TWEL Andrew Vitz Dissertation

Survivorship and habitat use by juvenile forest birds
 
Andrew C. Vitz, PhD 
Advisor: Amanda Rodewald
 
 
The post-fledging period is frequently considered the least studied and understood portion of the avian life cycle, and, as such, could arguably be described as a “frontier” in avian ecology. Our lack of understanding about the period contrasts sharply with the perception and recent empirical data showing that the post-fledging period has tremendous consequences for juvenile survival and population recruitment. 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, though variable, levels of mortality. One of the interesting patterns that biologists have detected during the post-fledging period is that many species show pronounced changes in habitat use relative to breeding season habitats. In particular, birds known to breed exclusively in mature forest commonly shift habitats after breeding is concluded and select habitat with dense understory vegetation, such as early-successional forest. Such habitat shifts are presumed to enhance survival or condition, but this has not been explicitly studied. My dissertation research used both observational and experimental approaches to rate the importance of successional habitats to mature forest birds during the post-fledging period. More specifically, this study aimed to (1) estimate fledgling survivorship of Ovenbirds (Seiurus aurocapillus) and Worm-eating Warblers (Helmitheros vermivorum), (2) identify habitat features selected by post-fledging birds, (3) quantify the extent to which habitat use influenced survival, (4) determine which habitat and landscape features best predicted fledgling behavior, especially as related to movements, and (5) describe the dietary trophic level of juveniles of 3 species and test the influence of diet on energetic condition.
 
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. Forested sites were located either adjacent to a regenerating clearcut or completely surrounded by additional mature forest. In addition, 85 independent juvenile Ovenbirds (ca. 4 weeks old) 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 (Global Positioning Systems) coordinates were collected, and habitat features were compared between actual fledgling and random locations. Known fate models in program MARK were used to estimate fledgling survival rates and evaluate the influence of understory vegetation and energetic condition on survival. For three species of forest songbirds, we examined whether fruit resources were heavily consumed during the post-fledging period using a stable isotope analyses of carbon and nitrogen.
 
Fledgling Ovenbirds and Worm-eating Warblers as well as independent juvenile Ovenbirds consistently selected habitats characterized by dense understory vegetation. Compared to random locations, birds not only used areas with 1.4-1.6x more understory vegetation, but use of these areas actually promoted survival. For fledgling Ovenbirds and Worm-eating Warblers and independent juvenile Ovenbirds, an individual’s energetic condition at the time of radio-tagging also was positively related to survival during the post-fledging period. Post-fledging survival rates were estimated to be 65% for fledging Ovenbirds and 67% for fledgling Worm-eating Warblers. No difference in survival was detected between independent juvenile Ovenbirds released in clearcut or mature forest habitat, and with all groups combined Ovenbirds had an 83% probability of surviving the 52 day period.
 
Although natal home range size for Worm-eating Warblers was nearly twice as large as those for Ovenbirds, both were substantially larger than typical breeding territories. Distances between daily locations averaged 1.5x farther for Worm-eating Warblers compared to Ovenbirds, and as fledglings aged the distance from the nest and between daily locations increased. 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 (> 1 km in a day), 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 may use regenerating clearcuts during the post-fledging period to gain access to abundant fruit resources, we quantified stable isotope values of retrix (grown as a nestling) and body feathers (grown after fledging) to evaluate the importance of fruit in the diet of young birds. Compared to retrices, basic plumage body feathers were more enriched in δ15N and δ13C values, suggesting that independent juvenile Scarlet Tanagers (Piranga olivacea), Wood Thrush (Hylocichla mustelina), and Ovenbirds primarily consumed predatory arthropods, whereas lepidopteran larvae were the principal food source when young were being fed by adults. We did not find evidence that diet influenced energetic condition nor did our data support the idea that mature forest birds specifically use clearcuts for fruit resources during the post-fledging period.
 
Although the breeding biology of most Neotropical migratory birds is well studied, information regarding the post-fledging period is generally lacking. By employing both observational and experimental approaches, this study makes a unique contribution to our understanding of post-fledging ecology by linking behavioral decisions regarding habitat use to energetic condition and survival. With respect to conservation, results suggest that dense understory vegetation is a critical feature defining suitable post-fledging habitat. Timber harvest, and clearcuts in particular, are 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.