected]

828



July 2007] Response to Experimental Edges 829

Tambien mapeamos las detecciones de las aves en seis parcelas control (10 ha,

n = 5; 25 ha, n =

1). Las especies focales fueron Empidonax minimus, Vireo olivaceus,

Dendroica coronata, D. virens, Seiurus aurocapilla, Oporornis Philadelphia y Zonotrichia

albicollis. En dos epocas reproductivas subsecuentes a los cortes experimentales, cuantificamos las respuestas de las aves a los bordes en ausencia de cambios

substanciales en la vegetacion inducidos por el borde, comparando la distribucion

de detecciones entre las parcelas control y de tratamiento. Predijimos que las

especies especialistas de borde serian atraidas a los bordes, que las especies de

interior de bosque los evitarian y que las especies generalistas de interior y de

borde tendrian una respuesta neutra. Ninguna de estas predicciones se

cumplio sistematicamente entre parcelas y anos, excepto para O. Philadelphia. Sin embargo,

ninguna de las respuestas significativas fue opuesta a las predicciones. Tambien

predijimos que la colonizacion post-cosecha de las parcelas de tratamiento

involucraria principalmente a

especies especialistas de borde, mientras que la

mayoria de las extinciones locales involucraria a especies especialistas de interior

de bosque. Solo la colonizacion de fragmentos de 10 ha siguio el patron predicho. La respuesta relativamente neutra a los bordes de bosque que observamos sugiere

que, en general, las aves de los bosques boreales no responden al borde en si ni a

sehales directas de la proximidad del borde. Las respuestas significativas podrian ocurrir mas tarde, una vez que los gradientes en la vegetacion desde el interior del

bosque hacia los bordes se hayan establecido.

Habitat edges influence the distribution of

many bird species: some cluster their territories

along edges, whereas others tend to avoid them

(Kroodsma 1984, Lanyon and Thompson 1986, Noss 1991, McCollin 1998, Renfrew et al. 2005).

Compared with the number of forest-edge "spe

cialists," relatively few species have been shown

to actively avoid forest edges (Baker et al. 2002), and even fewer exhibit consistent edge avoid

ance across studies (McCollin 1998, Villard

1998). It is critical to determine whether this lack of consistency is real, or whether it reflects

the lack of a general conceptual framework to

properly analyze, interpret, and compare pat terns in edge response (Ries et al. 2004).

Among the variety of mechanisms that

may explain significant positive or negative

responses to forest edges by woodland birds,

we distinguish three types: "access," "proxi

mate cues," and "ultimate cues." First, spe cies may be attracted toward edges because

they facilitate access to resources in adjacent

patches (Lanyon and Thompson 1986, McCollin

1998, Ries and Sisk 2004). Second, species may respond to proximate environmental cues

found near edges, such as

particular vegetation structures (McCollin 1998, Imbeau et al. 2003),

microclimatic conditions (Chen et al. 1999),

edge-related variations in food abundance

(Burke and Nol 1998, Ibarzabal and Desrochers

2004), or perhaps the presence of interspecific

competitors (Bollinger and Gavin 2004) or cer tain nest predators (Winter et al. 2000, Chalfoun

et al. 2002, Ibarzabal and Desrochers 2004,

Morton 2005). Third, ultimate effects of edges on

reproductive success may promote dispersal either away from or toward edges (e.g., Foppen and Reijnen 1994, Bollinger and Gavin 2004).

Thus, birds may respond either to proximate cues (the forest edge itself or conditions associ

ated with edge habitat) or to ultimate cues (e.g.,

edge effects on vital rates). When proximity to

an edge has consequences on individual fit

ness, an ability to use proximate cues would

represent a major advantage for individuals

possessing it.

To date, most studies investigating the spa tial response of forest birds to edges have been conducted in sites with pre-existing edges (e.g.,

Hansson 1983, Kroodsma 1984, Noss 1991,

Germaine et al. 1997, Flaspohler et al. 2001,

Manolis et al. 2002). In such cases, responses to proximate cues from the vegetation may be

more difficult to distinguish from responses to other proximate cues (microclimate, abun

dance of food, or nest predators), because the



830 VlLLARD, SCHMIEGELOW, AND TrZCINSKI [Auk, Vol. 124

influence of the vegetation is difficult to control

(but see Kristan et al. 2003). However, edge-to interior gradients in forest vegetation

are not as

strongly developed in the first years following clearcut harvesting (Harper and Macdonald

2002), providing opportunities to reduce con

founding effects.

