PARENT SESSION
Oral Session #75: Evolutionary Ecology -- Life histories, interactions.
Presiding: S. Armbruster
Thursday, August 8. 9:00 AM to 11:30 AM. Graham Meeting Room, TCC.
Clutch size evolution in a Neotropical bird: Experimental test of nest predation and individual optimization hypotheses.
Styrsky, Jennifer*,1, Robinson, Scott1, Brawn, Jeffrey1,2, 1 University of Illinois, Urbana, IL2 Illinois Natural History Survey, Champaign, IL
ABSTRACT- Avian clutch size varies along a latitudinal gradient such that tropical birds produce smaller clutches than their north temperate counterparts. One leading hypothesis to explain this pattern is that limited food supply in the tropics reduces the number of offspring parents can raise to reproductive maturity (Lack 1947). An alternative hypothesis is that in response to high rates of nest predation, tropical birds lay small clutches to reduce conspicuous nest visitation activity (Skutch 1949). We manipulated the clutch size of Spotted Antbirds ( Hylophylax naevioides ) in central Panama to evaluate these hypotheses. Spotted Antbirds build open-cup nests in tropical lowland forests and almost invariably produce a clutch of two eggs. From a sub-set of nests chosen at random, we removed one egg to reduce clutch size (n = 57) or added a third egg to increase clutch size (n = 40). Control nests were unmanipulated (n = 177). We compared nest predation rates, adult visitation rates to nests, and juvenile survival to dispersal among treatments. Neither adult visitation rates nor predation rates varied significantly among treatments during the incubation stage. During the nestling stage, adult visitation rates to the nest did increase significantly with brood size. Although predation rates during the nestling stage were significantly lower for reduced broods, predation rates did not differ for control or enlarged broods, which does not support the Skutch hypothesis. Enlarged broods produced significantly more fledglings than control broods, suggesting that food availability to nestlings does not limit clutch size. After fledging, however, juveniles from enlarged broods were less likely to survive to dispersal than were juveniles from control or reduced broods, which is consistent with the Lack hypothesis. Consequently, selection for small broods during the post-fledging period rather than during the nestling period may limit clutch size of tropical birds.
KEY WORDS: clutch size, life history evolution, Neotropical bird, predation