Body weight,energy metabolism,and time in the life of birds

The breeding of birds, both large and small, is affected by two specific factors: (1) hypometry of egg weight relative to female body weight and (2) seasonality of breeding, with the favorable period being limited and almost equal for birds of different body sizes. Dozens of published allometric formulas describing the dependence of energy parameters of eggs and nestlings at different growth stages and the energy cost of parental care on the body weight of parents, eggs, and nestlings, respectively, are reviewed. It is shown that birds, especially species with a large body weight, repeatedly change their metabolic parameters during ontogeny in order to shorten the period of breeding and growth. In most species, the energy costs of breeding in both sexes are approximately equal. Bringing food in the bill allows birds to supply nestlings with the amount of energy that is dozens of times greater than that expended for obtaining the food. In placental mammals, only females are involved in offspring development. Therefore, the growth rate of embryos and energy expenditures for milk feeding are limited by the metabolic potential of the mother. As a consequence, mammalian offspring grow ten times slower than bird nestlings, the body weights of females being equal.