If you’ve spent any time birding in the winter in the United States and Canada you know that in quiet winter-laden forests, the sound of a chickadee is like striking gold; where there is one chickadee there are bound to be more birds tagging along. During the fall and winter in the temperate zone chickadees busily forage alongside nuthatches, kinglets, and warblers creating a flurry of activity that instantly makes for a birding highlight. Year-round in the tropics, the incessant calling of some species summons a hungry flock of birds that flit from branch to branch gobbling up insects. Red-crowned Ant-Tanagers forage alongside, Buff-throated Foliage-gleaners, Olivaceous Woodcreepers, Slaty Antwrens, and other exotically named tropical species in what is none other than a phenomenal birding experience.
Joining up with a group of birds to forage has its advantages. Birds enjoy safety in numbers and share information about predators and where to find food. So, foraging in a group makes sense, but how these groups stay together and why some species lead while others follow has been somewhat of a mystery.
Often one species “captains” the mixed-species flock, keeping the birds together while guiding them towards resources or alerting them if a predator is nearby. But it’s not just any species that leads these mixed-species flocks, certain species seem to take the lead, while others tend to follow. White-throated Shrike-Tanagers for example, call with authority and watch with what seems like a protective eye over the flock rapidly snatch insects below them. What makes the White-throated Shrike-Tanager a good leader?
Emilio Pagani-Núñez at Guangxi University in China and his colleagues sought to determine what vocal characteristics make for a good leader using assets from the Macaulay Library. Pagani-Núñez and colleagues analyzed the call structures of species known to be mixed-flock leaders, followers, or occasional followers. They used 134 audio recordings from the Macaulay Library to assess the number of contact calls given, the amount of silence between calls, the frequency of calls, and call length.
They found that leaders of mixed-species flocks had more calls than songs in their vocal repertoire compared to occasional followers. Producing many calls can transfer different bits of information to the flock, from alerting members to food or giving the all clear after a predator has passed. The Willow Tit, a mixed-species flock leader, for example, has a variety of calls and gives one type of call when food is present and another type when food is absent (Suzuki 2012). Other species such as the Tufted Titmice, give different calls based on the perceived threat of predation (Sieving et al. 2010).
Leaders were also more vocal than species that followed along or only occasionally followed flocks. Indeed, leaders like chickadees and Red-crowned Ant-Tanagers tend to call a lot, which may make them more conspicuous and easier for others to follow. Mixed-flock leaders, as well as followers, gave more alarm calls than species that occasionally joined flocks. Species that tended to lead and follow, the authors suggest, are frequently species that occur in groups where giving alarm calls is part of group living. Hence, it is still up for debate whether alarm calls are associated with leadership or is simply because those species also tend to be more social.
Given the similar calling behavior among leaders of mixed-species flocks, researchers expected to find that leaders shared a similar call structure such as call length and pitch, but vocal characteristics of leaders were actually different from each other. Those differences could mean that leaders may share more complex information than we thought.
One thing is for sure, mixed-species flocks are exciting to watch and there is a lot more we could learn about them. The Macaulay Library is excited to see researchers utilizing the archive to help us better understand birds and their behavior. This research was made possible, in part, because recordists archived their sound recordings at the Macaulay Library. Maybe your recordings will help scientists uncover more fascinating things about birds and bird behavior. If you would like to learn how you can contribute sound recordings to the archive click here. Be sure to visit our How-to Guide as well, to learn more about sound recording.
Pagani-Núñez, E. X. Xia, G. Beauchamp, R. He, J. H. D. Husson, D. Liang, and E. Goodale (2018). Are vocal characteristics related to leadership patterns in mixed-species bird flocks? Journal of Avian Biology:e01674 doi: 10.1111/jav.01674.
Sieving, K. E., S. A. Hetrick, M. L. Avery (2010). The versatility of graded acoustic measures in classification of predation threats by the Tufted Titmouse Baeolophus bicolor: exploring a mixed framework for threat communication. Oikos 119:264–276.
Suzuki, T. N. (2012). Calling at a food source: context-dependent variation in note composition of combinatorial calls in Willow Tits. Ornithological Science 11:103–107.