Mixed Feeding Flocks
Mixed sparrow feeding groups are largely opportunistic by nature. For example a group of White-crowned Sparrows may have a Vesper Sparrow or a Field Sparrow mixed in. These groups are together for the sake of sharing a resource, and the more of them there are, the safer they are from predators. This opportunity may last only for a few minutes or hours.
There are, however, non-sparrow feeding groups that operate throughout the day as a socially coherent group. As we move into the winter months, we will be looking at some of these mixed flocks as a whole. As an example, a typical mixed feeding flock in Texas may include chickadees, titmice, creepers, possibly a nuthatch species, woodpeckers, and kinglets. In some places, it may include a warbler species or two. In this case, the North American mixed feeding group is almost entirely insectivorous. Fruit and berry-eating frugivore feeding groups are not well known in the Holarctic, but in Central and South America, they can compete right along with their insectivorous friends. It’s a good idea to actually observe a feeding group as a social unit, and if you have time, observe the individual roles each species might play.
Mixed feeding flocks are well-studied in the neotropics because of their incredible sophistication, but in North America… not so much. We know two important things about North American mixed feeding flocks. 1. They are almost certainly better protected as a feeding group than if they are feeding alone. 2. They increase their feeding efficiency by being in a group.
Safety is probably the most obvious reason to feed as a group. Food gleaners (those bird species that search leaves and bark for food) like the tits, wrens, nuthatches, creepers, and woodpeckers, spend much of their life staring at tree bark. They don’t often get to look over their backs to watch out for predators. They are helped enormously by the strategy of a feeding group. As it regards food, warblers and wrens frequently find good fortune in letting all the gleaners look for available food, and then taking advantage of left-overs or side dishes. Some warblers, like Pine Warblers, are fairly expert gleaners themselves.
As you might imagine, birds in mixed feeding flocks are very vocal. They rely heavily on calls that indicate a food bounty or warn of a potential threat. Many of these calls are studied with bioacoustical measurements in an effort to figure out what some bird calls mean. As you might expect, there are entirely different bird calls that separate alarm calls from, say, a food bounty or mate acknowledgement. A large amount of audible information is transmitted within a feeding flock.
For a few of you that travel to the American tropics, feeding flock ecology is a fascinating side discussion. In the tropics, many birds inside the feeding flock have specific roles. Some species are designated as sentinels – they watch out for predatory animals that might threaten the flock. Those birds will be replaced by other birds after a specific time frame so they can feed. Several species inside a flock are deemed to work at certain elevations – some on the ground, some in the understory, and some in the trees.
Some birds, like specific antbirds, follow only army ant swarms and treat them almost like a territory. They sleep when the ant swarm sleeps, and they move when they ant swarm moves. While you may think that has no relation to our Texas birds, I have seen many North American birds like the Wood Thrush actively engaged as a participant in an army ant feeding group in Costa Rica during the winter season. With neotropical migrants like flycatchers, thrushes, and warblers, feeding styles can change with the seasons. When you only observe them in breeding season, you really only know half their story.
How much of this role-playing is shared by North American mixed feeding flocks is still not fully understood, but it’s a fascinating discussion.
For more information on gleaners, gleaning strategy, and mixed flocks… http://en.wikipedia.org/wiki/Gleaning_(birds)