MORTALITY COSTS OF PARENTAL CARE AND SEXUAL DIMORPHISM IN BIRDS

By using a comparative analysis, we test for evidence of mortality costs associated with parental care and sexual dimorphism in birds. We find that changes in the extent and direction of sex bias in the provision of parental care are correlated with changes in the extent and direction of sex bias in the rate of mortality amongst adults. More specifically, we identify feeding and defence of chicks, rather than nest building or incubation, as the most costly forms of parental care. This is the first evidence from a comparative study of a direct mortality cost associated with providing parental care. Unlike previous comparative studies, however, we found no evidence of a mortality cost to adults associated with sexual dimorphism in either body size or plumage. This was true even when we controlled for the effects of sex bias in parental care and varied the phylogeny used in the analyses. Hence, amongst adults birds, caring for chicks appears to be more costly than does sexual dimorphism. Amongst juveniles, however, increases in size dimorphism are associated with increases in mortality amongst the larger sex compared with the smaller sex. This result may, however, be sensitive to the phylogeny used in the analysis. Nevertheless, our results suggest that, whereas the costs of parental care directly influence adult mortality, the costs of sexual dimorphism are incurred during juvenile growth.