An Investigation of Territory Quality in the Smooth-Billed Ani (Crotophaga ani)

Abstract

Territorial behaviour is exhibited in cooperative breeders. The quality of defended territories can vary, and high-quality sites, which enhance fitness, should be used preferentially over poorer-quality sites. This study was intended to address issues of territory quality within the plural breeding, joint-nesting, smooth-billed ani (Crotophaga ani). I tested the communal joint-nesting threshold hypothesis, which posits that independent pairs will pay the price of group living if compensated by acquiring a superior territory. My results quantified differences in territory quality in terms of time of first breeding and chances for re-nesting or second-brooding. Per capita territory quality correlated negatively with group size, in conflict with the communal joint-nesting threshold hypothesis. Another hypothesis to explain communal behaviour is the habitat saturation hypothesis. This postulates that offspring remain in their natal territory and delay reproduction because of a local absence of suitable breeding habitat. I tested the habitat saturation hypothesis, which was developed to explain the evolution of group living in cooperatively breeding birds. I compared occupied ani territories with vacant sites, using eight ecological variables that may be important to the fitness of a group. In addition, I compared occupied territories with sites that were occasionally occupied. I did not find the habitat of our population of smooth-billed anis to be saturated. Some sites were never occupied because there are territory characteristics that presumably limit breeding. In contrast with expectations of habitat saturation, there were suitable sites available for breeding that were not in use. Results from both hypotheses presented suggest that there are not obvious group-living benefits and that one must consider other explanations for the evolution and maintenance of this system.