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

Abstract

P. 2005. Differential migration of chiffchaffs Phylloscopus collybita and P. ibericus in Europe and Africa. Á/ J. Avian Biol. 36: 184 Á/190.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.

Morphological sexing of passerines: not valid over larger geographical scales

“…Morgan (2005) discussed the possibility of analysing multi-modal distributions of morphological characters to differentiate sexes accordingly. This approach was used, e.g., by Catry et al (2005) who separated male and female Chiffchaffs Phylloscopus collybita based on the bi-modal distribution of wing length. However, although this method is based on sound assumptions, there is no independent verification that the individuals classified as males or females do in fact belong to the respective sex cohort.…”

Sex differences in settlement behaviour and condition of chiffchaffs Phylloscopus collybita at a wintering site in Portugal. Are females doing better?

“…In our study area, chiffchaffs were present from early October to early March, being more common from October to late December. None of the individuals caught or heard in the field emitted the typical call of the summer breeding Iberian chiffchaff P. ibericus, and we believe those birds to have been absent from our study site in autumn and winter (see Catry et al 2005b for more details).…”

“…We used a maximum likelihood mixture model to separate the strongly bimodal distributions of wing lengths (see: MacDonald and Pitcher 1979;Catry et al 2005b). We used program MIX3.1a (Ichthus Data Systems) to estimate parameters allowing the development of sexing criteria (with estimated 99% confidence levels) for our samples.…”

“…Chiffchaffs show sex-based differential migration, with males more commonly found at higher latitudes and females being more frequent in the southernmost wintering areas (Catry et al 2005b). …”

“…We studied the settlement patterns and body condition of chiffchaffs at a site where the species is abundant during autumn and winter, in a region of the wintering range where females are more numerous than males (Catry et al 2005b). According to the dominance model, we predict that males (which are 14% larger than females, in chiffchaffs) would: (1) have better or at least similar body condition to that of females, and (2) be more likely to become sedentary at this study site.…”