Cleidoic, Amniotic Egg – Amazing Life Support System

“I think that if required on pain of death to name instantly the most perfect thing in the universe, I should risk my fate on a bird’s egg” Higginson 1863

Handling all metabolic requirements and byproducts independent of parent except temperature regulation

Egg Size, Shape and Color shaped by:

Morphology – grebes versus owls

Life History – cavity versus cup vs ground

Chick development – precocial vs altricial

Energy – YolkProtein – yolk and albumenCalcium/phosphorous – shellWater – AlbumenOxygen – Air Cell in part

Waste – uric acid (allantois)CO2

Outside

Chorion or Chorioallantois

Shell porosity adaptations

Burrow-nesting seabirdsMore pores

Black tern- wet nestsFewer pores

Shearwater – incubation 2.5X longer than chicken2 x fewer pores

Nest microclimate

Length of incubation

Fertilization, Egg production, Early Development

1) Sex Organs – Male – Homogametic ZZ Testis and Penis/Cloacal Protuberance CP

Female – Heterogametic WZ Ovary (ovaries)

2) Copulation

3) Internal Fertilization

Ova (Ovum) released via LH

Yolk = 1/3 Lipid, ¼ Protein, Water +

Associated compounds (androgens, carotenoids) = Maternal Effects

All Birds Internal Fertilization and Oviparous -

Fertilization

Magnum – 3hrs

Isthmus 1 hr

Uterus 19-20hrs

Cloaca minutes

Usually 1 egg/24 hrs. Laid in AM - Why?

Incubation of eggs – provide heat (below 26 devpt stops) and shade (>40 fatal) Brood patches – Prolactin, estrogen and progesterone in either or both sexes

Incubation periods

Time on nest

Time off nest

Given that 1 egg laid/day, brooding of multiple egg clutches can:

A) Begin immediately after first egg – asynchronous hatching of young Falconiformes, Strigiformes, Ciconiiformes, Pelecaniformes, Psittaciformes

B) Delay incubation until clutch is laid – synchronous hatching

Megapodes and Incubation via external heat sources - geothermal/biotic decay

Australian mallee fowl

Hatching -

Developmental Mode

Precocial (ancestral) Altricial

Table 18-6

Cute Ugly!

Contrasts of precocial vs altricial development

Precocial advantageous because once hatched, chicks can immediately fledge (nidifigous) feed on own and escape nestling predators.

Disadvantage – eggs take longer to develop, longer exposure to egg predators

Altricial allows greater growth (esp of brain) before maturation, so larger brain sizeDue to trade –off between organ growth and differentiation (maturation)

Disadvantage – although egg stage shorter, nestling phase longer, greater nestling predation risk, increased demands of parental feeding of nestlings

Fig 16-4 maturation of quail legmuscles reduces energy for growth early on, then costs of locomotion and feeding do so slower to reach adult size

Extremes of parental care

None Abiotic incubation Brood Parasitism

Single parent BothHelpersParents plusPrevious broods

Intraspecific brood parasitism “Egg dumping”Ducks, colonial swallows

Interspecific brood parasitism

Cowbirds, cuckoos, honeyguides

Figure 19-5

Evolution of brood parasitesIncreasing adaptationsIncreasing # host species

Common Cuckoo egg mimicry (on W chromosome)

Faster devpt, In utero incubation, earlier hatching, Egg ejection and host killing behaviors

Helper’s at the Nest - why forego reproduction and help?

Offspring perspective: Direct benefits:Indirect benefits: Ecological Constraints

Clutch Size

Determinate vs. Indeterminate Layers - will a bird replace eggs?

Clutch size variation: Correlated variables Lower latitude, lower altitude, body size, age

Evolution: Lack (1947) maximum # parents can feed and nourish

Alternatives: Trade-off hypothesis – long-term repro outputreduce clutch to inc adult survival

Predation – reduce time exposed , attractiveness and cost of loss where predation high (tropics)

Seasonality – greater the pulse in resources over baseline, the greater the clutch (latitude)