Jones G. (1987) Parent-offspring resource allocation in Swallows during nestling rearding: an experimental study. ARDEA 75 (2): 145-168

Precision electronic balances were placed underneath Swallow nests, and parental masses, nestling masses, load masses, feeding rates and time budgets were recorded to investigate parent-offspring resource allocation during nestling rearing. An experimental approach was adopted, whereby brood-size or brood age was manipulated, and a comparison was made with control nests containing natural broods. Simultaneous measurements of food abundance and body masses showed that the brood and both sexes of parents were heaviest when insect prey was plentiful. Body mass changes of adult Swallows during nestling rearing were analysed to determine whether mass losses reflected the costs of feeding or brooding the nestlings. Only females brooded, whilst both sexes fed the nestlings .in roughly equal proportions. Females lost mass most rapidly when brooding young chicks, but before the peak food requirements of the brood were attained. The rapid mass loss was not, therefore, caused simply by the effort of feeding nestlings. Since females did not significantly lose mass when their brooding time was experimentally increased, and since mass changes while foraging were on average relatively low or negative during the late brooding phase, rapid mass loss during brooding was interpreted largely as a 'programmed' anorexia, whereby females targeted their condition towards an optimal mass in time for a period of high nestling demand. Such an optimal mass probably minimised flight costs and allowed energy release for work without jeopardising female survival. The mass changes of female Swallows while foraging immediately after the chicks hatched were, however, not significantly different from those of heavy, incubating females, and it is possible that rapid mass losses during the early stages of the brooding phase may reflect the time constraints of brooding coupled with the cost of foraging for self-maintenance and for the nestlings by females. Adults responded to the demands of experimentally introduced nestlings by quickly changing their food delivery rates and often altering their load masses to meet the requirements of the freshly manipulated brood. Female brooding behaviour did not, however, consistently reach expected levels when younger chicks were introduced into the nest, and a 'programmed' diminishing female brooding response was postulated. After the termination of brooding, daily mass changes of both male and female parents were negatively correlated with their rates of food delivery to the brood, and so mass losses during late rearing were judged to be potentially costly to the parents in terms of future survivorship. Nestlings in enlarged brood-sizes grew more slowly and attained lower peak masses than those in control or reduced brood-sizes. The consequences of adult mass changes, nestling growth rates, and foraging in different levels of food abundance was discussed in relation to optimal brood-size for Swallows.