Energetics of free-living House Martins (Delichon Urbica) during breeding

Abstract

The D2O ^ technique is used to examine the daily energy expenditure of adult House Martins (Delichon urbica) raising broods of different size in order to extend the knowledge of the energetics of reproduction and to use energy expenditure as an estimate of the parental costs involved in reproduction. The Average Daily Metabolic Rate (ADMR) of House Martins rearing young, ranges from 2.22 - 5.27 x SMR, which is a similar level to that found in many other homeotherms. In the aerial feeding House Martin morphological and behavioural adaptations for flight help to reduce the costs of foraging and thereby help to minimise daily energy expenditure. The ADMR of male birds is positively correlated with nestling mass: this is probably a result of the rate at which food is delivered to the brood. Although the rate at which females feed the brood increased slightly with increasing nestling mass this is not reflected in their measured ADMR, which shows no correlation with brood mass. Experiments with artifically enlarged broods highlight the importance of the males' role in the fledging success of the brood. The energy expended by both parents is higher during second brood rearing than during first, and the possible proximate factors bringing about the increase are discussed. This increase is also discussed in the context of reproductive strategies: increased reproductive effort at a time of decreased residual reproductive value may increase fitness and be the ultimate factor responsible 4M for the higher energy expenditure during second brood rearing. Parental energy invested in reproduction is examined and it is shown that male and female House Martins invest equal amounts of energy in rearing the mean brood size. It is suggested that this is a result of the monogamous co- perative breeding system found in this species and the attempt by each parent to minimise the cost of reproduction by reducing their own energy investment to a minimum level compatible with successful reproduction.