We can improve our ability to assess population viability and forecast population growth under different scenarios by understanding factors that limit population parameters in each stage of the annual cycle. Postfledging mortality rates may be as variable as nest survival across regions and fragmentation gradients, although factors that negatively impact nest survival may affect postfledging individuals in different ways. We examined nest and postfledging survival of Ovenbirds (Seiurus aurocapilla) and Acadian Flycatchers (Empidonax virescens) in mature forest fragments in central Missouri. We used an information-theoretic approach to determine support for effects of factors intrinsic to the individual or nest site, temporal factors, local vegetation characteristics, and distance to edge on survival in both stages. We also examined the effect of incorporating the resulting survival estimates on population growth. In both species, survival increased from nest to postfledging stages (Ovenbirds: 0.27 ± 0.06 to 0.50 ± 0.09; Acadian Flycatcher 0.30 ± 0.03 to 0.89 ± 0.11). Age was by far the best predictor of survival in postfledging birds, with the majority of mortalities occurring in the first week out of the nest. We did not find support for survival tradeoffs of habitat used by nesting or postfledging birds. Acadian Flycatcher nest and postfledging survival were both related to variables associated with mature forest. Ovenbird nest survival was most affected by habitat characteristics associated with core mature forest, although postfledging survival may have improved near non-forest edges. We replaced an arbitrary estimate of juvenile survival (half of adult survival) with an estimate incorporating empirical postfledging survival estimates. With these revised parameters, Acadian Flycatcher population growth was more affected (13–26% increase in lambda) than Ovenbird population growth (3–6% change). Our results illustrate that species occupying similar nesting habitat do not necessarily face the same risks during the postfledging period.
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Vol. 118 • No. 3