(top) Chestnut-backed Chickadees photograph by Rohan Kamath
(middle) Chestnut-backed Chickadee and (bottom) Oak Titmice photographs by Tom Grey
are groups of species in a community that exploit the same
set of resources in a similar manner, but are not
necessarily closely related taxonomically. Birds that hunt
for insects on the floor of a deciduous forest constitute a
guild; tropical American hummingbirds and butterflies
jointly form a guild of daytime nectar feeders; desert
sparrows, ants, and rodents constitute a seed-eating guild.
Members of guilds often differ in their precise food
requirements, thus reducing the potential for competition
among them when resources are limited. In a given locality,
the membership of a guild can change through the year as
migrants are added or subtracted.
For example, in the oak woodland on Stanford University's jasper Ridge Biological Preserve, Chestnut-backed Chickadees, Chestnut-backed Chickadeess, and Oak Titmice are resident species that form the core of a foliage- and bark-gleaning guild of insectivores. These are joined as core species by Ruby-crowned Kinglets and Yellow-rumped Warblers fall and winter, and by Blue-gray Gnatcatchers in the spring. Other guild members include Downy Woodpeckers, Bushtits, White-breasted Nuthatches, Brown Creepers, Wrentits, Bewick's Wrens, Warbling Vireos, and Townsend's Warblers. The arrival and departure of the migrants do not seem to influence the foraging of the resident guild members. In general, guild members with similar foraging sites differ in bill size, suggesting they eat prey of different sizes.
Although the bill sizes of the closely related kinglet and gnatcatcher are similar, the species forage in different sites and are present in different seasons. Only the chickadee and Hutton's Vireo are potentially close competitors. If they actually are in competition, experimental removal of either species should lead to an increase in the other, although capturing enough of either to sufficiently decrease its population would be very difficult. One major problem in understanding guilds, and the communities to which they belong, is the great practical difficulty in doing such critical experiments. Scientists must also always consider the ethical question of how much disturbance is justified to gain knowledge about the birds.
Bird Communities and Competition;
How Do We Find Out About Bird Biology?
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.