Here, we measured the short-term response of forest bird species to experimentally cre

ated clearcut edges. We mapped bird territories

during the breeding season preceding clearcut

harvesting of the forest adjacent to our study

plots, and during the two subsequent breeding seasons. Seven species

were common enough

among plots and years to be included in the

analyses: Least Flycatcher, Red-eyed Vireo,

Yellow-rumped Warbler, Black-throated Green

Warbler, Ovenbird, Mourning Warbler, and

White-throated Sparrow (see Table 1 for scien

tific names). Collectively, these species represent a wide range of nesting and foraging strategies

(ground, shrub, and canopy nesters; ground for

agers, foliage gleaners, and aerial insectivores). We also examined population turnovers (local

extinctions and colonizations) for all other song bird species present in the study plots.

We tested the following predictions: after

clearcutting, (1) species classified as forest-inte

rior specialists would establish territories farther

from actual forest edges than from control-plot

boundaries, (2) species considered to be forest

edge specialists would defend territories located

closer to clearcut edges, whereas (3) territory location in relation to clearcut edges

or control

plot boundaries would not differ significantly for

interior-edge generalists. Finally, we

predicted that (4) in treatment plots, most postharvest col

onization events would involve forest-edge spe

cialists, and that most local extinctions would be

detected among forest-interior specialists. Ries

and Sisk (2004) pointed out that such a priori pre dictions should be made only when information

on habitat quality and resource distribution is

available for both sides of the edge. In the study area, only one of the seven species considered,

the White-throated Sparrow, is known to nest

and forage in recent clearcuts (Hannah 2001).

Experimental edges were sharp immediately

postharvest, with scattered shrubby patches and woody debris left in clearcuts. Therefore,

it is safe to assume that clearcuts represented nonhabitat for six of the seven species examined

during the present study. Preference or avoidance of forest edges by

certain species has important conservation

implications (Ries et al. 2004). For example, these phenomena would strongly influence the

amount and quality of habitat available for a par ticular species in a given landscape. Although

edge avoidance is routinely assumed by authors

Table 1. Comparison of predicted and observed responses of seven passerine bird species to

experimental clearcut edges in the first two years postharvest (see text for details).

Predicted Observed Observed

Species response (10-ha plots, P) (25-ha plots: R, P / F, P)

Least Flycatcher Neutral No data Yr 1: At (0.002) / N (0.90)

(Empidonax minimus) Yr 2: At (0.001) / N (0.22)

Red-eyed Vireo Avoidance Yr 1: N (0.94) Yr 1: N (0.27) / Av (0.076)

(Vireo olivaceus) Yr 2: N (0.33) Yr 2: N (0.42) / N (0.36)

Yellow-rumped Warbler Avoidance Yr 1: N (0.47) Yr 1: N (0.25) / N (0.51)

(Dendroica coronata) Yr 2: N (0.18) Yr 2: N (0.67) / N (0.78) Black-throated Green Warbler Avoidance No data Yr 1: Av (0.033) / N (0.63)

(D. virens) Yr 2: Av (0.055) / N (0.13) Ovenbird Avoidance Yr 1: N (0.17) Yr 1: N (0.11) / N (0.86)

(Seiurus aurocapilla) Yr 2: N (0.19) Yr 2: Insufficient data

Mourning Warbler Neutral Yr 1: N (0.29) Yr 1: N (0.44) / N (0.98)

(Oporornis Philadelphia) Yr 2: N (0.62) Yr 2: N (0.22) / N (0.24) White-throated Sparrow Attraction Yr 1: N (0.61) Yr 1: N (0.59) / N (0.37)

(Zonotrichia albicollis)_Yr 2: N (0.79)_Yr 2: N (0.68) / N (0.86) Note that there are two pairwise comparisons per year among 25-ha plots. Responses consistent with predictions are in bold.

See text for details on study design. Abbreviations: R = riparian fragment, F =

upland fragment, N = neutral, At = attraction,

Av = avoidance.




who refer to the concept of forest-interior spe

cies, this phenomenon remains largely untested

(Villard 1998). In spite of the fact that our study area is located in an

extensively forested land

scape with a recent history of anthropogenic

landscape change (Schneider 2002), forest types are still patchily distributed as a result of natu

ral disturbance regimes (mainly fire) and varia

tions in soils or drainage. Hence, a response to

forest edges could have evolved in bird species

occupying this landscape.

