A walk through the woodlands after a fresh snow can bring a needed meditative silence at the end of a busy day. But sometimes silence is short-lived, and if you’ve ever found yourself suddenly surrounded by a flock of scolding chickadees, then you know what I mean! The flipside is that, if you’re ever looking for something interesting in a woodland that seems devoid of birds, listen carefully and you might be rewarded with the buzzy alarm calls used by many of the resident winter birds that live in groups.
(A spectrogram of the “fee-bee” of a chickadee followed by two buzzy “chick-a-dee” calls, each with 4+ “dee” notes, suggesting a mobbing call)
These alarm calls are like an auditory advertising billboard; with a dominant mid-range frequency, they are repetitive and loud, and it is easy to home in on their source. And often, what you find at that source is something, well, alarming (at least to a chickadee), like a small owl or an accipiter (e.g., the Cooper’s Hawk below).
It turns out that birders aren’t the only ones using these advertisements to find potential predators. In fact, up to 50 different species have been noted to respond to chickadee alarm calls, and they typically join the group of scolding chickadees in a behavior called “mobbing“. The ultimate function of mobbing is unknown, but past studies have suggested that it may be a social tool to identify predators and/or an advertisement to a predator of the mobbing bird’s knowledge of the predator’s presence. Mobbed predators have been shown to depart more quickly (there’s a neat movie here) than un-mobbed predators, which suggests that resident birds can affect a predator’s foraging habits simply by ruining it’s chances of a surprise attack (and an easy meal). Other animals are known to perform advertisements of their knowledge of a predator’s presence, most notably the “stotting” performances of Thomson’s Gazelles that inform a lurking cheetah that it has been spotted, and a difficult, energy-intensive hunt will result if it proceeds.
Enter the newsworthy nuthatches…
A researcher at the University of Washington recently published a paper detailing the behavior of red-breasted nuthatches exposed to chickadee mobbing calls. Nuthatches are common members of winter flocks of forest birds, and you might expect that they would respond to the alarming calls of chickadees (which they do). The interesting part of the story is that nuthatches respond more strongly to alarm calls made while chickadees are mobbing a pygmy owl (an important predator of small birds in the West) than alarm calls broadcast while chickadees mobbed a great horned owl (a virtual non-predator). The structure of chickadee mobbing calls change with the threat level presented by a predator, with the number of “dees” and the calling rate increasing as the potential threat increases.
So next time you find yourself listening to the eponymous calls of the chickadee, listen carefully like a good nuthatch and you might find that you already know what all the fuss is about: “dee” is for danger!