Spring migration strategies of Whinchat Saxicola rubetra when successfully crossing potential barriers of the Sahara and the Mediterranean Sea

Abstract

The flexibility for migrant land birds to be able to travel long distances rapidly without stop-overs, and thus to cross wide inhospitable areas such as deserts and oceans, is likely to be a major determinant of their survival during migration. We measured variation in flight distance, speed, and duration of major stop-overs (more than two days), using geolocator tracks of 35 Whinchats Saxicola rubetra that migrated successfully from central Nigeria to Eastern Europe in spring, and how these measures changed, or depended on age, when crossing the barriers of the Sahara or the Mediterranean Sea. Thirty-one percent of Whinchats crossed at least the Sahara and the Mediterranean before a major stop-over; 17% travelled over 4,751 km on average without any major stop-overs. Flight distance and speed during, and duration of major stop-overs after, crossing the Mediterranean Sea were indistinguishable from migration over Continental Europe. Speed during a migration leg was lowest crossing Continental Europe and fastest, with longer duration major stop-overs afterwards, when crossing the Sahara, but there was much individual variation, and start date of migration was also a good predictor of stop-over duration. As the distance travelled during a leg increased, so major stop-over duration afterwards increased (1 day for every 1000km), but the speed of travel during the leg had no effect. There were no differences in any migration characteristics with age, other than an earlier start date for adult birds. The results suggest that adaptive shortening or even dropping of daily stop-overs may occur often, allowing rapid, long-distance migration at the cost of major stop-overs afterwards, but such behaviour is not restricted to or always found when crossing barriers, even for birds on their first spring migration. The results may highlight the importance of stop-over sites rather than barrier width as the likely key component to successful migration. Individual variation in spring migration may indicate that small passerine migrants like Whinchats may be resilient to future changes in the extent of barriers they encounter, although this may not be true of first autumn migrations or if stop-over sites are lost.