Biology 363/563 Ornithology
Dr. David Swanson, Office: CL 180
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.
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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.
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- 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
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CONCLUSION: EVOLUTION OF CLUTCH SIZE IS PROBABLY A RESULT OF A NUMBER OF SELECTIVE FACTORS (PARENTAL CARE, SEASONALITY, PREDATION) ACTING OVER TIME.
** PARENTAL CARE **
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.
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.
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