Parental recycling of nestling faeces in the common swift.

Several hypotheses have been proposed to explain why parent birds eat their nestlings' faecal sacs. Among them, the parental nutrition hypothesis suggests that faeces may provide alternative food to parent birds, 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 faeces could prevent contamination of the nest (nest sanitation). In the 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 regularly swallowed faecal sacs during the first 3 weeks after the nestlings hatched, but only occasionally thereafter until they fledged. Both sexes ingested sacs at a similar rate in the first week after hatching, and ingestion rate was directly related to the number of feeding visits. Females ate significantly more sacs than males overall. The parents consumed faeces after actively searching into and around the nest cup, also during brooding spells. However, brooding decreased with increasing brood size and nestling age, while faeces consumption increased. Our results are consistent with the hypothesis that parent birds ingest their nestling faeces to recycle water and nutrients, making the 'best of a bad job' during periods of high energetic requirements. In addition, ingestion of faeces may be an alternative strategy to delay hunger and to facilitate the allocation of food to the offspring. As the nestlings grew, parents ate fewer faecal sacs. The increased begging behaviour of the young appeared to be an important factor in determining the decline of faeces consumption, as it hindered parents approaching the nest. Copyright 1998 The Association for the Study of Animal Behaviour.