Pygmy Nuthatches Find Homes in Dead Snags: By JOE EATON

Special to the Planet
Friday August 27, 2004

The neighborhood keeps changing. One of my reliable sources tells me that pygmy nuthatches—relative newcomers to the East Bay—now nest in the Berkeley hills. A checklist of Berkeley hills birds compiled about 30 years ago doesn’t include this species even as an occasional visitor. But they’ve found a niche here, especially in places with the dead conifer snags they prefer for nest sites. 

Nuthatches look a bit like miniature woodpeckers, with stout beaks for prying up bark and digging into wood, and stubby tails. Unlike woodpeckers, though, they may work head-down. The “hatch” part of the name seems to be derived from “hack,” which is what nuthatches do to seeds: wedging them into crevices and hammering them with their bills. 

Pygmies are, no surprise, smaller than the other California species, the red-breasted and white-breasted. Their calls are different—higher-pitched peeps and squeaks rather than nasal “yanks”—and so is their behavior. It’s rare to see just one pygmy nuthatch. They’re highly social, almost always in small flocks. What’s most interesting about them is that their sociality extends to the nesting season. 

Ornithologists used to think that the family life of most birds was straightforward. A male finds a territory and attracts a female; they build a nest, she lays her eggs, they raise the kids. It was known that some males—hummingbirds, for instance—shirked nest construction and child care duties, and there were other notorious exceptions like the promiscuous prairie chickens and the polyandrous phalaropes. But the two-parent family was considered to be the norm—like an avian version of suburban life in the Eisenhower Era. 

Things proved to be a lot more complicated than that. Monitoring the actual behavior of birds, researchers found there was a great deal of extra-pair fooling around: male birds may not be the biological parents of the chicks they raise. Ducks and coots dump eggs in neighbors’ nests; swallows commit infanticide; grackles have harems; geese and gulls form same-sex pairs. And don’t even get me started about hedge-sparrows. 

One pattern that kept turning up was the presence of nest helpers—birds that aid the primary pair in defending the breeding territory and provisioning the nest. It’s more common in the tropics, especially in Australia for some reason, but occurs in a few North American birds as well: California’s own acorn woodpecker (with a particularly complex variant), the Southeastern red-cockaded woodpecker, the Florida scrub-jay, the Mexican jay, even the American crow.  

The pygmy nuthatch is part of that small group. Robert Norris, doing a field study on Inverness Ridge in Marin County in the 1950s, found that almost a quarter of nuthatch nests were attended by threesomes. The third bird, always a male, was never seen mating with the female or even coming on to her. But he helped excavate the nest cavity, feed the female while she incubated the eggs, feed the chicks, and sanitize the nest. Norris speculated that threesomes “might on the average succeed in raising more young than pairs,” although he didn’t have supporting data.  

That was before the rise of sociobiology, when scientists didn’t automatically look for genetic explanations of animal behavior. But sociobiologists had a field day with nest helpers. Once it was learned that helpers were usually related to the breeding pair, nest helping became a classic illustration of kin-selection—the notion that animals act to promote the spread of the genes they share with their near relatives. Here we had birds foregoing breeding themselves so their parents or siblings could have a successful season.  

But complications like nest-helping turned out to have their own complications. In Florida scrub-jays, helping appears to be not about perpetuating the family genes, but about waiting in line for prime real estate. Good territories are scarce in the Florida scrub, and helpers stand to inherit when the senior birds die. 

And something similar may be happening with pygmy nuthatches. It wasn’t until 1981 that anyone got around to testing Norris’s speculation that nuthatch nests with helpers were more productive than those without. In a four-year study of nuthatch nests in Arizona’s Coconino National Forest, William Sydeman found that having helpers—usually yearlings from the previous year’s brood—was a good deal for the primary pair, reducing the energetic costs of stuffing those hungry little mouths with insects. But there wasn’t a clear relationship with productivity. Nests with helpers outproduced the others in only one of the four years, which was a good year across the board. Without a direct productivity payoff, it’s hard to see how evolution could have been favoring helping behavior. 

What Sydeman suggested was that helping may be the helpers’ ticket of admission to winter roosts. Pygmy nuthatches, in northern Arizona at least, survive the cold by huddling together in tree cavities. Over 150 may share a space, stacked like cordwood. And Sydeman observed that helpers roosted with the parents they had assisted during the nesting season. There’s an obvious direct fitness value to making it through the rigors of winter. That seems to hold even in relatively mild Marin, where Norris’s nuthatches roosted in family-based winter groups. 

So helping behavior may not be as purely altruistic as scientists used to think. The whole thing should give pause to anyone looking for model family values in the lives of birds—let alone bees.