Examining Ashmole's hypothesis: Are life-history parameters of resident passerines related to the proportion of migrants?

Ashmole's hypothesis predicts that clutch size of birds is related to competition with migrants. This is because higher competition with migrants leads to greater winter mortality among residents, and the resultant lower competition in the breeding season enables the remaining birds to lay larger clutches. We examined the predictions of Ashmole's hypothesis by comparing the life-history parameters of passerines inhabiting Australia, Southern Africa and India, three regions where the proportion of migrants increases in the above order. Since the proportion of migrants is higher in open habitats (grassland and semi-desert) than in closed ones (forest and rainforest), we examined the predictions of Ashmole's hypothesis in both habitats after controlling for phylogeny. We found that body mass was positively correlated with the length of the breeding season, egg mass and the length of both incubation and fledging periods, but not with clutch size. Contrasts controlled for body mass showed that clutch size in both closed and open habitats was significantly larger in India than in both Australia and Southern Africa, as predicted by Ashmole's hypothesis. Furthermore, the reproductive effort (clutch size multiplied by egg mass) of genera inhabiting two or all three regions showed that it was significantly larger in India than in the other two regions, thus supporting Ashmole's hypothesis. The length of the breeding season in closed habitats was significantly different (Australia > South Africa > India) between the regions. We attribute the differences in clutch size to ecological factors, mainly the larger proportion of Palearctic migrants wintering in India. The differences in the length of the breeding season may be attributed to the different climates of the regions examined.