Population ecology of a long-distance migratory songbird in a changing environment
Stodola, Kirk Winslow
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We are living in a time of unprecedented environmental change where species need to respond to many different anthropogenic factors influencing the habitat they depend on. Understanding how species respond to changes in habitat quality is a critical component in understanding and mitigating the detrimental effects associated with future environmental change. Climate change has recently received a lot of attention because of its ability to alter habitat over large swaths of land. In birds, migratory strategy appears to be an important characteristic influencing the ability of a species to respond to climatic change. I investigate this relationship using 40 years of breeding bird survey data and compare and contrast how a community of birds is responding at opposite ends of the Appalachian mountain range. My analysis suggests that long-distance migrants are disproportionately declining in abundance in comparison to short-distance migrants and resident species, yet the disparity is only observed at the southern and not at the northern terminus of the Appalachian Mountains. Although the pattern appears clear, there is little information pertaining to the mechanisms driving these disparate population trends. The Black-throated Blue Warbler (Dendroica caerulescens) is experiencing population declines in the south, similar to many long-distance migrants. I studied its population ecology from 2002¬–2009 near the southern terminus of the Appalachian Mountains, to investigate the mechanisms driving the decline. This time period coincided with the southward advancement of the hemlock woolly adelgid (Adelges tsugae), which provided the opportunity to observe how habitat alteration influenced population demography. The decline of hemlock (Tsuga canadensis) had little influence on the reproductive biology or survival of Black-throated Blue Warblers, yet its loss eliminated an important nesting substrate. Lack of nesting substrate probably contributed to the ~70 percent decline in breeding pairs where hemlock was most prevalent and used most often. The decline in breeding pairs was attributable to the failure of new breeders to settle, suggesting that population declines following habitat alteration can be due to changes in the perception of habitat quality by new breeders, and not to outright changes in reproductive performance or adult survival as typically believed.