The annual cycle of a trans-equatorial Eurasian–African passerine migrant: different spatio-temporal strategies for autumn and spring migration

Abstract

The small size of the billions of migrating songbirds commuting between temperate breeding sites and the tropics has long prevented the study of the largest part of their annual cycle outside the breeding grounds. Using light-level loggers (geolocators), we recorded the entire annual migratory cycle of the red-backed shrike Lanius collurio, a trans-equatorial Eurasian-African passerine migrant. We tested differences between autumn and spring migration for nine individuals. Duration of migration between breeding and winter sites was significantly longer in autumn (average 96 days) when compared with spring (63 days). This difference was explained by much longer staging periods during autumn (71 days) than spring (9 days). Between staging periods, the birds travelled faster during autumn (356 km d ). All birds made a protracted stop (53 days) in Sahelian sub-Sahara on southbound migration. The birds performed a distinct loop migration (22 000 km) where spring distance, including a detour across the Arabian Peninsula, exceeded the autumn distance by 22 per cent. Geographical scatter between routes was particularly narrow in spring, with navigational convergence towards the crossing point from Africa to the Arabian Peninsula. Temporal variation between individuals was relatively constant, while different individuals tended to be consistently early or late at different departure/arrival occasions during the annual cycle. These results demonstrate the existence of fundamentally different spatio-temporal migration strategies used by the birds during autumn and spring migration, and that songbirds may rely on distinct staging areas for completion of their annual cycle, suggesting more sophisticated endogenous control mechanisms than merely clockand-compass guidance among terrestrial solitary migrants. After a century with metal-ringing, year-round tracking of long-distance migratory songbirds promises further insights into bird migration.

Long-distance bird migration within South America revealed by light-level geolocators

“…Red-backed Shrikes likewise first use the Sahel savanna of Sudan for ≤2 months after breeding, before moving farther south for the rest of the winter (Tøttrup et al 2012). Taken together, the results of these studies and our own suggest that large-scale (i.e., hundreds to thousands of kilometers) movements of overwintering migrants in the tropics may be more common than was previously thought.…”

“…(2) What is the location and extent of the wintering range of individual Forktailed Flycatchers, and how does the area they occupy in winter compare to that of other passerine migrants? Recent evidence from other migrant passerines demonstrates that at least some occupy more than one area (Heckscher et al 2011, Tøttrup et al 2012, or move throughout winter (Beason et al 2012).…”

Prolonged stopover and consequences of migratory strategy on local-scale movements within a regional songbird staging 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).…”

“…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.…”

“…Optimal migration theory predicts that birds balance the costs and benefits of different migratory decisions to minimize total time spent on migration, energy expenditure, predation risk, or some combination of these factors (Alerstam andLindström 1990,Åkesson andHedenström 2000). Many songbirds minimize migration time by seeking out optimal stopover habitats and accumulating large departure fuel stores rapidly in order to fuel long-distance flights (Hedenström 2008), while others minimize risk or energy expenditure by carrying smaller fuel loads, stopping more frequently, and making shorter flights (Bolshakov et al 2003,Åkesson et al 2012, Tøttrup et al 2012. The shorebird literature dubs these tactics ''skip'' and ''hop'' strategies, respectively, and outlines an additional strategy in which individuals accumulate extreme fat stores during long staging events to fuel subsequent ''jump'' flights .1,000 km (Piersma 1987, Warnock 2010.…”