Testing for sentinel coordination in Smooth-billed Anis (Crotophaga ani)

Abstract

Sentinel behaviour has been widely described, but whether or not it is coordinated among group members requires careful examination. The Smooth-billed Ani, Crotophaga ani, a joint-nesting cooperatively breeding bird, appears to have a sentinel system; however, it is unknown whether that vigilance is coordinated, the defining feature of sentinel behaviour. We followed social groups during two breeding seasons to observe and time their individual sentinel bouts. We then tested for coordination by quantifying the observed and expected proportions of time during which (1) no sentinel was present during observations, and (2) two or more sentinels were present. The majority of social groups spent less time than expected, under the coordination hypothesis, with at least one sentinel, indicating evidence against coordinated vigilance. This was significant for social groups observed in 2015, but not in 2002. Across both years, temporal overlap in sentinel behaviour between two individuals occurred significantly less than expected under the coordination hypothesis. This study contributes to a gap in our knowledge of sentinel behaviour and suggests that Smooth-billed Anis lack organized vigilance, possibly due to low within-group relatedness and group instability.