Nuthatches eavesdrop on variations in heterospecific chickadee mobbing alarm calls

Abstract

Many animals recognize the alarm calls produced by other species, but the amount of information they glean from these eavesdropped signals is unknown. We previously showed that black-capped chickadees (Poecile atricapillus) have a sophisticated alarm call system in which they encode complex information about the size and risk of potential predators in variations of a single type of mobbing alarm call. Here we show experimentally that red-breasted nuthatches (Sitta canadensis) respond appropriately to subtle variations of these heterospecific "chick-a-dee" alarm calls, thereby evidencing that they have gained important information about potential predators in their environment. This study demonstrates a previously unsuspected level of discrimination in intertaxon eavesdropping.

Fig. 1.

Chickadee alarm calls encode information about predator size and risk. The figure shows two chick-a-dee calls from the same individual chickadee: The first call was recorded in response to a large, low-threat, great horned owl, and the second was in response to a small, high-threat, northern pygmy owl. These signals vary in a number of acoustic features, including the calling rate, the number of broadband terminal D syllables in the call, and subtle variations in acoustic features related to the frequency and timing of the call (see ref. for more quantitative details on the acoustic differences).

Fig. 2.

Nuthatch approach behavior in response to different alarm call variants: chick-a-dee calls denoting large and small predators and control calls from a house sparrow (Cochran's test: P = 0.003). Each of the three playback treatments was standardized for length and amplitude (≈75 dB at 1 m).

Fig. 3.

Closest vertical-approach distance to the speaker varies with alarm call treatment for nuthatches that landed in the tree above the playback speaker (WSR: P = 0.006). Bars represent medians, and boxes delineate semiinterquartile ranges (25 and 75% of the data).

Fig. 4.

Percentage of trials in which nuthatches exhibited wing-flick behavior after the playback of large-predator and small-predator chick-a-dee alarm calls and control trials (Cochran's test: P = 0.0004).

Fig. 5.

Percentage of nuthatches that vocalized after the playback of control calls from a house sparrow and large-predator and small-predator chick-a-dee alarm calls (Cochran's test: P = 0.0004).