The evolution of body size in birds. II. The role of reproductive power

Given that body mass evolves non-randomly in birds, it is important to ask what factors might be responsible. One suggestion is that the rate at which individuals turn resources into offspring, termed 'reproductive power', mi ght explain this non-randomness. This is because, in mammals, the body mass with the highest reproductive power is the most common (modal) one. Reprod uctive power was estimated for birds from data on energetic content of eggs and population productivity. According to the formulation of Brown et al. (1993), reproductive power is composed of two component processes: acquisit ion (acquiring resources and storing them in reproductive biomass) and conv ersion (converting reproductive biomass into offspring). As with mammals, e stimates of reproductive power indicate that the most common body mass in b irds is also the body mass that maximizes reproductive power. The relations hip between reproductive power and diversity is different for species small er than this modal body mass when compared to those that are larger. The re lationship of body mass and reproductive power is different between birds a nd mammals in two ways: (1) the body mass that maximizes reproductive power is smaller in birds (33 g) than in mammals (100 g), and (2) mammals genera te more reproductive power than an equivalent-sized bird. Reproductive powe r is determined primarily by acquisition in small birds and mammals, while it is determined by conversion in the largest birds and mammals. It is like ly that reproductive power is closely tied to the evolution and diversifica tion of body masses because it constrains the ways in which traits affectin g fitness can evolve.