AUTHOR BLOG: Population-Specific Migration Patterns of Golden-Winged Warblers

condor-16-143

Gunnar Kramer

Linked paper: Nonbreeding isolation and population-specific migration patterns among three populations of Golden-winged Warblers by G.B. Kramer, H.M. Streby, Sean M. Peterson, J.A. Lehman, D.A. Buehler, P.B. Wood, D.J. McNeil, J.L. Larkin, and D.E. Andersen, The Condor: Ornithological Applications 119:1, February 2017.

Our study in The Condor: Ornithological Applications follows decades of research on the population dynamics of a declining songbird species, the Golden-winged Warbler.

Golden-winged Warblers have long been the focus of research and conservation interest due in part to sensationalized range-wide declines on the order of -2.5% per year; however, the true nature of these declines is more nuanced and complicated. When geographic populations are considered, Golden-winged Warblers demonstrate two very different stories. On one hand, the Appalachian population, breeding from Ontario through North Carolina and Georgia, experienced severe declines (up to -7% per year) over the past 50 years resulting in local extirpations and noticeable declines in the abundance and distributions of Golden-winged Warblers in the region. On the other hand, the western Great Lakes population, breeding largely in Minnesota, is home to ~50% of the global breeding population of Golden-winged Warblers and is maintaining historic population levels or possibly increasing over the past 50 years. When combined, these two very different stories blend into a general picture of overall population decline, spawning intensive efforts to identify breeding-grounds factors that might explain this decline, such as the loss of nesting habitat, poor nesting success, and competition with other closely related species (i.e., Blue-winged Warblers). Despite all the extensive research, no single breeding-grounds factor or combination of factors provides a parsimonious and consistent explanation for the differential population trends that exist in this species.

This is where our research comes in. Compared to the amount of research carried out in breeding populations of Golden-winged Warblers, relatively little has been done to link breeding populations to nonbreeding sites and identify migration routes in geographically isolated populations of Golden-winged Warblers. We developed a study to attach the smallest geolocators available (at the time in 2013 this was just under 0.50 g) to 9-10 g Golden-winged Warblers at multiple sites throughout their breeding distribution where they were experiencing variable population trends. Our main goal was simply to find out where these different populations went after they left the breeding grounds in North America and determine if the populations overlapped or occurred together in Central and South America during the nonbreeding period. If the declining Appalachian populations spent the winter in a region that was isolated from the stable western Great Lakes population, it is logical that the breeding-grounds population trends we observe might be caused at least in part by nonbreeding factors.

After all the hard work of redesigning the marking methods, getting the geolocators deployed, stressing over whether it would work, and then retrieving the geolocators, it was exciting to analyze the data and see entire year-long tracks of individual Golden-winged Warblers and to think about the distances these birds traveled and the places they spent their time when away from our study sites. We found that Golden-winged Warblers from Appalachian breeding populations spent the nonbreeding period in South America, mostly in a relatively small region on the border of Columbia and Venezuela. In contrast, Golden-winged Warblers from the western Great Lakes breeding population occurred throughout northern Central America in countries like Honduras, Nicaragua, Guatemala, Belize, and Mexico.

In the end, this geolocator study demonstrates a clear difference in nonbreeding locations and migration strategies among these different populations of Golden-winged Warblers. These differences are urgently meaningful from a conservation and management standpoint as they highlight a potential cause for regional differences in population trends observed across the breeding distribution. If nonbreeding factors are limiting Golden-winged Warbler population growth in the Appalachians, perhaps the most important implication of our work is to provide information that might help conservationists revise and refocus current strategies to better and target declining populations that spend the nonbreeding period in northern Colombia and Venezuela.

Read more at henrystreby.com.

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