Joint-nesting smooth-billed anis, Crotophaga ani, use a functionally referential alarm call system

• We tested responses of smooth-billed anis, a joint-nesting bird, to two types of alarm call: chlurp and ahnee alarms.
• Anis tended to produce chlurps in response to aerial predators and ahnee alarms in response to terrestrial threats.
• In playback experiments, anis hid in vegetation after chlurp stimuli, and flew up after ahnee alarm stimuli.
• Ani responses were consistent with evading attack by aerial (chlurp) and terrestrial (ahnee alarms) predators.
• Both calls meet the requirements (production specificity, context independence) of functionally referential signals.

Accurate, threat-specific alarm systems might play a role in the evolution of joint-nesting systems by offsetting the high costs associated with joint nesting. The smooth-billed ani, a joint-nesting cooperatively breeding bird, produces at least two distinct classes of vocal alarms. Anis give ‘chlurp’ calls in response to flying raptorial birds, and they give ‘ahnee alarms’ in response to terrestrial threats. We used acoustic playback to test free-living anis' responses to these alarm calls. Anis responded to chlurp playback by diving into vegetative cover or flying low along the grass and into cover. These behaviours are appropriate defensive responses to an aerial threat. Anis that perched in low vegetation or on the ground responded to ahnee alarms by flying up to higher perches, an appropriate response to a terrestrial threat. Thus, both calls elicited responses that were appropriate to evade predation in the contexts under which the alarms are typically given. We conclude that smooth-billed anis respond to both chlurp and ahnee alarm calls by taking evasive action consistent with aerial and terrestrial threats, respectively. Even in the absence of an actual threat, the calls alone are sufficient to elicit these responses. To our knowledge, this is the first report of a cooperatively breeding, joint-nesting species that uses a functionally referential alarm calling system. We hypothesize that direct benefits garnered from this sophisticated alarm system offset some of the fitness costs incurred by joint nesting.