When making decisions about how to navigate the world, many of us take cues from the people sharing our environment. If your neighbor departs on his morning commute carrying an umbrella, you might reconsider your choice of footwear. Similarly, overhearing your neighbor in a heated argument would probably thwart you from asking to borrow a cup of sugar or some pruning shears.
The act of eavesdropping and responding to the cues set out by neighbors is not unique to humans, nor is it species specific. Many of us have engaged in cross-species eavesdropping when we hear a barking dog and wonder what is eliciting such a reaction— a human intruder, a stray cat, or a garbage- plundering bear?
A more elaborate form of behavior-modulating eavesdropping is taking place on the grounds of the Institute of Ecosystem Studies. Beneath a forest canopy of oaks and sugar maples, IES scientists have revealed that small mammals shift their day-to-day activities based on the vocalizations of their avian neighbors.
Led by Dr. Kenneth Schmidt, an avian ecologist and IES visiting scientist, two different studies captured this phenomenon. One looked at grey squirrels and blue jays, the other at chipmunks and eastern tufted titmice. Both shed light on how mammals eavesdrop on birds to gain insight about environmental conditions.
In preparation for winter, grey squirrels collect and store food. Blue jays are known to pilfer squirrel pantries, consuming their hard- earned caches. This phenomenon is called kleptoparasitism. Through a series of audio playback experiments, Schmidt and IES animal ecologist Dr. Richard Ostfeld found that squirrels equate blue jay vocalizations with the loss of stored food.
When subjected to a range of bird calls, squirrels spent considerably less time storing nuts when blue jays were heard near their foraging site. Instead, they focused their efforts on eating food. Conversely, when calls from cardinals and goldfinches were broadcast, or when blue jay calls were perceived to be far away, squirrels allocated more time to burying nuts for future consumption.
Chipmunks also alter their feeding behavior based on bird calls. In this case, the birds in question are announcing the presence of a shared predator. Titmice are small grey songbirds that are vigilant and vociferous. When they identify a threat, they engage in high-pitched alarm calls.
When Schmidt and colleagues exposed foraging chipmunks to several titmouse vocalizations, they were able to discern day-to-day banter from alarm calls. When a mobbing call communicating a low flying hawk was heard, chipmunks abandoned foraging sooner than during non-alarm calls.
Remarkably, the titmouse mobbing call conveyed more risk to chipmunks than an actual hawk call. This may be because the titmouse call signals that hawks are both present and on the prowl. This study was one of the first to document how avian alarm calls regulate the behavior of a mammalian eavesdropper.
Results of the squirrel study will be published in American Naturalist; the chipmunk research has been submitted to Behavioral Ecology. Schmidt and his team will continue investigating information transfer between mammals and birds; this interface has the potential to reveal the evolution of communication systems.