Members of a number of species of birds display helping behavior in which individuals outside of a mated pair assist in caring for nestlings (see Emlen and Vehrencamp 1983; Brown 1987; Stacey and Koenig 1990). When ornithologists think of species that display helpers at the nest, species such as acorn woodpeckers (Melenarpes formicivorous)(see Koenig and Stacey 1990; Stacey and Koenig 1990), Florida scrub jays (Aphelocoma coerulecsens)(see Woolfenden and Fitzpatrick 1984), and white-fronted bee-eaters (Merops bullockoides)(see Emlen and Wrege 1988; Stacey and Koenig 1990) often come to mind, species that have been extensively studied for decades in the context of cooperative breeding. A species that does not as readily come to mind, but that nevertheless does display cooperative breeding, is the bushtit (Psaltriparus minimus). Sarah Sloane, in her dissertation research at the University of Michigan, researched the breeding of bushtits in the Chiricahua Mountains of southern Arizona and found a fascinating, highly flexible breeding system in which active nests frequently had helpers (Sloane 1992, 1996).

Bushtits, the sole New World members of the family Aegithalidae, were among the first three avian species listed as displaying cooperative breeding. Yet until Sloane's study in Arizona, they had been relatively under represented in the literature. Skutch (1935) observed frequent helping behavior among bush tits in Guatemala. Addicott (1938) and Ervin (1974), on the other hand, found that helping behavior in California bushtits was rare.

Sloane gathered extensive information on her study population in the arid, open oak woodlands of the Cave Creek basin in Arizona. She banded 607 birds between 1986 and 1990 and made 2,111 'nest watches' at 200 active nests to document which birds were at what nests at particular times of the breeding season. Sloane found that 39% of the active nests had helpers. Eighty-nine percent of these helpers were adults and 78% were males. Sloane established that cooperative breeding was common in the southern Arizona bushtits. She also found that some of the helpers in her population were females; this was the first study to observe female bushtit helpers at the nest.

If there are helpers at the nest, what advantages do helpers enjoy that would select for helping behavior? And what advantages would be enjoyed by breeding pairs that would select for tolerant behavior toward helpers among those breeders? Helpers may gain an indirect benefit in terms of inclusive fitness by helping raise related offspring; if they can help genes passed on by kin to survive, they gain some incremental selective benefit. By becoming part of a cooperatively breeding group, helpers may also gain a chance to take over a nest or gain a mate in the future. Thirdly, helpers may gain experience while helping that will later assist them in becoming successful parents.

Bushtit helpers enjoy an additional potential benefit; they have the opportunity to gain direct reproductive success by contributing genetically to the brood being raised. Female bushtits often contribute eggs to cooperative broods and because helping males are often alone with breeding females, males have many opportunities to gain genetic contributions through extrapair copulations (but see Bruce et al. 1996). In addition, the social system of the bushtits is so flexible that females helping at one nest have frequent opportunities to lay eggs in the nests of other breeding pairs (i.e, to engage in conspecific brood parasitism). Breeding pairs may enjoy advantages by tolerating helpers because nestlings at nests with helpers have a higher chance of surviving cold periods than pairs without helpers. The provisioning provided by helpers may also provide breeders with a savings in energetic expenditure, a benefit that may contribute to higher survivorship among breeders.

Bushtit helpers enjoy several potential benefits. Yet, they also suffer costs. Helpers have the opportunity to contribute genetically to the brood being raised, but they also must necessarily split this advantage with other individuals. If a female breeds in a solitary pair at her own nest, all of the female's offspring are her own; at a cooperatively breeding nest, however, only half or less of the offspring might be her own. Also, at cooperative nests with large numbers of nestlings (i.e., nestlings contributed by multiple females) some nestlings die of starvation because competition for food is intense.

