MORTALITY COSTS OF PARENTAL CARE AND SEXUAL DIMORPHISM IN BIRDS

By using a comparative analysis, we test for evidence of mortality cos ts associated with parental care and sexual dimorphism in birds. We fi nd that changes in the extent and direction of sex bias in the provisi on of parental care are correlated with changes in the extent and dire ction of sex bias in the rate of mortality amongst adults. More specif ically, we identify feeding and defence of chicks, rather than nest bu ilding or incubation, as the most costly forms of parental care. This is the first evidence from a comparative study of a direct mortality c ost associated with providing parental care. Unlike previous comparati ve studies, however, we found no evidence of a mortality cost to adult s associated with sexual dimorphism in either body size or plumage. Th is was true even when we controlled for the effects of sex bias in par ental care and varied the phylogeny used in the analyses. Hence, among st adults birds, caring for chicks appears to be more costly than does sexual dimorphism. Amongst juveniles, however, increases in size dimo rphism are associated with increases in mortality amongst the larger s ex compared with the smaller sex. This result may, however, be sensiti ve to the phylogeny used in the analysis. Nevertheless, our results su ggest that, whereas the costs of parental care directly influence adul t mortality, the costs of sexual dimorphism are incurred during juveni le growth.