Templeton, CN, E Greene, and K Davis. 2005. Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size. Science 308:1934-1937.

As we discussed in class, production of alarm calls when predators are detected is a common adaptation among many species. If a species is preyed upon by different predators that utilize different hunting strategies, alarm calls may be favored that have convey relatively complex information. These calls may communicate information about the type of predator, the risk it poses, or both. Researchers studied the alarm calls of the black-capped chickadee (Poecile atricapilla); a species forming flocks of six to eight individuals. Black-capped chickadee vocalizations include two alarm signals, a high frequency, low amplitude ‘seet’ vocalization and a loud, broad band ‘chick-a-dee’ alarm call. The ‘seet’ call is used when a flying raptor is detected and warns other members of the group to scatter. The ‘chick-a-dee’ call is used when a perched raptor is detected and invokes a mobbing response in members of the flock.

Researchers studied alarm calls in black-capped chickadees through presentation of predators in captivity. The authors presented chickadees with 13 species of live, perched raptors, 2 species of live mammalian predators, a bobwhite quail control and a control without any animal. Raptors varied in size by a factor of 20 from small, maneuverable species that have a large proportion of songbirds in their diet to larger species that eat fewer songbirds.

The number of calls was highly variable with smaller predators eliciting significantly more calls than larger predators or controls. The number of syllables per alarm call differed based on predator treatments, with the number of D syllables in the chickadee call being particularly variable. Both wingspan and body length of predators were inversely related to the number of D syllables. Numerous other factors differed between small and large predators including the length of the D section of the call (longer for smaller predators). Encounters with larger predators elicited higher energy, larger bandwidth D notes than those where small predators were encountered, the greater locatibility of these signals indicate that larger predators are viewed as less of a threat. Researchers played mobbing calls to chickadee flocks to determine their reactions to ‘chick-a-dee’ calls elicited by different predator presentations, an experiment similar to other studies described in lecture. Calls associated with smaller predators resulted in more calls and more intense, longer duration responses by listening individuals than did calls responding to larger predators. The authors concluded that chickadees encode information about predator size into their call as smaller, more maneuverable predators generally pose a greater threat. Researchers noted that two types of information can be transmitted, the size of the predator, and its state (flying versus perched or ground-based). This sort of complex signaling is similar to the meerkat example discussed in class.

The identification of possible size information in this study is particularly interesting as it seems to indicate that referential signaling is occurring in this species. The designs of these two calls also match what we would expect based on what we learned about alarm signals in class. The ‘seet’ vocalization is used to convey imminent danger and to incite members of the group to flee. Having properties one would expect for such a call, it is short and simple, has low amplitude and a relatively small bandwidth with a lower overall frequency. These features make the caller less easily located. The ‘chick-a-dee’ call has the hallmarks of an assembly call: it is longer, more complex and has a larger bandwidth, making the signaler more easily located. The conspecific predator alarm calls of black-capped chickadees may be direct benefits, indirect benefits or both, a concept introduced in lecture. A possible direct benefit is the maintenance of optimal group size, while indirect benefits may be important if black-capped chickadees form flocks with related birds as alarm calls would increase the survival rate of kin.