The use of time and energy by aerial-feeding birds

Abstract

The breeding and feeding ecology of the Swallow, Hirundo rustica, and the Sand Martin, R. riparia, in Central Scotland is described, with particular reference to the constraints imposed by environmental conditions. The time and energy investment in reproduction was examined for all stages of the breeding season in a wide variety of environmental conditions. Brood sizes were manipulated to change the investment of each parent in the brood. The D2018 technique was used to measure flight costs (0.0848 kcal g-l h-1 for the Swallow, 0.1288 kcal g-l h-1 for the Sand Martin) and the rate at which each species collected food under a variety of conditions was examined (mean values were 0.14 assimilable kcal min-1 for the Swallow, 0.05 assimilable kcal min-1 for the Sand Martin). These data, along with measurements of the nestlings' daily energy requirements, were used to investigate energy balance and the consequences for the timing and level of each breeding attempt. It is suggested that the Swallow lays later than the Sand Martin because (a) it takes larger insects and does not lay until these become abundant and (b) the female Swallow incubates alone whereas both Sand Martin sexes incubate, hence the Swallow defers laying until the risk of encountering bad weather during incubation is low. On 65% of the days on which measurements were made Sand Martins were unable to feed adequately a brood of five or more nestlings (31% of days for the Swallow) because foraging rates were depressed by bad weather. The risk of encountering bad weather is thus likely to limit the upper level of brood size. Energetics data are used to show that parents feeding nestlings maximise the net intake rate of energy rather than that of nutrients (protein, sulphur or calcium). The Swallow's diet includes nonpreferred small items, especially when these are relatively abundant, even when the preferred large items are available - contrary to predictions of optimal foraging models. It is shown, however, that foraging effiency is high when small items are taken. In agreement with central place foraging theory the bolus size of Sand Martins is mainly determined by the distance travelled to the feeding site. This is not always the case, however, since patch and prey quality and the search methods employed have a greater influence on the bolus size of the Swallow. Overall, the study allowed the scope for breeding activity for hirundines under different conditions to be defined and showed the importance of each species' reproductive and foraging strategies in optimising the number of offspring produced.