Parent:offspring resource allocation strategies in birds : studies on swallows (Hirundinidae)
The use of precision automated electronic balances has allowed remote quantification of instantaneous and long term body mass changes in breeding swallows, Hirundo rustica. By means of observation, experiment and optimality modelling, the extent of mass changes during incubation and nestling rearing are described, and the fitness consequences of mass changes discussed. An understanding of the causes and consequences of mass changes in swallows is developed from laboratory investigations of short term mass changes in canaries Serinus canarius, and from carcass analysis of breeding sand martins R. riparia, and swallows. Parent: offspring resource allocation was investigated during incubation in swallows. A model is developed which assumes that fitness is maximised in individuals which spend most time on the nest as a result of maximising the difference between net gain while foraging and clutch reheating costs, measured in units of energy. The model is tested, and the most frequently observed inattentive period proves, to be similar to that predicted to be the most energetically profitable. The early decline in swallow body mass during nestling rearing is likely to represent a 'programmed' anorexia in females during the brooding phase, whereby mass loss is beneficial in reducing flight costs and releasing energy available for work. After termination of brooding, however, mass losses were associated with rapid feeding rates to the brood for both sexes, and were judged to be potentially costly in terms of adult survival. (ii) By concurrent monitoring of resources for parents and offspring, investment in self-maintenance relative to investment in offspring is calculated, and the results interpreted in the wider context of lifehistory tactics and parental investment theories. Both sexes of swallow invested more in 'self' relative to 'offspring' when food was scarce or when feeding broods of small metabolic mass. Females appeared to risk their body mass falling to lower levels than that of their mates when feeding conditions were poor. overall, the study showed that the costs and benefits of mass changes in swallows differed according to the stage of the breeding cycle, and that detailed knowledge of the causes of mass changes allows insight into the evolution of reproductive strategies of birds of both sexes in relation to individual quality and resource availability.