Home & Garden Columns
I was surprised to learn earlier this year that western bluebirds have been nesting in South Berkeley. I suspect this is pretty uncommon; in the 1920’s, Joseph Grinnell and Margaret Wythe reported “rare cases” of nesting in Berkeley (and elsewhere this side of the East Bay Hills.) Local birder Rusty Scalf caught the bluebirds in the act last May, using a tree cavity at San Pablo Park. In January, a city tree maintenance crew removed the dead limb that contained the cavity. After some prodding by Scalf and other local bluebird advocates, a nest box was installed on the tree; the birds adopted it and successfully reared four nestlings.
Scalf says he was surprised by his discovery as well; he would have expected any suitable bluebird nest site to be preempted by house sparrows. These ubiquitous aliens have been known to stage hostile takeovers of bluebird nests, pecking the nestlings to death. Scalf theorizes that Berkeley’s house sparrows have become imprinted on terra cotta roof tiles as nest sites and don’t bother with tree cavities.
This year a second pair nested in a street tree on Parker Street. A couple of weeks ago I saw what appeared to be two males, both with intensely blue plumage, foraging together around the corner on Sacramento. Scalf writes: “A second male is hanging out in that area, sometimes right next to the nest tree. The parents go on about their business, feeding the young in the box and ignore this male.” The literature on western bluebird behavior suggests that the supernumerary male may be a sibling of the nesting male, tolerated by the normally territorial pair.
The family life of western bluebirds can, in fact, be somewhat complicated. Several field studies have documented the role of helpers that assist a pair in caring for their young. A lot of the data on cooperative breeding in this species has come from a multi-year study by Janis Dickinson, now at Cornell, at UC’s Hastings Reserve in the Carmel Valley. According to a 1996 article by Dickinson, Walter Koenig (best known for his acorn woodpecker research), and Frank Pitelka, all the adult helpers and most of the juvenile helpers observed over a twelve-year period were male. Almost three-quarters of the adult helpers assisted birds that were presumed to be their parents.
Having helpers is an obvious boon to the parents, and seems to favor nestling survival. In the Hastings study, nests with adult helpers fledged 12 percent more young than unassisted nests. Nestlings attended by adult helpers received more feeding visits per hour, had higher growth rates, and were more likely to fledge successfully. In Oregon, assisted parents lived longer, fledged more nestlings, and had more offspring live to adulthood than those without helpers.
But what’s in it for the helpers? Dickinson and her colleagues reported that their rates of survival, future mating, and breeding success are not significantly different from those of nonhelpers.
In an evolutionary fitness sense, though, they benefit from the increased survival of their siblings, whose genes they share. It’s the logic of the anthill and the beehive, where sterile female workers care for sisters produced by the queen.
Helpers may also be in a position to take over a territory when their parents die. And recent work out of the Hastings Reserve shows that some young males are allowed to set up their nesting territories on the periphery of their parents’—the avian equivalent of an in-law apartment. (It’s not clear from what I’ve read whether these birds are former or current helpers. Some males do attend their parents’ nests while raising their own broods.)
The catch to this starter-home scenario is that the senior male sometimes mates with his daughter-in-law. The cuckolded son winds up rearing his father’s offspring instead of, or at best along with, his own. Still, from a genetic standpoint, that would be better than expending your energy on nestlings sired by a rank stranger.
Researchers are using DNA paternity studies to assess the fitness costs and benefits to the younger males.
Biologists used to state confidently that most bird species were monogamous. Genetic studies have shown that while many are socially monogamous, they still manage a fair number of extra-pair copulations (EPCs for short.) Bridget Stutchburys’ new book The Private Lives of Birds is a good source for the multitude of variations on this theme. The family arrangements of western bluebirds are in fact fairly conventional compared to those of their oak-savannah neighbors, the acorn woodpeckers, who carry on like Sixties communards.