Time and energy constraints on the brood size of swallows, Hirundo rustica, and sand martins, Riparia riparia.

The daily energy requirements of swallow Hirundo rustica and sand martin Riparia riparia adults with broods of different sizes were compared with the energy (food) gain rates of adults. Energy requirements of broods were obtained from food delivery rates, growth rates and from published data. The energy requirements of adults were obtained from D2O18 measurements and from time budgets. Energy gain rates were measured from food delivery rates to collared nestlings. These energy gain rates were reduced in bad weather when food abundance was low. In good weather, energy gain rates were theoretically high enough to supply the energy needs of larger than normal broods. In bad weather, however, the adults would only be able to collect sufficient energy for a normal brood of four to six. I suggest that this effect of weather and feeding conditions on energy gain rates and, hence, on reproductive effort is an important factor in limiting the upper level of brood size in these species.