AVIAN LIFE-HISTORY EVOLUTION IN RELATION TO NEST SITES, NEST PREDATION, AND FOOD

Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Picifo rmes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related t o adult survival, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to fo raging sites, whereas these traits varied strongly among nest sites. I nterspecific differences in nest predation were correlated with much o f the variation in life history traits among nest sites, although ener gy trade-offs with covarying traits also may account for some variatio n. For example, increased nest predation is associated with a shortene d nestling period and broth are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibl y due to an energy trade-off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary f ecundity trait on which selection is acting. Ultimately, food limitati on may cause trade-offs between annual fecundity and adult survival, b ut differences among species in fecundity and adult survival may not b e explained by differences in food abundance and instead represent dif fering tactics for partitioning similar levels of food limitation. Var iation in fecundity and adult survival is more clearly organized by ne st sites and more closely correlated with nest predation, species that use nest sites with greater nest predation have shorter nestling peri ods and more broods, yielding higher fecundity, which in turn is assoc iated with reduced adult survival. Fecundity also varied with migrator y tendencies; short-distance migrants had more broods and greater fecu ndity than did neotropical migrants and residents using similar nest s ites. However, migratory tendencies and habitat use were confounded. m aking separation of these two effects difficult. Nonetheless, the conv entional view that neotropical migrants have fewer broods than residen ts was not supported when nest site effects were controlled.