Why defaecate on your doorstep? Investigating an unusual behaviour in Africa’s smallest falcon

Abstract

Depositing faeces at the nest should be expected to carry risks such as increased parasite loads and disease exposure. This perplexing behaviour is unusual in birds but is consistently shown by a handful of species, and has been demonstrated to function in predator deterrence, thermoregulation and prey attraction. Pygmy Falcons Polihierax semitorquatus, which occupy the massive, communal nests of Sociable Weavers Philetairus socius, consistently defaecate at the entrance of their nest chambers, creating a conspicuous ‘doormat’ of white faecal matter. We addressed two potential adaptive hypotheses, proposing that antibacterial and/or antifungal functions in Pygmy Falcon faeces may explain this behaviour, such that faecal mats could reduce bacterial and/or fungal loads within the nest. Fresh and dry organic faecal extracts were tested, using the disc diffusion method, against Staphylococcus aureus and two of numerous mould species cultivated from samples collected in situ. We found no evidence for antibacterial or antifungal effects of Pygmy Falcon faeces against these test organisms. Further investigation into alternative hypotheses such as boosting falcon immune systems, anti-parasitic properties, conspecific signalling, thermoregulation and/or predator deterrence are therefore required to shed further light on this unusual behaviour.

Keywords: communal breeding, faecal sacs, faeces, immune response, nest materials, nest sanitation