By Mitchell Walters
Linked paper: Do similar foragers flock together? Nonbreeding foraging behavior and its impact on mixed-species flocking associations in a subtropical region by H.H. Jones, M.J. Walters, and S.K. Robinson, The Auk: Ornithological Advances.
If you were to go birdwatching along a forested trail during the Florida winter, chances are you wouldn’t encounter much at first. You might hear the distant rolling warble of a Carolina Wren defending its winter territory, or maybe the harsh “caw” of an American Crow flying overhead, but after a few minutes the trail would seem quiet and virtually bird free. That is until, all of sudden, you are surrounded by a chorus of high-pitched calls coming from a cavalcade of songbird species — a mixed-species flock.
Mixed-species bird flocks are defined as groups of two or more species that move and forage together. In these foraging associations, specific “nuclear” species such as the Tufted Titmouse lead the flock and contribute to its cohesion, while other “attendant” species follow them (see above). Research has shown that these attendant species forage more efficiently when joining flocks, largely due to the decreased need to watch for predators. Thus, flocking increases a bird’s chances of survival and reproduction, which may explain why upwards of 70% of species in some bird communities and roughly 20% of bird species worldwide join mixed-species flocks.
What determines the structure of these foraging flocks? Some say flocking increases competition for food and prevents species with similar foraging strategies from occurring together in the same flock (the “competition” hypothesis). However, other researchers say the opposite, that flocks are more often made up of species that share similar foraging behavior and body size (the “similarity” hypothesis). Given that the jury is still out on the drivers of flock formation, our team took to the hardwood forests of north central Florida to tease apart these contradictory hypotheses.
For two winters, we collected data on the foraging ecology and mixed-species flock membership of an insectivorous bird community to see whether similarity in foraging behavior drives flock structure and abundance. We were able to look at the fine-scale foraging ecology of each species, such as what “attack maneuvers” it used and what kind of vegetation (or “substrate”) it preferred to forage on. We also determined the proportion of time each species spent in flocks and how often each possible pair of species joined a flock together (the pair’s association strength). After completing 91 point counts and flock surveys, as well as over 1200 foraging observations, we found evidence for both foraging hypotheses at different scales.
At a broad scale, we found that the association strength of a species pair was positively correlated with foraging similarity, meaning species that occurred together in flocks more often shared similar foraging behaviors. In addition, species that flocked together had a greater overlap of foraging substrates, attack maneuvers, and foliage density than would be expected by chance.
However, at a finer scale, we found evidence of niche competition, particularly on the basis of foraging substrate. Flock-joining species were clustered into four distinct foraging categories based on their foraging maneuvers and substrates. These species also exhibited relatively low overlap of foraging substrate (~40%). Many species specialized heavily on subtropical substrates, including the live leaves of evergreen trees, dead leaf clusters, and epiphytic plants such as Spanish moss. We also found that species abundance in foraging flocks was best explained by foraging substrate use, with live leaf and aerial foragers having higher abundances in flocks and at our study sites.
So, does foraging similarity help or hinder a flock-joining bird? This seems to be largely a matter of scale. Of course, not all bird species join flocks, and a certain level of similarity in terms of body size, food items, and microhabitat is needed to effectively forage with the group. Among these flocking species, however, dividing the foraging niche into specialized foraging substrates is likely a way to minimize competition with flockmates. We’d argue that both foraging competition and foraging similarity can structure flocking interactions when they act at different scales and on different dimensions of the foraging niche.