Methods

Study area. ? The present study

was conducted

from 1993 to 1995 near Calling Lake, Alberta

(55?N, 113?W), as part of a broader study on

the effects of experimental forest fragmentation on boreal birds (Schmiegelow et al. 1997). We

selected 11 study plots of two different sizes

(eight 10 ha, three 25 ha) in mature-to-old boreal

mixed-wood forest. In each plot, the dominant

tree species were trembling aspen (Populus

tremuloides) and balsam poplar (P. balsamifera), with scattered white spruce (Picea glauca) and

paper birch (Betula papyrifera). All plots were characterized by

a dense shrub layer dominated

by alders (Alnus tenuifolia, A. crispa), wild roses

(Rosa spp.), and lowbush cranberry (Viburnum

edule). Stand age varied from 90 to 130 years, and density and height of white spruce and alders increased with stand age (for details, see

Schmiegelow et al. 1997). The experimental design is illustrated in

Figure 1. Plots were located within continuous

forest (C: controls), or forest-fragment sites of

10 ha (three plots: F2-F4) or 40 ha (Rl and Fl) surrounded by clearcuts >100 m wide. We used

study plots of two sizes: (1) 10-ha plots within continuous forest (controls; n =

5) or surrounded

IKSp^BBaMHFw ^^MP^"- JgWiMiWlMllW 8hk I9BW8BHB BMPa. ffiB

1 0.5 0 1 2 3 ^î=^^^^H^=i^=i^^^^B Kilometers

Fig. 1. Map of study area, 200 km north of Edmonton, Alberta. Calling Lake occupies the north

east corner. Plot codes refer to controls (C), upland fragments (F), and riparian (lakeshore) frag ments (R). Smaller plots are 10 ha in size and larger ones 25 ha.



832 Villard, Schmiegelow, and Trzcinski [Auk, Vol. 124

by clearcuts on all sides (fragments; n = 3), and

(2) a 25-ha control plot (n =

1) and others within

larger (40-ha) fragments (n =

2). Plot Rl was con nected to a

riparian buffer strip, whereas all other

treatment plots (F1-F4) were completely isolated from nearby mature forest. It should be noted

that forest structure and composition within plot Rl was very similar to that of other plots because

it was located in upland forest, 100-300 m from

the riparian edge (Fig. 1). Experimental clearcut

ting took place during the winter of 1993-1994,

following the flagged site boundaries. Seven of the 11 plots

were surveyed

one year before har

vesting and two years after, whereas four were

only surveyed one year postharvest.

Because distance to the nearest edge varies as

a function of the number of edges and plot size

(10 ha and 25 ha), we performed separate analy

ses for each plot size and conducted pairwise

comparisons between 25-ha plot CI and plots Rl and Fl, respectively (see below). We mod

eled each year separately to allow within-year

spatial comparison with our controls. Pre- and

postharvest data were also used to examine pat terns in local extinctions and colonizations.

Bird surveys. ?

Every year, observers visited

each plot eight times. We mapped the loca

tions of all birds seen or heard and recorded all occurrences of countersinging among conspe

cific males following the procedure described in Bibby et al. (1992). For each species,

a mini

mum of two detections separated by >10 days was

required to register a territory (Bibby et al.

1992). In most cases, the number of detections

within a given territory exceeded three (Fig. 2).

For a given plot-year, we conducted analyses

only on species occupying at least two territo

ries. In two cases (Ovenbird; Fig. 3E, F), too few

detections fell within the control plot to permit statistical analysis.

Statistical analyses.?To quantify responses to

forest edges bordering experimental clearcuts,

we measured the distance from the location of

each bird detection within a defined territory to

the nearest forest edge in treatment plots, and

to the nearest plot boundary in controls. In the

case of 25-ha plots, we matched actual clearcut

edges with corresponding boundaries imposed on the control plot (Fig. 1). For example, for sta

tistical comparison between plots Fl and CI, we

measured the distance from each bird detection

to the nearest clearcut edge in Fl, and to the

nearest plot boundary in CI to the west, north,

or east. Because territories are nested within

study plots and individual bird detections are

nested within territories, we modeled responses to edge using linear mixed models (Pinhero and Bates 2000). In 25-ha plots, each species'

response to forest edges was estimated using the following model:

