Nuthatches and chickadees - along with several other bird species - frequently move in mixed species flocks. The mixed species flocks allow a few individuals of each species to take advantage of different foraging opportunities in a small area. They also serve as a form of mutual protection. A paper published this week shows that the nuthatches in those flocks are able to understand the chickadees' alarm calls.Earlier research had established that black-capped chickadees use different calls to indicate the degree of danger. Chickadees give longer calls for more dangerous predators - typically smaller raptors that are more likely to prey on chickadees. Less dangerous raptors merit shorter calls.
The researchers played chickadee calls of different lengths in areas inhabited by red-breasted nuthatches but not by chickadees. When the longer call was played, nuthatches mobbed the speakers. When the shorter call was played, nuthatches reacted with less vigor. This suggests that nuthatches not only recognize the call as an alarm call, but also understand its meaning.
Abstract here.The contemplative nuthatch comments as well.