Booby clutch size
     Nazca boobies have a variable clutch size: sometimes females lay two-eggs per reproductive attempt while at others they lay only a single egg.  This pattern of clutch size variation is particularly interesting in Nazca boobies because it is an obligately siblicidal species, and only one chick is ever raised to fledging.  Nazca booby females typically lay their eggs 5 days apart and begin incubating immediately, resulting in a five day hatching asynchrony.  The hatching asynchrony sets up a size hierarchy between the chicks, with the first hatched chick ("A") being larger than the second-hatched chick ("B").  Within a couple of days, the A-chick pushes its sibling from the nest scrape, and the B-chick dies from exposure or is eaten by a predator.  Because of obligate siblicide, each Nazca booby pair can only produce one offspring per reproductive attempt.  The question arises, then, why do Nazca boobies produce a second egg if they will not raise two chicks to fledging?
Through experimental manipulations of clutch size, Ph.D. student Leslie Clifford found that two-egg clutches have higher reproductive success than do one-egg clutches because of the insurance value of the second-laid egg.  If the A-egg fails to hatch (Nazca booby eggs have a hatching failure rate of 25-40%) or the first chick dies in its first few days of life, the B-chick may be raised to maturity in its place, providing a sort of insurance policy against reproductive failure. 
Given the reproductive advantage that B-eggs confer, why don't all Nazca boobies produce two-egg clutches?  Part of the reason involves food limitation.  Leslie provided 50 pre-breeding females with as much fish as they would eat and compared the proportion of supplementally fed females that produced two-egg clutches with the proportion of control, unfed females that produced two egg-clutches.  92% of the supplemented females laid two-eggs, while only 70% of the control females did so, clearly implicating food limitation as an important determinant of clutch size in Nazca boobies.
Leslie tested two other hypotheses about clutch size variation in the Nazca booby that were not supported.  The first is known as the Indvidual Optimization Hypothesis, which proposes that individuals' optimal clutch sizes vary according to their circumstances.  The cross-fostering design of the above insurance egg experiment decoupled parental quality and clutch size, and allowed us to test the hypothesis that some individuals' optimal clutch size is one, while others have an optimal clutch size of two.   Experimentally enlarged clutches had a higher probabilty of producing a fledgling than did control-one egg clutches, indicating that all individuals would do better with a two-egg clutch.  Therefore, the Individual Optimization Hypothesis does not appear to explain why some individuals lay a single egg, although it should be noted that the birds with experimentally enlarged clutches did not have to incur the cost of actually producing the second egg.
The Egg Quality Hypothesis proposes that clutch size variation is adaptive, and that one- and two-egg clutches are equivalent strategies that trade egg quality for egg number.  This hypothesis predicts that one-egg clutches consist of a single, large highly hatchable egg, while two-egg clutches consist of two small low quality eggs.   Using our long-term data, Leslie showed that the first-laid egg from two-egg clutches is equal in size or larger than the single egg of one-egg clutches, suggesting that A-eggs from two-egg clutches are higher or equal in quality to eggs from one-egg clutches.  Therefore differences in egg quality, mediated by egg-size, can not explain clutch size variation in this species.
 

For more on this work see:

L. D. Clifford and D. J. Anderson. 2000.  Experimental demonstration of the insurance value of extra eggs in an obligately siblicidal seabird.   Behavioral Ecology, in press.

L. D. Clifford and D. J. Anderson.  2000.  Food limitation explains most clutch size variation in the Galápagos masked booby.    Journal of Animal Ecology, in press.

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