American Robin

Turdus migratorius

                                  

              

“Birds Lay Eggs” …Aristotle

3

2 11. Embryo2. Shell

3. Shell Membrane

5

5. Yolk Sac

4

4. Air Cell

6

6. Albumen

7

7. Chorion

8

8. Amnion

9

9. Allantois

Clutch Size – the number of eggs a bird lays in one set

Wild Turkey Eggs

1. Average clutch sizes range from 3 to 12 among waterfowl and 2 to 23 among gallinaceous birds

2. Clutch size can also vary within a single species, for example, from 4 to 14 in the Northern Flicker

Great link to a short summary paper on clutch size

Clutch Size Relationships

1. Latitudinal Variation – Generally larger clutches in northern areas. Why?

2. Size?

3. Nest Size

4. Thermoregulatory effects

5. Habitats

Which particular # of eggs maximizes reproductive success for a particular species?

Nutritional requirements for eggs formation seem to limit clutch sizes of precocial birds

Feeding abilities of parents limit clutch sizes of altricial birds

General Theories of Clutch Size

1. The maximum number of eggs a bird can cover

2. Lack: The maximum number of young the parents can successfully raise. Mortality is determined by the rate of reproduction

The #’s of survivors per brood in relation to brood-size in Great Tits. Note that the most productive brood-size is around 10 while the the average clutch size is only about 9

Brood Size

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per

bro

od

General Theories of Clutch Size

3. Skutch: Theory of Adjusted Reproduction (Reproductive rate determined by annual mortality)

a. Reduction in clutch sizeb. Reduction in number of broods per yearc. Failure of male to assist in rearing of youngd. Deferment of reproductive maturitye. territorialism

Lesser Scaup (15 eggs in this nest, but usually lay 8-12).

Reproductive potential of this female is decreased.Why? Or is it?

Intraspecific Brood Parasitism and Clutch size

1. Intraspecific nest parasitism increases when there is a shortage of nest sites and population density is high

2. Usually results in larger clutches

Intraspecific Brood Parasitism and Clutch size 1. Female Common Goldeneyes produce fewer eggs when parasitic

eggs are added to nests early in the incubation period. However, they do not compensate for the parasitic eggs added late in the incubation period and incubate unusually large clutches.

1 4 1 4

Earlyparasitism

Lateparasitism

Shaded portion denotes the number of eggs added and white portion denotes eggs laid by incubating female

What Makes a Robin’s Egg Blue

Pigments result in Colors1. Color is added to the eggshell from pigments secreted by

cells in the oviduct wall.

2. The timing of pigment deposition affects color

3. Pigments added right before the egg is laid form the patterns or markings on its surface

-If the egg remains still while pigments are applied, spots appear-If the egg is moving, lines or scrawls appear.

American Robin Eggs

American Robin Eggs

Only three pigments are responsible for the tremendous diversity in egg color and markings

1. Protoporphyrin - produces colors ranging from yellow and pink to reddish buffs or browns.

2. biliverdin and 3. zinc chelate of biliverdin - result in blue and green colors

When these three pigments are applied in different proportions, colors ranging from violet-blue to olive-green may result.

Why should the color of an egg matter to a bird?

Why have any color at all?

Egg colors and markings have strong adaptive values• Eggs laid on the ground or in open nests in trees

often exhibit cryptic coloration.

The eggs blend in with their surroundings and are much less visible to potential predators. This “nest” belongs to a Killdeer, a shorebird.

Egg colors and markings have strong adaptive values• Distinctive patterns help females identify their

own egg

Each female of the Common Murre lays a single egg during each nesting with very different markings. Distinctive patterns help females identify their own egg in a colony where thousands of eggs may dot a cliff face.

Egg colors and markings have strong adaptive values

a) The brightness of the eggs may help the parents of cavity-nesting birds to more easily locate eggs in the cavity, But…

b) Could it be that cavity-nesting bird eggs are colorless because there was no selective pressure for camouflage since the eggs are concealed in a cavity?

• Eggs of cavity nesting birds, such as woodpeckers, are often white

Northern Flicker Eggs

American Robin

Turdus migratorius