The Great Chickadee Invasion

By Joe Eaton
Tuesday December 21, 2010 - 09:02:00 PM
The northern chestnut-backed chickadee can be seen around northern California in the holiday season.
Thom Quine
The northern chestnut-backed chickadee can be seen around northern California in the holiday season.
Santa Cruz chestnut-backed chickadee, our local variety (gray sides.)
Linda Tanner
Santa Cruz chestnut-backed chickadee, our local variety (gray sides.)

The weather and a persistent head cold have kept me indoors for most of December. Contact with the natural world has been pretty much limited to killing ants in the kitchen (yes, the Argentines are back) and watching the bird feeders. Ron decided to hang up a suet feeder this year, and the chestnut-backed chickadees have responded—up to four at one time. We’ve also had visits from a ruby-crowned kinglet. 

Those chickadees are an interesting biogeographical case. They’re relative newcomers to the East Bay. When Joseph Grinnell and Alden Miller wrote their classic Distribution of the Birds of California (1944), there were only a handful of records: sightings in Berkeley in the 1890s and 1913, a few reports from the Hayward area. 

Grinnell and Miller recognized three subspecies of Poecile (formerly Parus) rufescens. The northern chestnut-backed chickadee (P. r. rufescens) was resident from Alaska south to the vicinity of Sebastopol in Sonoma County. The Marin chestnut-back (P. r. neglectus) was restricted to southwestern Marin County, including Point Reyes and Mount Tamalpais. The Santa Cruz chestnut-back (P. r. barlowi) occurred from San Francisco down to Cambria. All three preferred conifer forests not far from the coast. 

The three forms can be distinguished in the field by the coloration of their sides. Rufescens has reddish sides as well as a chestnut back; barlowi has gray sides. Neglectus is intermediate, with the chestnut on the sides paler and duller and forming a narrower band. 

All the historic East Bay sightings were attributed to the gray-sided barlowi

As early as 1914, Grinnell, the legendary director of UC’s Museum of Vertebrate Zoology, speculated that chestnut-backed chickadees might do well in the East Bay if they ever managed to cross the treeless gap from their northern or southern ranges: “Judging from the climatic peculiarities of the immediate vicinity of Berkeley, it would appear consistent with our knowledge of geographical distribution to expect that, with the aging of our planted groves of conifers, chickadees will find conditions favorable to the establishment of permanent colonies…” 

The first detected East Bay beachhead for the chestnut-back was near Sunol in southern Alameda County, where Henry Carriger saw a barlowi pair prospecting for nest sites in 1938 and collected five eggs in 1940. By 1945, Alden Miller documented nesting along Wildcat Creek in Tilden Regional Park. A lone bird was observed on the UC Campus in 1946, but nesting wasn’t confirmed until 1950 when Keith Dixon and Harrison Ryker found an active nest in a bay laurel near Strawberry Creek. 

Some birders were concerned that the presence of the chickadees would adversely affect their close relative, the oak titmouse (Baelophus [formerly Parus] inornatus.) However, the two did not appear to compete directly for food, habitat, or nest sites. In the early 1960s, Richard Root did field research on chickadee-titmouse interactions in Las Trampas Canyon near Moraga. He found chickadees and titmice occupying overlapping territories. Although the species had similar seed-and-insect diets, Root reported that they had different foraging strategies. The chickadees, lighter and more agile, were more likely to hang below twigs while searching the foliage for insects. The heavier-billed titmice were better equipped to hammer open hard-shelled seeds. Chickadees also foraged higher in the canopy than titmice. 

So peaceful coexistence reigns in the East Bay, where populations of both chestnut-backed chickadees and oak titmice appear stable. 

This may change: studies by P K Kleintjes and Donald Dahlsten in the 1990s showed chickadees in Contra Costa County relying on planted Monterey pines for breeding habitat and on Monterey pine pest insects for food. The loss of Monterey pines to pitch canker disease may be affecting the chickadees. 

Meanwhile the chestnut-sided northern race rufescens had at some point crossed the gap from Sonoma into the Marin range of neglectus. Rich Stallcup, writing in the newsletter of what was then the Point Reyes Bird Observatory (now PRBO Conservation Science) in 1995, reported recently seeing “very bright individuals of the northern race, rufescens, well within the historic range of neglectus.” 

“It appears clear that the invasion will broaden and spread amoebically over the whole range of neglectus,” wrote Stallcup. “It also seems clear that the two will freely interbreed, diluting the ‘purity’ of both forms. How long can it be until the Marin Chestnut-backed Chickadee will be totally absorbed by the Northern Chestnut-backed Chickadee, and the subspecies neglectus become extinct?” 

Good question. I recall someone bringing that up recently on the North Bay Birds listserve, but couldn’t find the post. Given 20 years, I would imagine that the neglectus gene pool has been pretty thoroughly muddied by now. 

Who mourns for neglectus? Is the death of a subspecies more acceptable when it’s the result of genetic swamping by a near relative? (Tell that to the American black duck or the golden-winged warbler.) Maybe it’s easier to get worked up about a full species than about a subspecies, which can be a pretty arbitrary designation. 

Along with the Marin takeover, rufescens has also colonized the Sierra and now nests as far south as Yosemite, where it co-occurs with the mountain chickadee. But that’s another story.