An occasional cooperative breeder sticks close to home
Western Bluebirds, year-round residents in central California’s oak woodlands, are occasional cooperative breeders. They usually breed as individual pairs, as do Eastern and Mountain bluebirds, but Western Bluebirds occasionally have helpers at the nest.
A Western Bluebird nestling wears a U.S. Fish and Wildlife Service leg band and colored leg bands.
In autumn, most Western Bluebird daughters disperse, but most sons and a few daughters remain with their parents. These families are joined by immigrant females—sometimes that year’s young from nearby territories, and more often unbanded females from farther away. The resulting groups usually remain together through winter, sharing mistletoe berries, sleeping together on cold nights (as many as 13 birds huddled into a single nest box), and defending the territory against intruders. Benefits of winter group life are clear, but what benefits exist for parents, sons, brothers, and other close relatives living in “kin neighborhoods” during the breeding season? Ever since a field assistant discovered an adult male bluebird feeding young at another male’s nest in 1983, our field researchers have been exploring the behaviors of this fascinating species and collecting critical conservation information on factors influencing its survival and reproductive success in a long-term study supported by the National Science Foundation and the staff of the University of California’s Hastings Reserve.
A high-quality winter territory provides males with berries all season, and also with access to potential mates and a breeding territory the next spring. The unrelated immigrant females that join the group in the fall often pair up with stay-at-home sons in midwinter and breed with them in spring. Unlike males that do not live in family groups, sons that spend the winter with their parents on a “wealthy” territory do not have to endure the challenges of locating a mate on their own. In spring, when a son and his mate edge their way onto a nearby territory, his parents may help them carve out space; parents may appear to have their own, exclusive territories, but they often share an outer sliver of the home territory with their newly independent sons and “daughters-in-law.”
When a young male is unable to find a mate, helping his parents can be an excellent option. Parent Western Bluebirds with helpers on average fledge one more chick than parents without helpers, and these chicks share many genes with the helpers. Thanks to this process of “kin selection,” helping provides an unmated male the equivalent of having two-thirds of an offspring (because he shares about half of the genes with an average of 3 chicks), which is better in the long-run than producing no offspring at all. Remarkably, a few helpers may have a nest of their own, moving back and forth to feed young at both nests on the same days. Simultaneous breeding and helping is extraordinarily rare in cooperative breeders, suggesting that helper Western Bluebirds must be getting something out of helping—otherwise why would they work so hard to feed additional young, potentially at the expense of their own offspring?
If there are some benefits for male bluebirds remaining near their kin’s territory, there are also costs. If females are in short supply, relatives may compete for mates. Preliminary evidence suggests that helpers may lose in another less obvious way: DNA fingerprinting of the entire population over several years indicates that fathers occasionally sire offspring in their sons’ nests. Even when sons feed only the young in their own nest, they are sometimes effectively helping to raise their father’s nestling in place of their own! Fathers have an advantage over their sons in the genetic game of cuckoldry because, based on DNA evidence, sons do not mate with their mothers. In contrast, fathers are completely unrelated to their sons’ mates and, at least sometimes, are the actual fathers of some of their supposed grandchildren. Do sons lose more than they gain by nesting near them? Might raising some chicks sired by their father be a better option than occasionally raising a chick sired by an unrelated neighbor? DNA-paternity data will help us learn whether living near their fathers provides more long-term costs or benefits for male bluebirds.
Theoretically, birds living in cooperative groups may spot predators and even help one another in dangerous circumstances. Unfortunately, predation events are rare and difficult to monitor under ordinary circumstances; it could take years to assemble enough data to understand whether critical life events provoke responses by kin. However, determining the consequences of such aid could help to unravel the mystery of kinship and its importance in a wide array of behavioral interactions, including when family members live on independent territories and appear to ignore each other.
The variation in Western Bluebird behaviors and strategies makes them valuable study organisms for teasing apart the complexities of how natural selection shapes social behavior.
Caitlin Stern is a Ph.D. candidate in the Department of Neurobiology and Behavior at Cornell University. Elise Ferree is a postdoctoral associate at the Cornell Lab of Ornithology.