PARENTAL RECYCLING OF NESTLING FECES IN THE COMMON SWIFT

Several hypotheses have been proposed to explain why parent birds eat their nestlings' faecal sacs. Among them, the parental nutrition hypot hesis suggests that faeces may provide alternative food to parent bird s, while the economic disposal hypothesis proposes that eating faeces represents an economical alternative to carrying them away when birds are engaged in parental activities. In both cases the elimination of f aeces could prevent contamination of the nest (nest sanitation). In th e present study, we tested these hypotheses on the common swift, Apus apus. The behaviour of parents at the nest was videotaped and analysed in relation to sex, brood size and nestling age. The parents regularl y swallowed faecal sacs during the first 3 weeks after the nestlings h atched, but only occasionally thereafter until they fledged. Both sexe s ingested sacs at a similar rate in the first week after hatching, an d ingestion rate was directly related to the number of feeding visits. Females ate significantly more sacs than males overall. The parents c onsumed faeces after actively searching into and around the nest cup, also during brooding spells. However, brooding decreased with increasi ng brood size nd nestling age, while faeces consumption increased. Our results are consistent with the hypothesis that parent birds ingest t heir nestling faeces to recycle water and nutrients, making the 'best of a bad job' during periods of high energetic requirements. In additi on, ingestion of faeces may be an alternative strategy to delay hunger and to:facilitate the allocation of food to the offspring. As the nes tlings grew, parents ate fewer faecal sacs. The increased begging beha viour of the young appeared to be an important factor in determining t he decline of;faeces consumption, as it hindered parents approaching t he nest. (C) 1998 The Association for the Study of Animal Behaviour.