Are blackcaps sylvia atricapilla differential distance migrants by sex?

Aims: To investigate if male and female blackcaps show differences in the amplitude of migratory movements. Methods: The extent of movements of male and female British blackcaps were analysed using ringing recovery records. Furthermore, through a literature review and the collection of original field data, the sex-ratios of blackcap samples taken at several latitudes in Europe and Africa were compared. Results: There were no differences between British male and female blackcaps in relation to wintering latitude or distance moved during migration. Sex-ratios of blackcap samples were quite even across a wide range of latitudes. However, there was a significantly larger proportion of females in samples of birds that wintered in Africa. Conclusions: Laboratory data and morphological studies have yielded inconclusive results in relation to the question of whether or not European blackcaps are differential distance migrants. The results from the present study suggest that British blackcaps are not differential distance migrants. Furthermore, there is, at best, only a weak latitudinal segregation of the sexes of blackcaps wintering in Europe and Africa. The slightly higher proportion of males in European samples, when compared to Africa, could result from a differential behaviour of males and females in some blackcap populations, or simply result from differences in the sex-ratios of blackcap populations breeding and wintering in different areas. An absence of a pronounced latitudinal segregation in the size-monomorphic blackcap is interesting, and contrasts with the pattern found in more sexually dimorphic species, such as the chiffchaff Phylloscopus collybita or the robin Erithacus rubecula

Differential migration of chiffchaffs Phylloscopus collybita and P. ibericus in Europe and Africa

Differential migration is a widespread, but poorly understood, phenomenon in birds. In this paper, we present the first detailed field study of differential migration in the Old World warbler (Sylviidae) family. We studied two chiffchaff Phylloscopus [collybita ] semispecies: the common chiffchaff P. [c. ] collybita and the Iberian chiffchaff P. [c. ] ibericus. Using data collected at several latitudes in Europe and Africa, we present convincing evidence for differential distance migration of sexes in chiffchaffs, with females moving further than males. Interestingly, while there was a pronounced gradient in the sex-ratios in Europe and North Africa (with an increasing proportion of females with declining latitude), no clear pattern was found south of the Sahara, where sex-ratios were more male-biased than predicted by a simple latitude model. This suggests that, amongst the chiffchaffs wintering in West Africa, a large proportion is composed by Iberian birds, and provides support to previous suggestions that Iberian chiffchaffs are long distance migrants. Results from detailed studies in Senegal also show that chiffchaffs display differential timing of spring migration, with males leaving the winter quarters considerably earlier than females. The results are discussed in the framework of the three main (non-mutually exclusive) hypotheses attempting to explain the latitudinal segregation of the sexes. Given the relative failure of standard comparative studies to discriminate between competing single-factor hypotheses to explain differential migration, it is argued that the chiffchaff species complex might be particularly suited to study this issue using a new approach suggested by Cristol et al. (1999): detailed (further) comparisons between closely related species (such as the common and the Iberian chiffchaffs) could help identifying the key factors to be incorporated into optimality models that can predict relative distance of migration of different sex or age classes