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

Catry, Paulo, Bearhop, Stuart, Lecoq, Miguel
Journal für Ornithologie 2007 v.148 no.2 pp. 241-249
Passeriformes, animal performance, birds, captive animals, females, gender differences, habitats, intraspecific competition, males, migratory behavior, molting, muscles, overwintering, physiological response, prediction, sex ratio, Portugal
In passerine birds, males are generally larger and dominant over females. In line with the dominance theory, in all known differential migrant passerines, females migrate further than males. However, there are alternative explanations to the dominance hypothesis, including the specialisation hypothesis, predicting that females should do better than males in habitats and/or regions to which they are specially adapted, and where they predominate in numbers. Few studies have aimed at comparing the behaviour and condition of males and females wintering in locations largely dominated by female birds. We studied a partly nomadic species, the common chiffchaff Phylloscopus collybita, at a wintering site in Portugal. Most individuals caught at this site were transients. The only consistent and relatively powerful predictor of settlement behaviour was sex. Almost all the birds that settled in the study area were female, while there was an almost even sex ratio in the transient sample. Females had higher muscle and post-juvenile moult scores when first captured. In addition, there was a significant sex-based difference in mass trajectories, with females maintaining body mass and males losing body mass when settled at the study site. Results from this study suggest that the poor performance of males was either due to inferior male competitive ability in a region/habitat where females predominate and/or it resulted from the fact that males that reach these wintering grounds are of inferior individual quality.