Avian life-history evolution: Explaining variation among species, populations, and individuals

Explaining the diversity of life history strategies adopted by organisms is a central goal in evolutionary ecology. However, the goal of understanding life history evolution is complicated by the fact that variation exists at many different levels of organization, and the sources of variation at one level often fail to explain variation at another. I explored this problem by examining the causes of life history variation at three levels: among species, populations, and individuals.; First, I used a comparative analysis of 70 bird species to test the hypothesis that sibling competition favors the evolution of rapid development. All three measures of sibling competition (extra-pair paternity, brood parasitism, and hatching asynchrony) covaried with incubation period in the direction expected under the sibling competition hypothesis, but only extra-pair paternity explained significant variation in incubation period after controlling for phylogeny. This suggests that interspecific variation in development may be an adaptive response to the evolutionary pressure of sibling competition.; Life history variation also can arise as a consequence of environmental constraints and hence need not be adaptive. To explore these potential roles of constraint and adaptation, I collected data on life history traits of Chestnut-collared Longspurs (Calcarius ornatus) breeding in habitats that differed in nest predation risk. Contrary to the expectations of life history theory, nestlings in the high-risk habitat, which consisted of monocultures of an introduced grass, grew more slowly and had longer post-natal developmental periods than did nestlings in the low-risk, native habitat. In this case, life history variation is not adaptive but instead reflects constraints imposed by the environment, most likely reduced food availability in the exotic habitat.; Averaging life history traits across habitats may mask smaller-scale variation. In particular, variation in nest site choice within a habitat may be an important source of life history variation among individuals. Indeed, I found that Longspurs choose nest sites that created an amenable radiative environment for offspring, and by experimentally manipulating nest orientation I found that the direction a nest faces, by modulating insolation, has a significant effect on growth and development of nestlings.