Vijk =

Pi + bj(i)

+ Zijk

i = l,...3,j

= l/...j,k

= l,...k

where y^k

= distance of an observation from the

nearest clearcut edge, i is site, j is territory within

site, and k is a detection within a territory. This

model specifies territory within site [bj(i)]

as a

random effect, and site as a fixed effect. The bj(i)

and etjk

values are assumed to be independent random variables with N(0, o2b), N(e, o2). The |3, values estimate the mean response of a

species to the forest edge at a site. Because the distribu

tion of possible distances from the nearest clear

cut edge differed between the two 25-ha plots, we ran separate models to compare each plot with the 25-ha control plot.

Smaller (10-ha) plots were replicated within treatments and controls. Our replicate plots

may have differed for many reasons; thus, this

nested structure allowed us to estimate the

between-site variability as a random effect and

to test for a treatment effect (T = forest fragment

or control) on the distance of bird detections

from the edge:

yTijk=PT +

bi(T) +

bj<i) +

EW

However, the fact that 10-ha plots contained

fewer territories than larger ones made it more

difficult to estimate the variance in the response

of territory holders at a site, b:(i).

Using every spot-mapped bird detection in

the analysis provides a more detailed assess

ment of response to edge than using average

distance-to-edge per territory, or average dis

tance from territory centroids to nearest edge. Detections within a territory are modeled as

independent measures of an individual bird's

activity in relation to the nearest edge. We

explicitly modeled the correlation in observa

tions collected within territories by estimating a random effect of territory distance from the

nearest edge (bj(i)). In other words, we assumed

randomness within a territory and modeled a

territory effect. Considering distance to nearest




Clearcut ....-"" . "*"

/^^ / \ - '""" a-1 ~\~?^-.(.

, *y One year i T \i_^

^^r>> preharvest R1 f*?500m?*

Clearcut _(. <y

;

Clearcut .'" ... \ \\ \ .^^^

( *\[^/[-' \ \ v postharvest

I- / /^"^^x Two years K| ^>=-^^ / .' 'V ".'J- postharvest

w rrVtiln Fig. 2. Changes in the number and distribution of Black-throated Green Warbler territories in

plot Rl before and after experimental harvesting. The location of the spot-mapping grid in rela

tion to experimental clearcuts is shown (top left). Each dot (right) represents a bird detection on

spot maps.

edge as a random effect could be problematic for abundant species because of the saturation

of available habitat but, typically, <50% of a

study plot was occupied by a given species. We

could also have compared the observed distri

bution of detections to randomly generated ones, but a

comparison to actual control data

seemed more biologically relevant.

Predictions. ?

Each species' response to experi mental forest edges was

predicted using existing classifications (Freemark and Collins 1992, Miller et al. 2004). Because species' assignment to cat

egories differed among authors in some cases,

we reclassified them as follows: (1) generalists: Least Flycatcher and Mourning Warbler; (2) forest-edge specialist: White-throated Sparrow;




25-ha plots: one year preharvest

A B 300 -i 300 -i

225 -L 225 J

:

pîhjipiii:

U n ij ij \-i

,i 0 J 19 6 12 17 5 3 9 11 2 3 14 7 14 10 Q J 19 15 12 15 5 4 9 8 2 2 14 15 14 18

C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1

LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP

^ 25-ha plots: one year postharvest

>? C D CD 300 -i 300 -.

O) P = 0.002 P = 0.03 P = 0.08

"? G) 225 -J J T 225

-j

fV. 0 -I 18 10 11 12 12 11 9 10 4 3 13 9 16 13 Q J 18 14 11 14 12 9 9 8 4 2 13 16 16 16

co "?^?r~~'?"?"?^?r? ?|?r~n? " ?r~n?|~~'?"?r?1 .? C1 R1 C1 Rl C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1

Q LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP

25-ha plots: two years postharvest

E F 300

-| 300 n

P = 0.001 P = 0.06

225 J J T 225 -J

0 J 14 12 10 11 5 14 9 7 2 3 5 10 9 14 q J 14 20 10 14 5 9 9 5 2 2 5 15 9 13

C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 R1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1 C1 F1

