By Lee R. Franks
Western Bluebirds (Sialia mexicana) are small thrushes that live and breed in Edgewood County Park. Males are easily identified by the brilliant blue plumage of their heads, wings, and tails, and by their rust-colored breasts. Females are duller and have more brown and gray in their feathers. The superficially similar Lazuli Bunting also has blue upperparts and chestnut on the breast. The smaller Lazuli Buntings, however, have conical bills and prominent white wing bars.
Western Bluebirds are predominately insectivores (92% animal, and 8% vegetable). The typical insects they consume include grasshoppers, beetles, ants, flies, and caterpillars. Vegetable items they like include small fruits like currants, grapes, elderberries, mistletoe, and poison oak berries.
These birds are socially monogamous. Both partners usually care for their young, but they also seek matings outside the pair bond, with the result that offspring are not always related to their attendant male. Western Bluebirds are also cooperative breeders, which is only true of about 3%, or approximately 300 species, of bird species worldwide.
Cooperative breeders, as the name implies, help other breeders (usually parents or stepparents) to raise their young while waiting for an opportunity to breed themselves. These helpers are young from the breeding pair’s previous broods, and their helping tasks include defending territories and bringing food to nestlings. Scientific studies show that breeding pairs with helpers fledge more young than those without helpers, primarily because they suffer less stress, and hence survive longer and are more likely to re-nest.
How do helpers finally achieve breeding status? Waiting for an opening is the first step. Females monitor nearby groups and move quickly to replace females that disappear. Males, on the other hand, inherit breeding positions on their natal territories in relation to their age and status. The dominant (usually oldest) son replaces his (deceased) father, stepfather, or brother. Helpers may also take over a separate portion of the family territory for their own breeding purposes.
Although the helpers may appear to act altruistically (i.e. one bird seems to put itself at risk to help another, like a bird in a flock that spots a predator and gives an alarm call, alerting the rest of the group), they actually act in their own best interest. Achieving status on an exclusive territory is difficult when occupied territories saturate the habitat. By helping to raise another’s brood, the helpers enhance their own chances for breeding through inheritance of a territory or through other forms of territory acquisition.
Why has evolution produced cooperative breeding? Current thinking amongst members of the scientific community is that cooperative breeding arises when environmental constraints severely limit the opportunities for younger birds to breed independently. These limitations may include a shortage of territory openings because higher quality habitats are saturated with established breeders, or an unpredictable availability of resources that could make it too risky for individual pairs to commit themselves to reproduce in any given year.
Western Bluebirds reside in stable environments that have specialized habitats (low-growing grasslands sprinkled with oak trees), so there is little marginal or secondary habitat for young individuals to occupy. Consequently dispersal of young is limited and cooperative breeding becomes a way, perhaps the only way, for individuals and the species to survive.
We know, for example, that development along California’s Central Coast—from Santa Barbara County to Monterey County—is leading to the clear-cutting of oaks, upon which Western Bluebirds depend, changing the oak woodlands into treeless acreage for agricultural use, particularly vineyards. Hopefully, because they are cooperative breeders, the Western Bluebirds will successfully survive this challenge as they have so many others.
The Birders Handbook; Paul R. Ehrlich, David S. Dobkin, Darryl Wheye
Ornithology; Frank R. Gills
Photos for this article were taken at Edgewood Park and Natural Preserve, San Mateo County, CA, USA.
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