These costs of acting as a helper may seem high in comparison to the benefits. In a situation in which nest sites and nests were easy and inexpensive to obtain, helping might not be worth it. However, cooperatively breeding birds often live in habitats where nest sites or nests are not easy to obtain. In Florida Scrub Jays, for example, cooperative breeding is partly a result of habitat saturation; there are limited numbers of suitable breeding territories in the habitat and as a result, individuals that cannot find a territory can gain a chance of future benefits by joining a cooperatively breeding group. For bushtits, the limiting factor seems to be the cost of building a nest. Bushtit nests are elaborate affairs, made of spider webs and a thick lining of feathers. Addicott (1938) observed that building a bushtit nest could take 58 days. Therefore, if an individual finds itself without a nest during the breeding season, it cannot quickly build a new one. Because nest building is so expensive, failed breeders and unmated males often attempt to take over existing nests from other groups. They sometimes succeed, they sometimes are driven away, and they sometimes fail to usurp the resident breeders but stay on as helpers anyway. For solitary males, the difficulty of the situation is heightened by the fact that, because female bushtits disperse and thus suffer higher mortality than males, the sex ratio is strongly biased toward males. Therefore, if a male has not found a mate by the beginning of the breeding season, his chances of doing so are low and he may be better off helping at a nest where he can enjoy some chance for future, and even current, benefits.

Now that a wealth of information has been provided by Sloane's comprehensive study, perhaps ongoing work will be instituted on bushtits in Arizona so that long-term information about the population can be gathered as it has on many other cooperatively breeding species. Comparisons of the behaviors and ecologies of different cooperatively breeding birds are sure to continue to yield exciting new insights into the evolution of helping behavior. Bushtits offer a particularly interesting example of cooperative breeding, in part because their habitat saturation seems based on the cost of constructing a nest and also in part because they are one of the cooperatively breeding species that sometimes displays multiple breeding strategies, including monogamy, polygamy, polyandry, and polygynandry (see also Davies 1992; Mumme et al. 1988).
 
 

LITERATURE CITED

Addicott, A. B. 1938. Behavior of the bust-tit in the breeding season. Condor 40:49-63.

Brown, J. L. 1987. Helping and Communal Breeding in Birds. Princeton Univ. Press, Princeton.

Bruce, J. P., J. S. Quinn, S. A. Sloane, and B. N. White. 1996. DNA fingerprinting reveals monogamy in the bushtit, a cooperatively breeding species. Auk 113:511-516.

Davies, N. B. 1992. Dunnock Behavior and Social Evolution. Oxford Univ. Press, Oxford.

Mumme, R. L., W. D. Koenig, and F. A. Pitelka. 1988. Costs and benefits of joint nesting in the acorn woodpecker. Amer. Nat. 31:654-677.

Emlen, S. T., and S. L. Vehrencamp. 1983. Cooperative breeding strategies among birds. Pages 93-120 in: Perspectives in Ornithology (A. H. Brush and G. A. Clark, eds.). Cambridge Univ. Press, Cambridge.

Emlen, S. T., and P. H. Wrege. 1988. The role of kinship in helping decisions in white-fronted bee-eaters. Behav. Ecol. Sociobiol. 23:305-315.

Ervin, S. 1974. Flock integrity, pair maintenance, and the occurrence of supernumerary birds in the bushtit Psaltriparus minimus. Ph.D. dissertation, Univ. of California, Santa Barbara.

Koenig, W. D., and P. B. Stacey. 1990. Acorn woodpeckers: Group living and food storage under contrasting ecological conditions. Pages 413-454 in: Cooperative Breeding in Birds: Long Term Studies of Ecology and Behavior (P. B. Stacey and W. D. Koenig, eds.). Cambridge Univ. Press, Cambridge.

Skutch, A. F. 1935. Helpers at the nest. Auk 52:257-273.

Sloane, S. A. 1992. Supernumeraries at Bushtit (Psaltriparus minimus) Nests: Incidence, Origins, and Proximate Causes. Ph.D. dissertation, University of Michigan, Ann Arbor.

Sloane, S. A. 1996. Incidence and origins of supernumeraries at bushtit (Psaltriparus minimus) nests. Auk 113:757-770.

Stacey, P. B., and W. D. Koenig (Eds.) 1990. Cooperative Breeding in Birds: Long Term Studies of Ecology and Behavior. Cambridge Univ. Press, Cambridge.

Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay: Demography of a Cooperatively-breeding Bird. Princeton Univ. Press, Princeton.