Stopover ecology and the importance of two apalachicola bay barrier islands for neotropical migrant songbirds during spring migration

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Nearctic-Neotropical migrant passerines are experiencing population declines due to habitat loss and degradation on both the wintering grounds and breeding grounds. A decline in suitable habitat along the migration route may also be limiting populations. Stopover habitats are essential along an ecological barrier, such as the Gulf of Mexico, so that birds can rest and refuel after a long nonstop flight. In many cases, barrier islands represent the first piece of land a northbound migrant passerine may encounter after crossing the Gulf during spring migration. However, barrier islands are under significant threats due to climate change, sea-level rise, and other anthropogenic stressors. The goal of this study was to understand the use of these barrier island ecosystems by migrant songbirds during spring migration in order to better inform management decisions as barrier islands face increasing threats. During the 2015 and 2016 spring migration seasons, I operated banding stations on two barrier islands, St. George Island and St. Vincent Island located in Apalachicola Bay, Florida. I took measurements from all birds captured and took a blood sample from Gray Catbirds (Dumetella carolinensis) to be used in plasma metabolite analysis. The number of birds captured per net hour was greater on St. George Island, the smaller and more developed of the two islands. I found the two islands to be relatively equal in terms of measurements of fat scores, muscle scores, and body mass of captured migrants, suggesting that both islands, although different in many respects, provide important stopover habitat for migrant songbirds that is nearly equal in quality. This was especially true of the Northern Waterthrush (Parkesia novaboracensis), which seemed to find adequate resources on both islands. I found no significant effects of sex on fat scores, muscle scores, body mass, capture date, and refueling performance of Gray Catbirds. However, I found that second-year birds were larger than after second-year birds in 2016. Age had a significant effect on capture date on St. George Island with younger birds arriving before older birds in 2015 and older birds arriving before younger birds in 2016. Refueling performance for Gray Catbirds during the entire study period was best explained by year and fat scores. This study validates previous findings that Apalachicola Bay barrier islands are important for Neotropical migrant songbirds and that more research is needed in this geographical area. In particular, these islands may play a globally important role for transient Northern Waterthrush given the availability of isolated freshwater wetlands on both islands. Overall, both islands appear to provide important stopover habitat for migrant songbirds, although their relative importance may vary depending on annual effects of climate. In some years, one island may provide better stopover habitat for some species over the other island. Management efforts should focus on conserving these barrier islands, particularly their freshwater habitats, and other coastal habitats that are important for Neotropical migrant songbirds, considering they will be increasingly affected by sea-level rise.

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