Dividends of Diversity

A feeding station with seeds (left) and a novel hopper feeder attached to an empty feeding station (right) were used to test feeding success rates between single-species and mixed species groups of songbirds.

Todd M. Freeberg, DOI: 10.1038/SREP43014

Some birds regularly travel in mixed-species flocks. This diversity is thought to increase the birds’ chances of obtaining food and avoiding predators, either because mixed species groups tend to be larger, or because different species use different problem-solving strategies, increasing the group’s ability to handle new situations.

Behavioral ecologist Todd Freeberg of the University of Tennessee and his colleagues investigated these hypotheses in naturally occurring flocks of over-wintering Carolina chickadees (Poecile carolinensis), tufted titmice (Baeolophus bicolor), and white-breasted nuthatches (Sitta carolinensis). They evaluated the ability of bird groups containing one or more individuals from each species to approach and explore a new platform feeder at four geographically distinct sites. Specifically, they noted flock size, the proportion of species in each flock, the length of time until the first bird pulled a seed from the feeder, and the amount and frequency of seed-taking by each species. 

The researchers found that larger group size was not associated with greater seed acquisition. Instead, the most successful flocks were more diverse than unsuccessful flocks, with chickadees and titmice more likely to acquire seeds from the feeders in mixed groups. Chickadees in particular tended to take seeds more quickly and at a higher rate when flocks had proportionately more titmice and nuthatches, and when diversity was highest. The researchers were surprised by this result, as chickadees are a subordinate species in the flock, and most likely to back down from an aggressive encounter.

Freeberg was also surprised that the nuthatches were relatively unsuccessful at obtaining food. Though generally more inclined to climb trees and explore than the other two species, in this experiment they did not take many seeds.

Freeberg would like to repeat the study with tagged individuals to understand the role of personality traits, such as whether a given bird is more shy or assertive. “There’s a lot of individual variation in how they respond to predators and competitors,” he says, adding that diversity is not simply defined by the mixing of species, but also by differences in personalities. (Scientific Reports)