photograph of Acorn Woodpeckers by Rohan Kamath
or "communal" breeding occurs when more than two birds of
the same species provide care in rearing the young from one
nest. About 3 percent (approximately 300 species) of bird
species worldwide are cooperative breeders. There are two
types of cooperative arrangements: those in which mature
nonbreeders ("helpers-at-the-nest" or "auxiliaries") help
protect and rear the young, but are not parents of any of
them, and those where there is some degree of shared
parentage of offspring. Cooperative breeders may exhibit
shared maternity, shared paternity, or both.
The best-studied North American cooperative breeders, the Scrub-Jay, Gray-breasted (Mexican) Jay, Groove-billed Ani, and Acorn Woodpecker, differ from each other
|in the details of their breeding
biology. Scrub jays in Florida represent a group of
populations that probably were once in contact with the
widespread western populations but are now totally isolated.
Only in Florida are Scrub jays cooperative breeders, and
there they reside in permanent, group-defended territories.
Ornithologists Glen Woolfenden and John Fitzpatrick have
found that groups consist of a permanently bonded monogamous
pair and one to six helpers, generally the pair's offspring
of previous seasons. About half the territories are occupied
by pairs without helpers, and most other pairs have only one
or two helpers. Although pairing and breeding can occur
after one year spent as a helper, birds often spend several
years as nonbreeding auxiliaries. Males may remain in this
subsidiary role for up to six years; females generally
disperse and pair after one or two years of helping. Helpers
participate in all nonsexual activities except nest
construction, egg laying, and incubation. Pairs with helpers
are more successful -- they fledge one and a half times more
young than pairs without helpers.
Florida Scrub-Jays are largely restricted to the scattered and now much reduced oak scrub habitat; reproductive success outside of oak scrub is very poor. All available habitat is occupied, and populations appear to be stable from year to year, which means young birds are unlikely to find vacant space to set up territories of their own. In contrast, Western Scrub-Jays generally are not space-limited, and the probability of a young bird leaving home and finding a territory in which to breed is high.
Like the Florida Scrub-Jay, the closely related Gray-breasted Jay of the southwestern U.S. lives in permanent group-defended territories, and breeding adults are monogamous. Studies by ethologist Jerram Brown and his colleagues have shown that the cooperative system of this species is more complex than that of its southeastern relatives in several ways. Gray-breasted Jay groups are much larger, ranging from 8 to 18 individuals; thus, they usually include offspring from more than just the preceding year. Within each group, two and sometimes three breeding pairs nest separately but simultaneously each season, and some interference among them often occurs. Interference usually involves the theft of nest-lining materials, but can include the tossing of eggs from nests by females of rival nests. Although the laying female does all the incubating, she is fed on the nest both by her mate and by auxiliaries. Nestlings receive more than half of their feedings from auxiliaries.
Although the Groove-billed Ani breeds in southern Texas, our knowledge of its breeding biology comes from the work of sociobiologist Sandra Vehrencamp and her colleagues who studied the species in Costa Rica. The groups defending permanent territories consist of one to four monogamous breeding pairs that occasionally include an unpaired helper. All members of the group participate in building a single nest into which all females lay their eggs. Incubation and care of the young are shared by all members of the group. Beyond a certain clutch size, some eggs tend to be buried and fail to receive proper incubation, leading to a decreased probability of any given egg hatching.
Unlike the "cooperative" breeders that they appear to be, female anis engage in behaviors that increase the probability of their own eggs being the successful ones in the communal clutch. The most effective of these behaviors is the tossing of other females' eggs from the nest. In spite of the increased competition and conflict, multipair groups manage to fledge more young per individual than do single pairs in similar habitats.
Long-term studies of Acorn Woodpeckers have been conducted by a succession of ornithologists, including M. H. and B. R. MacRoberts, Walter Koenig, Ron Mumme, and Frank Pitelka at the University of California's Hastings Natural History Reservation in central coastal California. There Acorn Woodpecker groups are composed of up to 15 members whose territories are based on the defense and maintenance of granaries in which they store acorns. Groups consist largely of siblings, their cousins, and their parents. Some of the sexually mature birds are nonbreeding helpers. Within each group, up to four males may mate with one (or occasionally two) females, and all eggs are laid in a single nest. Thus paternity and sometimes maternity of the communal clutch is shared.
Per capita reproductive success generally increases with group size up to 7 or 8 members, and then declines. Clutches produced by two females are somewhat less successful than those of single females due to behavioral interference between the two females and some egg tossing. Although there is some geographic variation in the size of groups and other aspects of the Acorn Woodpecker system, it breeds cooperatively throughout its range.
Why has evolution produced cooperative breeding systems? Initial hypotheses were based on kin selection (seemingly "selfless" behavior like helping at the nest being favored because it increases the reproductive success of relatives genetically similar to the helper) or on maximizing of reproductive output. As more cooperatively breeding species have been examined worldwide, these explanations generally have not been supported. Instead, cooperative systems appear to arise when environmental constraints force birds into breeding groups because the opportunities for younger birds to breed independently are severely limited. Limitations may include a shortage of territory openings because higher quality habitats are saturated with established breeders; a shortage of sexual partners (generally females), indicated by the skewed sex ratios that are common in groups; and unpredictable availability of resources, which could make it too risky for individual pairs to commit themselves to reproduce in any given year. That cooperative breeding is a common strategy in arid and semiarid portions of Africa and Australia lends strong support to this line of reasoning. Cooperative breeding may be viewed primarily as a means by which young adults put off the start of their own breeding in order to maximize their lifetime reproductive output, and in the process occasionally promote genes identical with their own via kin selection.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.