Biology 363/563 Ornithology

Dr. David Swanson, Office: CL 180


  • 1. Content

    • a) Yolk = serves as energy-rich food supply for embryo; consists of 21-36% lipid + 16-22% proteins. Yolk makes up 22-66% of total egg mass.

    • b) Albumen = egg white, 90% water + 10% protein; serves as water supply for embryo, acts as shock absorber, insulator, protects against bacteria; comprises 33-71% of total egg mass

    • c) Blastoderm = thin region of cells on top of yolk, forms embryo and extraembryonic membranes

      • (1) Amnion = surrounds embryo with watery environment, allows terrestrial development

      • (2) Chorion = outermost, protective and gas exchange functions

      • (3) Allantois = outgrowth of gut, fuses with chorion to form chorioallantoic membrane, functions in gas exchange and waste storage (uric acid)

      • (4) Yolk Sac = outgrowth of gut, surrounds and digests yolk

  • 2. Shell Structure

    • a) Inner fibrous shell membranes (2) composed of keratin; serve to contain contents during shell formation and anchor sites of shell crystal formation

    • b) Eggshell composition = inorganic calcium and magnesium salts embedded in network of collagen-like fibers. Main crystal is CaCO3 (98%).

    • c) Thickness of Shell = increases with Egg Mass0.456. Thickness decreases through incubation period (<10%) as embryo uses calcium from shell for skeletal development, also facilitates hatching.

    • d) Shell Texture = gas exchange between embryo and environment occurs by passive diffusion, but can't occur through shell. Pores form during shell production as CaCO3 crystals don't grow completely together (0.3-50 _m diameter) - allows gas and water exchange.

  • - Pores occupy only a small area of eggshell surface (approx. 0.02%).

  • - Pores may be funnel-shaped (wider at top), branched, or incomplete, and are often partially or completely covered with accessory materials (wide variety: iron, proteins, etc.), leaving only thin passages for gas exchange. Accessory materials are thought to prevent plugging of pores by mud, nest debris, etc.


  • - The number of pores per egg and pore diameter increase as egg mass increases, but pores/cm2 decreases as egg mass increases (Pores/cm2 Mass-0.231). Growth rate may be related to porosity: more pores allow increased oxygen uptake which may increase growth rate.

II. CLUTCH SIZE = number of eggs laid per nest. Varies greatly among birds: single egg (albatross, puffins, larger doves, some swifts, goatsuckers) to 8-12 eggs (ducks, Galliformes). Champion = Gray Partridge (9-23 eggs).

- Clutch size is a heritable trait acted on by natural selection over many generations. Constant in some bird species, but plastic in others.

- Why such variation??

Tradeoffs exist between costs and benefits of producing larger clutches, so theoretically an Optimal Clutch Size should exist that produces the maximum number of young surviving to reproduction. This clutch size should exist in all local populations.


Altricial Species = young hatch naked, blind, and helpless.

Precocial Species = young hatch feathered and self-supporting

- FOUR MAJOR HYPOTHESES FOR THE EVOLUTION OF CLUTCH SIZE (Assume that natural selection acts to optimize clutch size)

1. Lack's Hypothesis - proposed by David Lack (1947-1948) = clutch size is adjusted to maximize the number of young that the parents can feed or care for. Seems to work for many altricial birds (require greater parental care), but not for all, as some lay smaller clutches than those which appear most productive.

- Not as applicable to precocial species since they don't feed the young.


2. Tradeoff Hypothesis = Balance between current reproductive effort costs (production of eggs, providing for young, etc.) and future reproductive potential. Assume: Increased clutch size results in increased adult mortality.

- According to this hypothesis, clutch size should maximize the parents lifetime reproductive contribution, rather than the number of young surviving to breed each year.

- Some evidence suggests that large broods are more stressful than small broods (larger broods result in increased mass loss in parents), but clutch size and adult survival are not correlated in species tested so far. Thus, this hypothesis is apparently not important to the evolution of clutch size.

3. Predation Hypothesis = Nest predation selects for smaller clutches which are less likely to be found and preyed upon.

- Evidence = (1) Cavity nesters lay larger clutches than open-nesters (probably because they are safer from predators).

(2) Lower clutch size in tropical birds, where predator numbers are increased relative to temperate or arctic habitats.

- But, clutch size does not appear to vary consistently with predation level, so this hypothesis is probably not greatly important to the evolution of clutch size either.

4. Seasonality Hypothesis = clutch size reflects seasonal availability of food resources relative to population size. Adult populations are regulated at certain levels in the nonbreeding season by resource availability.

- Resource levels increase in summer in seasonal environments which leads to a "surplus" (over the nonbreeding season) that is available for reproduction. The greater the surplus, the greater the clutch size relative to the adult population size.


- This hypothesis appears to be the best explanation of geographic variation in clutch size among birds; supported by data from a number of bird species




I. Incubation = provides warmth and protection for developing eggs. Parent(s) that incubate have brood patch = bare, edematous, highly vascular skin on abdomen and/or breast, present only during the breeding season.

- Two patterns of incubation are present in birds:

1. Incubate as soon as first egg is laid = most types of birds; Provides greater protection of eggs from adverse weather and enemies.

  • * Results in Hatching Asynchrony = young hatch out at different times. Older young generally survive better and may abuse or kill younger siblings. In some cases, the parents may also kill younger offspring (Brood Reduction).

  • * Why lay >1 egg if resources aren't available for >1 offspring??

    • a) Insurance in case first egg or offspring dies

    • b) Adaptive in species faced with irregular or unpredictable food supply, since allows brood size to be matched to environmental conditions.

2. Incubate only after clutch complete = Practiced by waterfowl, Galliformes, most Passerines.

  • * Results in Hatching Synchrony = all young hatch at approximately the same time.

  • * Allows immediate switch from incubation to care of young. May allow increased numbers of young to survive. Occurs in species with predictable food supply.

II. Care of Young

- Amount of time and energy devoted to parental care varies depending on whether the species is altricial or precocial.

- Actually, there is a spectrum of maturity at hatching, ranging from extremely altricial to completely independent (able to walk, feed themselves, etc.).

- The most precocial birds are the Moundbuilders from Australia (Galliformes, Megapodidae). Adults build compost mound in which the eggs are buried. Heat from the fermenting vegetation is used to "incubate" the eggs; temperature is controlled by modifying the mound (adding or removing vegetation) by the male. The young hatch completely independent.

Parental Investment in Care of Young

(a) Precocial - ranges from none to feeding young. More parental investment in eggs in these species. Eggs are larger with a higher yolk content.

(b) Altricial - young are fed and brooded by parents.