Life history traits of Palaearctic and Nearctic migrant passerines

Abstract

I studied the life history consequences of tropical migration in passerine birds from Europe, and eastern and western North America. Tropical migrants were compared with more sedentary birds within a region . Analyses were conducted at two different taxonomic levels . First, at the generic level, I investigated whether there was any difference between migratory habit groups in general. If a genus consisted of species with different migratory habits, these were treated separately . Second, analysis at the within-family level (between-family differences eliminated) was used to assess the role of phylogeny in relation to life history differences observed at the generic level. I also examined the effect of body weight on life history features, using regression . Features common to tropical migrants on both continents were lower body weight and a smaller number of broods than in more sedentary birds. For a given body weight, tropical migrants also laid smaller clutches than more sedentary birds, but this differ ence was accounted for by between-family differences and did not indicate a linkage with migration per se . Mean annual survivorship did not differ between tropical migrants and more sedentary birds in Europe, whereas eastern Nearctic tropical migrants had a significantly higher survival rate at the within-family level, which may therefore be a direct consequence of wintering in the tropics. In the Nearctics, migratory habit seemed to be a more important factor, affecting other life history features than in Europe, where body weight explained between-genera differences better . The implications of the between-continent differences for conservation are briefly discussed.