WETERING H VAN DE & JUKEMA J (2008) Spatial segregation of Common Swift Apus apus age groups during the breeding season?. LIMOSA 81 (1): 11-16.

In 1998-2007 Common Swiftswere ringed near breeding colonies in two villages in NW Friesland to study their population structure and distribution during the breeding season. Birds were caught using tape lures and mist nets. From 2003 onwards adult and second-calendar year ('first-year') were identified on the basis of differences in coloration and wear of the primaries. A total of 2520 Swifts were trapped, mainly adults in May, and mainly first-year birds in June and July (table 1). Up to 2007 20 ringed birds were reported from the same province (fig. 1), and two from Finland and Poland. Under certain weather conditions, large numbers of Common Swifts forage at low altitudes along the verges of a highway built on the dyke enclosing lake IJsselmeer, c. 3 km from the ringing site. Here, 36 birds were collected that fell victimto collisions with traffic, in the first half of July. All were adults and five of them had been ringed at the nearby breeding location, at least four years before. This contrasts strongly with the age distribution of the catches near the colonies in July (4% adults). Three causes are suggested for this difference: a difference in capture probability between adults and subadults near the breeding sites, a weaker bond of subadults with breeding colonies which leads them to wander and forage farther away than adults, and a higher propensity of adults to forage at low altitudes. The latter might be related to a higher abundance of larger flying insects (suitable as staple food for the young) low over the vegetation, which must be traded off against a higher risk of collision. The overall picture suggests a spatial segregation of foraging adult and subadult Swifts during the breeding season, both in height and in foraging area.

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