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Body size, first year breeding, and extra-pair paternity in an island endemic, the Island Scrub-Jay




Desrosiers, Michelle A., author
Angeloni, Lisa M., advisor
Ghalambor, Cameron K., advisor
Funk, W. Chris, committee member
Sillett, T. Scott, committee member
Crooks, Kevin R., committee member

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Body size is a fundamental characteristic that shapes all aspects of an organism's biology. The advantages of large body size may include increased probability of territory acquisition, a critical component of fitness for species that require a territory to breed. Large body size, an indicator of quality, may also be advantageous to males in acquiring mates, including matings outside of a pair bond. Such advantages could be especially important in island systems because habitat saturation may result in strong intra-specific competition for territories, and females may be especially motivated to seek large extra-pair mates to increase the body size of their offspring. We tested the role of body size in determining the ability of an island-endemic bird, the Island Scrub-Jay, Aphelocoma insularis, to acquire a territory and breed in their first spring, as well as to sire extra-pair offspring. We compared the body size of individuals that obtained a territory and bred to those that did not, as well as the body size of social fathers to the extra-pair sires to whom they lost paternity. We found that large body size was important in the siring of extra-pair young. However, body size did not predict the ability of male or female Island Scrub-Jays to acquire a territory and breed in their first year. We suggest that year-to-year variation in environmental conditions and chance may be more important than a large body size or weapon performance in early territory acquisition. Our study provides evidence for a mechanism, specifically female preference for a large body size in males, that supports the observed rates of extra-pair paternity, and demonstrates the general difficulty, even for individuals with a relatively large body size, of acquiring a territory as a yearling in an island system with saturated habitat.


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territory acquisition
body size
extra-pair paternity
Island Scrub-Jay


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