Backyard Gatherings
by Kevin J. Cook
Beyond signaling the end of breeding season, autumn also changes the way many birds eat. Once-solitary birds readily associate with other birds in a more collective-style feeding. This is easily seen in the backyard: gatherings of crows, aggregations of grackles, troupes of robins, hordes of starlings.
As you watch your yard birds adjust their feeding to the season, three different ecological concepts are worth knowing.
- Guilds. These are assemblages of different species that survive by exploiting similar foods in similar places and doing so at the same time. True guilds cross taxonomic boundaries. Thus, topminnows, bats, dragonflies, and swallows comprise a mosquito-eating guild at a marsh.
The species minimize or avoid competition by specializing. Two examples: Topminnows eat larvae and bats feed at dusk.
Subguilds comprise all the species in a single taxonomic group: seed-eating birds, fruit-eating birds, insect-eating birds.
- Commensal feeding. The word "commensal" derives from Old English and means "to share the same table." In symbiotic terms it means some organism derives benefit from associating with another organism but without imposing any harm or loss.
Some birds form specific associations to their feeding advantage: Cattle egrets follow large grazing mammals; American coots tag along with swans; common ravens shadow golden eagles.
- Mixed flocks. Birds that defended feeding territories during breeding season and worked diligently to exclude interlopers willingly join ranks during late autumn and winter. Whereas guilds and commensal feeding are year-round behaviors, mixed flocks are seasonal in temperate climates.
Typical backyard mixed flocks include chickadees, titmice, nuthatches, kinglets, juncos, house finches, pine siskins, and American goldfinches plus, quite often, a downy woodpecker or two. Actual species mixes vary from place to place.
A mixed flock may include as few as a dozen or more than 100 individual birds. They typically sweep through an area, feeding as they go, and may or may not reappear on subsequent days.
Come spring, the mixed flocks disintegrate as the individual birds once again assume breeding-season feeding behavior.
