Mother knows best: functionally referential alarm calling in white-tailed ptarmigan

Functionally referential alarm calls have stimulus specificity, distinct acoustic structure, and elicit different escape responses that are appropriate to the threat. The mechanisms by which escape responses are evoked are not fully understood and may range from eliciting innate responses to conveying representational information. White-tailed ptarmigan (Lagopus leucurus) are a long-lived alpine tundra grouse, which are preyed upon by aerial and terrestrial predators. We investigated the hypothesis that alarm calls of ptarmigan hens with chicks are functionally referential. We recorded hens' alarm calls in response to naturally occurring and model predators in California's Sierra Nevada alpine tundra for two summer seasons. We conducted playback experiments in the field to determine chick responses to alarm calls. Alarm calls commenced with an extended 'alerting' note followed by a series of staccato notes grouped into elements. Fundamental and dominant frequencies in element notes were significantly higher in terrestrial compared to aerial threat alarm calls. Playbacks of terrestrial threat alarm calls elicited an upright/alert position by chicks (75 % of responses). In response to aerial threat alarm call playbacks, chicks flattened to the ground and froze (80 % of responses). To our knowledge, this study provides the first empirical evidence of functionally referential alarm calling, including the responses of the receivers, in an avian species in the wild.