Influence of Condition and Habitat use on Survival of Post-Fledging Songbirds

Habitat quality of a bird's breeding grounds has been typically evaluated by investigating patterns in nesting success, whereas events that follow fledging have been largely ignored. One especially overlooked aspect of breeding-habitat quality is how habitat affects the survival of young birds after they leave the nest, a period when mortality is notoriously high. We studied survival of fledglings of two mature-forest species, the Ovenbird (Seiurus aurocapilla) and Worm-eating Warbler (Helmitheros vermivorum), to identify intrinsic (e.g., age, condition) and extrinsic (e.g., habitat structure) factors that influence survival. From 2004 to 2007, we radio-tagged 51 Ovenbird and 60 Worm-eating Warbler fledglings in southeast Ohio. We recorded the birds' locations daily and compared vegetation structure at the fledglings' and paired random locations. Using known-fate models in program MARK, we calculated post-fledging survival to be 65% for the Ovenbirds (51 days after fledging) and 67% for the Worm-eating Warblers (31 days after fledging). Fledglings' condition at the time of radio tagging was positively related to survival after fledging, implying carryover effects from the nestling period. Fledglings of both species used dense vegetation with 40–60% more woody stems in the understory than at random locations. Moreover, use of dense vegetation actually promoted survival. Although riparian thickets and tree-fall gaps within some forests may provide abundant habitat for fledglings, other forests may lack the structural attributes that promote fledglings' survival. Our findings highlight the importance of both breeding and post-fledging requirements being considered in avian conservation plans.