Migratory Connectivity of Vermivora Warblers
Since 2013, the Minnesota Cooperative Fish and Wildlife Research Unit in partnership with collaborators across the US and Canada have been investigating the migratory behaviors and nonbreeding distribution of golden-winged warblers (Vermivora chrysoptera) and blue-winged warblers (V. cyanoptera) from across their breeding range. Golden-winged and blue-winged warblers are small (9-gram), Nearctic-Neotropical migrant songbirds that migrate between North American breeding sites, and tropical wintering sites every year. Recent genetic evidence suggests these two species are extremely closely related and, despite significant differences in plumage coloration, these “species” may represent color morphs of the same species. Golden-winged warblers and blue-winged warblers breed in similar landscapes, sing the same songs, and hybridize regularly where their breeding ranges overlap. In general, golden-winged warblers breed throughout the Great Lakes states and provinces whereas blue-winged warblers breed more generally throughout the eastern portion of the US and portions of southern Canada.
Golden-winged warblers have long been a focus of research and conservation efforts as standardized population monitoring (through the national Breeding Bird Survey) has shown that populations of golden-winged warblers have declined by ~60% across their breeding range since 1966. However, these declines are not consistent across the breeding distribution. In fact, the overall declining trend is driven entirely by the almost complete loss of golden-winged warblers from the eastern, Appalachian Mountains population whose numbers have dropped 98% since 1966. Meanwhile, populations of golden-winged warblers breeding in the Great Lakes region have remained numerically stable. For decades, researchers have been trying to understand the drivers of these different population trends (stable in the Great Lakes, decreasing in the Appalachian Mountains) and have been unable to find a parsimonious explanation based on breeding-grounds factors like reproductive success, breeding habitat availability, hybridization, etc., hindering conservation efforts aimed at slowing and reversing declines.
Our research focuses on using relatively new technology to track these migratory songbirds from their breeding sites in North America, to their nonbreeding sites in the tropics of Central American and South America to attempt to uncover whether factors outside of the breeding period may be linked to these different population trends that are observed in golden-winged warblers on the breeding grounds. To collect the data required to answer these basic questions, we (and collaborators from the US and Canada) attached data-loggers called light-level geolocators to golden-winged warblers, blue-winged warblers, and their hybrids at 23 sites across their breeding range in the US and Canada. Geolocators are attached to the warblers using a backpack-style leg-loop harness and record the level of ambient light every 2 minutes and store that information on a computer chip onboard the unit. Geolocators are small and simple and do not transmit data like satellite tags. Therefore, geolocator-marked birds must be recaptured 1 year later in order to get data from the geolocators. The year following geolocator deployment, we returned to the same sites and recaptured the same geolocator-marked birds that were returning to their breeding territories. Upon recovering a geolocator, we are able to use computer programs to estimate the daily location (within ~150 km) of the geolocator-marked individual throughout the year and uncover the area the bird spent the winter and the migration routes used to navigate between breeding and wintering sites.
Results from the geolocators we have recovered from Vermivora warblers across their breeding sites show that golden-winged warbler populations winter in isolation from each other with the declining, Appalachian Mountain population wintering in northern South America in Colombia and Venezuela whereas the numerically stable Great Lakes population winters in Central America from southern Mexico, Guatemala, and Belize, through Honduras, Nicaragua, Costa Rica, and Panama. Moreover, forest loss in northern South America where the declining Appalachian Mountain population of golden-winged warblers overwinter has outpaced deforestation rates in Central America where numerically stable populations of golden-winged warblers and blue-winged warblers overwinter. As a result, conservation efforts focused on creating nesting or breeding habitat in North America may be ineffective if the amount or quality of winter habitat is limited in historically declining populations of golden-winged warblers. Instead, conservation and management to improve or create appropriate wintering habitat may be what is required to reverse long-term population declines in golden-winged warblers. This research demonstrates the importance of studying migratory species throughout their annual cycle to inform effective conservation strategies and identify factors that limit the populations of wild animals. Tracking these warblers clearly establishes the need for cooperative international conservation efforts as these, and other migratory species rely on many different geographic and sociopolitical regions to complete their annual cycle.
As we continue to work with data collected during this project, we will investigate population-level differences in migration behavior, factors influencing migratory decisions, immunological capability, and the genetics of migration.
See this document for more information on Gunnar's research!