Nesting and postfledging ecology of Neotropical migrant songbirds in Missouri forest fragments
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The postfledging period, after fledging and before migration, is a critical stage for Neotropical migrant songbirds. The postfledging period encompasses an interval of high mortality that can greatly affect population growth models. Several species of mature forest nesting birds have been documented using very different habitat late in the summer, suggesting shifting habitat requirements during the postfledging period. I monitored nests and used radio-telemetry to observe postfledging juveniles of two species, the Ovenbird (Seiurus aurocapilla) and Acadian Flycatcher (Empidonax virescens), which overlap in nesting territories but differ in natural history. Monitoring was done in Missouri mature-forest fragments from 2012 to 2015. I document juvenile ecology and investigate how risks and habitat selection vary from the nesting period to the postfledging period and observe if trends are preserved across guilds. In chapter 1, I describe postfledging juvenile behavior, parental care, and space use. I used generalized linear mixed models within an information theoretic framework to evaluate the relationship of postfledging movement rates to intrinsic, temporal, and local-habitat variables. Fledgling Acadian Flycatchers (n = 45) utilized more vertical space and had 59% smaller natal home-ranges than fledgling Ovenbirds (n = 62). I found strong positive effects of age on movement distances for both study species and found a negative effect of foliage density on Ovenbird movements. My work provides a new source of support for the theory that habitat quality for postfledging Ovenbirds and other ground foraging forest songbirds increases with forest understory or groundcover foliage density. The few postfledging survival rates published to date are as variable as nest survival across regions and fragmentation gradients. However, factors that negatively impact nest survival may benefit postfledging individuals or not be as important postfledging. In chapter 2, I used an information-theoretic approach to determine support for effects of intrinsic, temporal, edge and local vegetation factors on survival in each stage and examined the potential effect of the resulting survival estimates on population growth. I did not find support for survival tradeoffs in habitat between stages. Nest period survival was comparable between species (~0.30) while postfledging period survival was 43% lower for Ovenbirds (~0.50) than for Acadian Flycatchers (~0.89). Projected population growth was sensitive to estimates of postfledging survival in our populations. In chapter 3, I compared resource selection for nest-sites and by postfledging juveniles using Bayesian discrete choice resource selection models evaluated with an information theoretic approach. Resource selection models indicated that Acadian Flycatcher habitat selection requirements relaxed from nesting to postfledging, with only canopy cover positively contributing to selection postfledging. Resource selection for Ovenbirds shifted from a preference for open understory mature forest nest sites, to increased selection for high understory foliage density and sapling density. Habitat management based upon nesting requirements would likely be sufficient for postfledging Acadian Flycatchers, but insufficient for postfledging Ovenbirds.
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