Dramatic intraspecific differences in migratory routes, stopover sites and wintering areas, revealed using light-level geolocators

Abstract

Migratory divides are contact zones between breeding populations that use divergent migratory routes and have been described in a variety of species. These divides are of major importance to evolution, ecology and conservation but have been identified using limited band recovery data and/or indirect methods. Data from band recoveries and mitochondrial haplotypes suggested that inland and coastal Swainson's thrushes (Catharus ustulatus) form a migratory divide in western North America. We attached light-level geolocators to birds at the edges of this contact zone to provide, to our knowledge, the first direct test of a putative divide using data from individual birds over the entire annual cycle. Coastal thrushes migrated along the west coast to Mexico, Guatemala and Honduras. Some of these birds used multiple wintering sites. Inland thrushes migrated across the Rocky Mountains, through central North America to Columbia and Venezuela. These birds migrated longer distances than coastal birds and performed a loop migration, navigating over the Gulf of Mexico in autumn and around this barrier in spring. These findings support the suggestion that divergent migratory behaviour could contribute to reproductive isolation between migrants, advance our understanding of their non-breeding ecology, and are integral to development of detailed conservation strategies for this group.

Prolonged stopover and consequences of migratory strategy on local-scale movements within a regional songbird staging area

“…Using an array of automated radio-telemetry receiving stations, we confirmed long-held predictions that blackpolls exhibit prolonged stopover in the GOM during fall migration (Nisbet et al 1963, 1995, Nisbet 1970 Delmore et al 2012). Blackpolls and vireos also exhibited lower movement rates throughout the study area than typically reported for long-distance migrants from banding (60 km day À1 ; Ellegren 1993) or geolocator studies (68-473 km day À1 ; Fraser et al 2013), which suggests that birds were predominantly stopped while in our study area.…”

“…Although staging activity is typically attributed to shorebirds and waterfowl (Dunne et al 1982, Newton 2008, numerous geolocator studies have recently reported passerines making stops ranging from 8 to 36 days (Heckscher et al 2011,Åkesson et al 2012, Delmore et al 2012, Tøttrup et al 2012, Callo et al 2013, Fraser et al 2013, Jahn et al 2013, Kristensen et al 2013, Renfrew et al 2013, Wolfe and Johnson 2015, Van Loon et al 2017 that are analogous in length to staging events. Stops of .7 days surpass the amount of time theoretically expected for migratory refueling under optimality models (Alerstam 1991) and have been classified as ''prolonged stopover''-a behavior distinct from typical songbird stopover (McKinnon et al 2013).…”

“…Samplesize restrictions of geolocators have also limited the study of how prolonged stopover differs between individuals. However, intraspecific differences in prolonged stopover are likely, given that factors like capture date and breeding latitude can markedly influence stopover duration (Gómez et al 2017, Van Loon et al 2017, migration rate (La Sorte et al 2013, Brown and, and route choice (Delmore et al 2012, Fraser et al 2013. Understanding intraspecific differences is important from a conservation perspective.…”

“…Stops of .7 days surpass the amount of time theoretically expected for migratory refueling under optimality models (Alerstam 1991) and have been classified as ''prolonged stopover''-a behavior distinct from typical songbird stopover (McKinnon et al 2013). Although it is still unclear why songbirds exhibit this behavior, these extended stops may be an adaptive strategy for accumulating large fat stores at food-rich sites to fuel long migratory flights (Tøttrup et al 2012, Callo et al 2013), especially just before or after a barrier (Delmore et al 2012, Fraser et al 2013, Gómez et al 2017). Regardless of their purpose, effective conservation requires identifying prolonged stopover areas (McKinnon et al 2017, Van Loon et al 2017) and understanding behavioral patterns at these sites, because localized loss of stopover resources can pose an ecological bottleneck (Myers 1983, Buehler and Piersma 2008, Gómez et al 2017.…”

Prioritizing tropical habitats for long-distance migratory songbirds: an assessment of habitat quality at a stopover site in Colombia

“…Density estimates, considered in combination with other factors, may therefore be one of the most cost-effective and efficient means of assessing habitat quality. This does not preclude, however, the initial establishment of evidence for multiday stopovers (Delmore et al 2012) and active energy deposition (Bayly et al 2013), because high densities can occur at sites rarely used for fueling, e.g., sites used in emergencies, often referred to as "fire escapes" (Mehlman et al 2005). Our finding that densities were not uniform but varied between transects, presumably because of flocking behavior or clumped resources, e.g., fruiting trees, emphasizes the importance of surveying a representative area of the habitats of interest (Thomas et al 2010).…”