The post-fledging period is frequently considered the least studied and understood portion of the avian life cycle and could arguably be described as a frontier in avian ecology. For Neotropical migratory passerines, the post-fledging period begins with young fledging from the nest and extends until the onset of fall migration and is characterized by high mortality and changes in habitat use. For Ovenbirds and Worm-eating Warblers, our objectives were to calculate fledgling survivorship, identify patterns of habitat selection, determine whether habitat use influenced survival, and examine fledgling movements and diets.
Between 2004-2007 we radio-tagged 51 Ovenbirds and 60 Worm-eating Warblers immediately prior to fledgling from nests located in mature forests of southeast Ohio. In addition, 85 independent juvenile Ovenbirds were radio-tagged and randomly assigned to one of three experimental treatments: 1) released in original clearcut of capture, 2) moved to and released in a different clearcut, and 3) moved to and released in a mature forest habitat. All radio-tagged birds were tracked daily, GPS coordinates were recorded, and habitat measurements were collected. For three species we examined the diet of juvenile birds using stable isotopes.
Fledgling Ovenbirds and Worm-eating Warblers as well as independent juvenile Ovenbirds all demonstrated selection of habitats characterized by dense understory vegetation. For all three groups, use of areas with dense understory vegetation and an individual's energetic condition were positively related to survival. Post-fledging survival rates were estimated at 65% for fledging Ovenbirds and 67% for fledgling Worm-eating Warblers. Independent juvenile Ovenbirds had an 83% survival probability with no difference between treatments.
Natal home range size and distance between daily locations were larger for fledgling Worm-eating Warblers compared to Ovenbirds. For both species, movements of young fledglings were best explained by their energetic condition at the time of fledging. The ability of fledglings in high energetic condition to move farther distances may improve survival by facilitating the location of suitable post-fledging habitat. In contrast to young fledglings, independent juvenile Ovenbirds frequently undertook substantial movements, and birds released into mature forest habitat were documented moving farther from their release location than those released into clearcuts.
Because others have suggested that birds use regenerating clearcuts during the post-fledging period to gain access to fruit resources, we used stable isotope values of feathers to evaluate the dietary trophic level of juvenile birds. Our data did not support the idea that mature forest birds specifically use clearcuts for fruit resources during the post-fledging period.
Our results suggest that dense understory vegetation is a critical feature defining suitable post-fledging habitat. Timber harvest, and clearcutting in particular, is one way to create areas with dense understory and may be necessary to maximize juvenile survival in landscapes that lack suitable post-fledging habitat. However, because clearcutting also reduces availability of breeding habitat, managing for alternate post-fledging habitats (i.e., riparian thickets, tree-fall gaps) within the mature forest may be preferable in some landscapes. In the end, conservation strategies can be greatly improved by taking into account habitat requirements during the post-fledging period.