Female-Biased Mortality in Nestlings of a Bird with Size Dimorphism

Abstract

1. Explanations for sex-biased mortality include higher vulnerability of the heterogametic sex and greater susceptibility of one sex to food shortages. 2. We recorded daily mortality of male and female nestlings in 376 broods of the blue-footed booby, Sula nebouxii, a species where females are 32% heavier than males. We predicted: (i) higher mortality of last-hatched females than last-hatched males; and (ii) greater increase of mortality for last-hatched females than last-hatched males through the season. 3. Last-hatched females suffered greater mortality than last-hatched males, although the difference was only significant in three-chick broods. Male and female mortality was: 19% and 26%, and 24% and 44% in two-chick broods and three-chick broods, respectively. 4. Last-hatched females suffered a greater increase of mortality than last-hatched males as the season progressed, but the difference was only significant in broods of three chicks. 5. Pooled data from broods of two and three chicks showed that the risk of death increased substantially with hatching sequence and hatching date, although the increase was different for male and female chicks. Male and female mortality was similar in early broods, however, as the season progressed female mortality increased significantly with hatching sequence, while the increase in mortality of first-, second-and third-hatched males did not differ. 6. These results support the idea that blue-footed booby females are more vulnerable to food-related stress than males because of their larger size.