Seasonal interactions between migration and winter in a migratory songbird, the Magnolia Warbler (Dendroica magnolia)
Aaron T. Boone, MS
Advisor: Paul Rodewald
Populations of many species of Nearctic-Neotropical migratory songbirds have been declining over the past several decades. Investigations of these declines have historically been focused on events occurring in temperate breeding grounds and tropical wintering grounds. Recent research on Nearctic-Neotropical migratory songbirds has emphasized the importance of events occurring during migration and interactions among breeding, wintering, and migratory phases of the annual life cycle. Stable isotope analysis has been in instrumental in revealing seasonal interactions and migratory connectivity in migratory songbirds, but little research has addressed interactions between events occurring in tropical wintering and migratory stopover areas.
From May to early June in 2004 and 2005, I investigated seasonal interactions within the annual cycle of Magnolia Warblers (Dendroica magnolia) captured during migratory stopover in northwestern Ohio (Ottawa and Lucas counties). Birds were captured using mist nets placed in dogwood shrubland, mature deciduous forest, and beach-ridge forest habitat. A total of 615 Magnolia Warblers were captured, banded, measured, weighed, aged and sexed. In 2005, approximately 200 individuals were sexed using molecular techniques, and the extent of pre-alternate molt (plumage-status) was assessed by counting the number of freshly replaced wing coverts and quantifying the amount of dark breast streaking with digital photo analysis. Feathers grown in pre-alternate molt in wintering areas were collected from 555 individuals and used for stable-carbon isotope analysis.
Stable-carbon isotope ratios (Δ13C ) of feathers ranged between -25.4‰ and -19.9‰ for both years, and this range corresponds to Δ13C values reported other species of wood-warblers that wintered in mesic tropical forest (generally considered high quality) and more xeric scrub. I tested for relationships between winter-specific habitat use (i.e., Δ13C values) and capture date, energetic condition (size-adjusted body mass), and plumage-status of transient Magnolia Warblers. I predicted that warblers that had wintered in more xeric habitats (enriched Δ13C values) would migrate later, be in lower energetic condition, and have lower plumage status (i.e., completed less molt in tropical wintering areas). Overall, results suggested that migration timing, energetic condition, and extent of pre-alternate molt completed by Magnolia Warblers were not influenced by wintering in either mesic or more xeric tropical habitats. I believe that Magnolia Warblers that wintered in more xeric locations may not have been negatively impacted or were able to regain mass during migration. Little published information exists on the wintering ecology of Magnolia Warblers. Additional information is needed to understand how winter habitat associations impact survival, energetic condition, migratory timing, and feather molt. Establishing the quality of various tropical habitats used by Nearctic-Neotropical migrants is important for the effective conservation of these species. A better understanding of the interactions among phases in the annual cycle of Nearctic-Neotropical migratory songbirds will be useful for developing comprehensive conservation planning that spans geographic and political boundaries.