Evolution of Growth Rates in Eagles: Sibling Competition Vs. Energy Considerations

Nestlings of birds that raise only one young (B1 species) grow slower than those of birds that raise more than one young (B2 species). B2 species may be fast growing because sibling competition exerts a strong selective force for rapid growth. Other hypothesis have been proposed to explain the slow growth of B1's. To overcome the problem inherent in previous investigations of comparing species widely divergent in ecology and phylogeny, I analyzed patterns of variation in growth of one group only: eagles of the world. I fitted growth curves to data obtained from the literature for some species, but most eagles were compared using the length of the nestlings period as an indicator of growth rate. I also collected data on the growth of wild Bald Eagle (Haliaeetus leucocephalus) nestlings. I investigated how postnatal growth may be related to brood size, clutch size, nestlings mortality, body size, and prenatal growth. B1 eagles were found to grow slower than B2 eagles. Differences in foraging ecology, diet, and mode of development are unlikely to account for the observed variability in the rate and form of growth. Contrary to the sibling competition hypothesis, growth rate was not correlated with intensity of sibling competition. Eagles that regularly lay two eggs but always lose one chick because of sibling competition (obligate fratricide) grow slower than those species in which such mortality may or may not occur (facultative fratricide). Species characterized by facultative fratricide grow at a rate expected for their adult body size, whereas to B1 species do not. B1 eagles are characterized by growth curves with early inflection points compared to B2 eagles. The shape of the growth curves and slower growth rates of B1 species and contrary to predictions of the sibling competition hypothesis and may instead have resulted from selection to reduce peak energy requirements of the nestlings. I present a model of the effect of hatching interval, species—specific growth rate, and relative growth of siblings, on the probability of fratricide, which suggests that sibling competition is not an important factor in selecting for rapid growth.