LEFL REVI YRWA BGNW OVEN MOWA WTSP LEFL REVI YRWA BGNW OVEN MOWA WTSP

Fig. 3. Mean distance (? SE) from bird detections to nearest clearcut edges or

corresponding control

plot boundaries in three 25-ha plots (Fl =

upland fragment, Rl = riparian fragment; CI =

control; see

Fig. 1), one year before harvest (A and B) and 1-2 years postharvest (C and D; E and F). Number of

territories within each study plot is indicated below mean distances, and P values are indicated above

when significant. The number of Ovenbird detections per territory in plot CI (two years postharvest)

was insufficient to allow statistical analyses. See Appendix for bird species codes.




and (3) forest-interior specialists: Red-eyed

Vireo, Yellow-rumped Warbler, Black-throated

Green Warbler, and Ovenbird. We reclassified

Red-eyed Vireo as a forest-interior specialist because it consistently avoided edges in other

studies (reviewed in Villard 1998). Accordingly, we

predicted that (1) territories of forest-interior

specialists would be located farther from the forest edge, when accounting for random varia

tions observed in the control plot, and that (2)

territories of forest-edge specialists would be

located closer to edges, whereas (3) territory locations of interior-edge generalists would be

similar in control and fragments. Finally, we

predicted that (4) most postharvest colonization

events would involve forest-edge specialists and most local extinctions would be detected

among forest-interior specialists. We compared the proportions of local extinctions and coloni

zations between forest-edge and forest-interior

specialists, pooled separately for 10-ha frag

ments, 40-ha fragments, and controls. All sites

were combined, whether they were monitored

one or two years postharvest. Species present in

a given plot before harvest and absent in the first or second years postharvest

were considered

locally extinct. Similarly, species absent before

harvest and present in either year postharvest were considered to have colonized.

Results

The classification of species according to

their perceived response to forest edges was

not a good predictor of their observed response

to experimental clearcut edges. On the basis of

statistical significance, 11 of the 36 responses observed were consistent with predictions

(Table 1). Six of these 11 responses were asso

ciated with the predicted neutral response of

the Mourning Warbler. Even when responses matched predictions, this was not the case for all

plots and years considered (Table 1). Significant edge avoidance was observed only in three

cases involving two species (Red-eyed Vireo

and Black-throated Green Warbler). Ovenbird

responses also suggested edge avoidance, but

the differences were not significant (Table 1;

Figs. 3 and 4). Our predictions assumed that published clas

sifications of species' responses to edge are based

on solid empirical evidence, which may not be

the case (Villard 1998). If we disregard these

A 100

| One year postharvest

80 - I

::h '! lni q, 43 3 3 4? 43 43 O) ? "

,_r_^_.T_^. 7__....-,_,_, g? CF CF CF CF CF

REVI YRWA OVEN MOWA WTSP

2

o B 2 10? i Two years postharvest

Q 80 - ~T~

"I* _T ,T tt 20 -

3 3 53 5 2 33 53

CF CF CF CF CF REVI YRWA OVEN MOWA WTSP

Fig. 4. Mean distance (? SE) from bird detec tions to nearest clearcut edge

or corresponding

control-plot boundary in 10-ha plots, (A) one

year and (B) two years after harvesting. The

number of territories within the study plot is

indicated below mean distances, and P val

ues are indicated above when significant. See

Appendix for bird species codes.

predictions and consider instead that the seven

species examined form a representative sample

of avian life histories in the boreal mixed-wood

forest, our results suggest that most boreal

forest bird species respond weakly to clearcut

edges per se and to environmental conditions

associated with recently created forest edges

(access to resources in adjacent patches, sharp

gradients in microclimate, and possibly changes in biotic interactions). However, because more

territories could straddle control-plot boundar

ies than actual edges, bird detections should

have been closer to control-plot boundaries by chance alone. Thus, our

analyses provide a very conservative assessment of attraction to edges and a liberal test of edge avoidance.

Our last prediction pertained to local extinc

tion and colonization events. Proportions of

local extinctions and colonizations differed




significantly when comparing 10-ha plots and

controls for both forest-interior specialists and

forest-edge specialists (G test, Gad]

= 9.77, P =

0.021), whereas the difference between 40-ha

plots and controls was not significant (P =

0.20). As predicted, most colonization events in frag ments involved forest-edge specialists (Fig. 5), but this was evident only in 10-ha fragments, where species turnover was greatest. There

were relatively few local extinctions, either in

fragments or controls, and the proportions of

forest-interior and forest-edge specialists going

locally extinct were similar. Interestingly, the

most striking pattern was the large proportion of colonization events in control plots involv

ing forest-interior specialists. These paralleled a

sharp increase in overall abundance in these

sites in the second year following harvest

(Schmiegelow and Hannon 1999).

