Faecal sac energy in common swifts (Apus apus) as a function of mass and nest identity

Nest sanitation is thought to be an important component of reproductive success because it can reduce pathogen loads and ectoparasites in the nest. Coprophagy - the ingestion of faeces - includes consumption of nestling faecal sacs by adults (allocoprophagy) and is therefore often discussed in the context of sanitation. In several bird taxa, this behaviour has also been linked to the "parental nutrition hypothesis", whereby adults may recover part of the energetic and nutritional costs of provisioning. In the Common Swift (Apus apus), faecal sac ingestion has been documented, yet the energetic content of faecal sacs has rarely been quantified directly, limiting evaluation of potential energetic recycling. Here we quantify the energy content of nestling faecal sacs using bomb calorimetry and test how gross energy relates to sac mass while describing among-nest variation. We collected 224 faecal sacs from nine nests in central Israel after fledging and measured gross energy and (desiccated) mass of each sample. Mean gross energy was 1.94 kJ per sac, with substantial variation among nests. In a mixed-effects model, gross energy increased with sac mass, and sacs from the same nest were more similar in energy content to each other than to sacs from other nests. Because samples were collected post-fledging and we did not measure ingestion frequency or assimilation efficiency, our results quantify the gross energetic substrate potentially available for recycling, but do not by themselves demonstrate a net energetic benefit of coprophagy. We outline the need for studies linking faecal-sac energy to nestling age, provisioning dynamics, and adult energy budgets to evaluate ecological payoffs and potential selection on this behaviour.