Discussion

In spite of the diverse life histories of the seven

species considered, the dominant pat tern was one of neutral short-term response to

clearcut edges, especially in the smaller frag ments (10 ha). Neutral responses were predicted

in only two of those seven species. At the level

of species assemblages, the predicted influx

of forest-edge specialists was observed in 10

ha fragments, but numerous colonizations by

20 j-1

18 10-ha fragments H

16 El 40-ha fragments I 14 Controls r?i

8 12 I I 10 I

Is ^ I

1 1 LE(I) LE(E) C(l) C(E)

Fig. 5. Number of local extinctions (LE) and

colonizations (C) recorded in the study plots over the first two years following experimen tal clearcut harvesting. Data are

grouped by habitat-use category (I

= forest-interior special

ists, E = forest-edge specialists).

forest-interior specialists were also observed in

control plots. This latter result mirrors patterns in overall abundance, where total numbers of

individuals detected by point counts in the control plots increased significantly in the sec

ond year following harvest (see Schmiegelow et al. 1997, Schmiegelow and Harmon 1999). By contrast, overall number of individuals detected

in the treatment areas remained stable, despite

apparent colonization by forest-edge special ists (F. K. A. Schmiegelow unpubl. data). Thus,

experimental harvesting resulted in shifts in bird distribution across the study area in the first two

years postharvest (see Schmiegelow et al. 1997),

but these shifts were not reflected in predicted short-term responses by individual species on

the basis of existing edge-response classifica

tions. However, none of the patterns observed

was the opposite of predictions (i.e., avoidance

instead of attraction or vice versa). Therefore,

one could attribute the low predictive success

of the edge-response classification we tested to

alternative factors, such as low statistical power

(Murcia 1995). Nevertheless, to our knowl

edge, the present study is among the few field

experiments measuring forest birds7 response to

edges that feature spatial and temporal controls

and, at the 10-ha scale, replication. How can we

interpret these results? Four

types of explanation come to mind: (1) environ

mental conditions in our study region

or study sites are

atypical, (2) our results reflect a lack of

sensitivity in our approach to the measurement

of species' responses or low statistical power,

(3) existing classifications of bird species with

regards to their response to edges are invalid,

or (4) many boreal bird species do not or cannot

use proximate cues to detect edge habitat.

Disturbance regime and response to forest

edge.?As noted above, the study area is

characterized by an active fire regime, and

large-scale anthropogenic disturbance (for

estry, exploration for oil and gas) is recent.

Hence, one could hypothesize that forest

bird populations of north-central Alberta

are relatively unresponsive to the treatment

because they have evolved in a fine-grained,

highly dynamic landscape (Mutch 1970, Perera et al. 2004) where habitat edges dominate "natural" landscape mosaics. Longer-term

monitoring of responses at a regional scale is

necessary to determine whether newly created

anthropogenic edges elicit stronger responses




as exposure increases in conjunction with

expansion of development activities.

Sensitivity of our approach. ?

Although we are

confident in our quantitative approach to the

detection of edge response, we recognize that

our results are conservative with respect to the

detection of edge attraction. In addition to the

fact that territories could straddle the boundary of control plots, bias might arise from the spa

tial location of territorial males. Because males

spend large amounts of time patrolling terri

tory boundaries, differences in bird locations

between control and treatment plots could be

reduced if territory boundaries in the latter tend to correspond to clearcut edges. Only

one of the

seven species, the White-throated Sparrow,

was

predicted to exhibit significant attraction toward

edges, but we may have overlooked such

responses in other species, owing to the conser

vative approach we used. While spot mapping,

only once did we detect an individual of one of the other species considered here in an adja cent clearcut (male Mourning Warbler <10 m

from forest edge; M.-A. Villard pers. obs.).

Thus, White-throated Sparrow was the only

species likely to frequently use supplementary resources (sensu Dunning et al. 1992) in experi

mental clearcuts during the time-frame of this

study (see also Hannah 2001). Another complicating factor is the variabil

ity in abundance among species examined.

Although we

registered few territories for some

species (e.g., Ovenbird), contributing to low

power with which to detect effects, other spe cies were much more abundant, which may also

have reduced the probability of detecting shifts in response to experimental edges because of

territory packing. Mourning Warbler would fit

this pattern. On the other hand, Least Flycatcher still showed a

significant attraction to edges in

Rl, even though its density reached 10-18 ter

ritories in 25 ha (Fig. 3). The present study also points to an

impor tant tradeoff in sampling design when inves

tigating edge effects. It is necessary to sample several sites to quantify among-site variability.

However, maximizing the number of sites sam

pled may incur a cost in estimating a site-level

response, because the number of individuals

sampled at each site decreases when plot size

is reduced. Further, in the present study, vari

ability in distance from forest edge was greater

among territories than among 10-ha sites

(M.-A. Villard et al. unpubl. data), which indi

cates that larger plots containing more territories

provide a more accurate assessment of a spe

cies' response to edges. Future studies should

include replicated plots large enough to analyze

complex, additive edge effects (Fernandez et

al. 2002). For example, Black-throated Green

Warblers appeared to avoid the northeastern

corner of plot Rl to a greater degree than the

southeastern corner (Fig. 2), probably because

the northern and eastern plot boundaries were

directly adjacent to clearcuts, in contrast to the

southern plot boundary. Prediction of species response to forest edges.?

Villard (1998) questioned the validity of classifi cations of bird habitat-use with respect to forest

edges. His argument was based on the lack

of empirical evidence provided in support of those classifications, and on their poor perfor

mance when confronted with such data. Since

then, a few studies have provided empirical evidence to classify species (e.g., Porneluzi and

Faaborg 1999, Brand and George 2001, Bollinger and Gavin 2004). Here, we used existing clas

sifications strictly to formulate predictions. Our

results confirm that such classifications may not

be consistent across the range of a given species

and in variable landscape contexts, especially in

the case of forest-interior specialists. Use of direct or indirect cues. ?The general lack

of response of forest bird species to the abrupt forest edges that were created in this experi

mental study suggests that direct cues of prox

imity to edge play a minor role in breeding-site

selection. Unfortunately, we did not document

possible edê effects on reproductive success

and, therefore, cannot investigate potential costs of the observed lack of response to edges.

However, other investigations of edge effects

conducted in the same study area confirm

that edge effects on reproductive performance

were either weak or inconsistent (Cotterill and

Hannon 1999, Song and Hannon 1999, Lambert

and Hannon 2000). In summary, our results confirm that clas

sifications of species' habitat-use with respect to forest edges require further empirical test

ing. More replication at the larger plot scale

would also be desirable. The relative neutral

ity we observed in forest birds' responses to

sharp, recent forest edges is consistent with the

suggestion that birds may respond to edge-to interior gradients in vegetation (McCollin 1998,




Imbeau et al. 2003) rather than to edges per se.

Thus, a species7 response to edge will vary both

with landscape context and with time since

edge creation.

Acknowledgments

This research was supported by discov

ery grants from the Natural Sciences and

Engineering Research Council of Canada

to M.A.V and F.K.A.S., the Alberta/Canada

Partnership Agreements in Forestry (F.K.A.S.), and the Alberta Wildlife Enhancement Fund

(F.K.A.S.). We thank S. J. Hannon for her col

laboration in designing the broader experimental

study and for her assistance with field logistics. This research was made possible by the excellent

collaboration of the staff at Alberta-Pacific Forest

Industries. We are grateful

to our field assistants

R. Brown, S. Diggon, M. Eggen, A. Jansen, K.

Hannah, P. Heaven, K. Lisgo, M. Lomow, H.

Lefebvre, M. Ruebel, I. Schmelzer, J. Wojnowski, and to N. McVarish, T. Morcos, I. Robichaud, and

K. St. Laurent for geographic information system

(GIS) assistance. E. K. Bollinger and three anony mous reviewers made insightful comments on a

draft of the manuscript.

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Associate Editor: E. K. Bollinger




Appendix. Bird species investigated in the present study.

Code Species Classificationa

LEFL Least Flycatcher I/E REVI Red-eyed Vireo I YRWA Yellow-rumped Warbler I BGNW Black-throated Green Warbler I

OVEN Ovenbird I MOWA Mourning Warbler I/E WTSP White-throated Sparrow E

a Modified from Freemark and Collins (1992) and Miller et al. (2004); see text.

Classification key: I = forest-interior specialist, I/E = interior-edge generalise E =

forest-edge specialist.



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