Focus On...the Hairy and Downy Woodpeckers
Few more striking examples of parallel evolution can be found among North American birds than the Downy and Hairy Woodpeckers. Again we have a pair of species whose plumage is remarkably similar but whose size, shape, voice, habitats, and general biology differ considerably. San Diego County is at or near the edge of the range of both species, so neither is common here. Both species have vast overlapping transcontinental distributions, but the differences in their habitats and distributions are accentuated here on this edge. The recent and sudden spread of the Downy, though, is on the verge of bringing it into contact with the Hairy even at the southern tip of its range. The San Diego Bird Atlas allows us to see this change far better than was previously possible.
Both the Hairy and Downy Woodpeckers are readily distinguished from other black and white woodpeckers by their white backs. The backs of the Nuttall's and Ladder-backed Woodpeckers have distinct bars. The Red-naped Sapsucker has a large white wing patch that might be mistaken for the white back of the Hairy and Downy in a split-second glance, but its back is irregularly barred or mottled with white.
The main external difference between the Hairy and Downy is in size, especially of the bill. There is no overlap. Compared with our more numerous woodpeckers, the Downy is smaller than the Nuttall's and Ladder-backed, the Hairy is larger, almost as large as an Acorn Woodpecker. The Hairy is almost a third again larger than the Downy in wing length, and its bill is nearly twice as long as the Downy's. The difference in bill length affects the apparent shape of the birds' heads: the Downy's stubby little bill looks like it has been punched into the bird's puffy little head; the Hairy's long bill looks like it tapers from a long face.
The plumage patterns of the two species are almost identical, though, except for the inconspicuous small black bars on the white outer tail feathers of the Downy, lacking on the Hairy. Both species vary considerably over their broad ranges, and it's a marvel of nature how this variation runs in parallel throughout North America. Both species tend to be larger in the north, smaller in the south, exemplifying a well-known biogeographic principle. The California subspecies of the Downy, Picoides pubescens turati, is the smallest of North America's woodpeckers. Both species have the wings heavily spotted with white in the east, less so in the west, and in the dark forests of the Pacific Northwest have the "white" parts of the plumage deeply tinged brown. The California subspecies of both the Downy and the Hairy (Picoides villosus hyloscopus) have the underparts lightly tinged smoke gray. The difference from white is difficult to appreciate in spring and summer when the birds are in worn plumage and more or less stained from months of contact with trees, but it can be seen in a good view in fall and winter when the birds are clean and fresh.
The adult males of both species have a similar red patch on the nape, where females are black. The juveniles have red and white stippling on the forecrown, in the same location as do the Nuttall's and Ladder-backed Woodpeckers. Otherwise, though, they resemble the adults, so there is little opportunity for confusion with other species. The crown patch of the juvenile Hairy Woodpecker can be yellowish rather than red, recalling the Three-toed Woodpecker of the far north. As a beginning teenage birder I fell into this trap, identifying a juvenile Hairy in the San Jacinto Mountains as the Three-toed--a species unknown from southern California.
The voices of the two species distinguish them easily as well. The Downy Woodpecker's "pick" call is often extended into a multinoted whinny that drops in pitch. It is nearly identical to the similar call of the Ladder-backed Woodpecker but differs clearly from the ascending rattle of the Nuttall's. We are fortunate that it is the species with the divergent calls that overlap, while the species with the similar calls don't. The Hairy Woodpecker's "peek" call sounds like a rubber squeeze-toy--when squeezed very quickly. The calls may be run together in a series but don't trail off in pitch and volume the way the Downy Woodpecker's whinny does.
The Hairy Woodpecker has long been associated in San Diego County with pine forests in the mountains, and our atlas results show an almost perfect correspondence with this habitat during the breeding season. The few squares where the species has been recorded in winter but not in the breeding season are almost all adjacent to the breeding range, suggesting that the Hairy's rare appearances outside its breeding range result from just short-distance dispersal of the local population.
These records of dispersal to the coastal lowland don't include Hairy Woodpeckers seen in far northern San Diego County. These, I believe, represent the southern extreme of the species' low-elevation distribution as a resident. Though in San Diego we are accustomed to thinking of the Hairy Woodpecker as a mountain bird, it is not so restricted over most of its range. Along the coast of California, it is resident if uncommon in lowland riparian and oak woodland south to Santa Barbara County. Occasional nestings are known at low elevations farther south, as along the Santa Ana River in northwestern Riverside County.
In northwestern San Diego County, we now have three records. Evelyn Ashton reported single individuals from Los Jilgueros Preserve, Fallbrook (D7), on 21 January and 7 May 1998. Most remarkable, Robbie Fischer observed an adult feeding a fledgling along the Santa Margarita River near Rifle Range Road in Camp Pendleton (F5) on 28 June 1998--the southernmost known breeding of the Hairy Woodpecker at a low elevation on the Pacific coast.
Another unexpected low-elevation site for the species, though one much closer to the core population, was along the San Diego River above El Capitan Reservoir (M17). Bob Sanger reported single individuals there on 21 March and 6 June 1998. Note that all of these Hairy Woodpeckers out of their usual range occurred in a single year.
Almost nothing was known about the nesting of the Hairy Woodpecker in San Diego County before we initiated the atlas. Now we are up to 11 confirmations of breeding. These suggest the species is not an early nester: the seven observations of fledglings range from 28 June to 29 July. On Hot Springs Mountain (E20), Ken Weaver and Clark Mahrdt had a nest with nestlings as late as 15 July in 2000, though Ed Hall and Clark Mahrdt reported a Hairy Woodpecker feeding young on Palomar (E15) as early as 28 May in 1999. The Hairy excavates a typical woodpecker hole in trees, especially dead snags. The nest Clark and Ken found was about 25 feet up from the ground.
Unlike the Hairy, primarily a mountain species, the Downy is basically a species of lowland riparian woodland, especially of willow trees. Indeed, the California subspecies turati of the Downy was long called the Willow Woodpecker. The Downy typically excavates its nest in dead snags of willows and can be seen foraging on remarkably slender twigs as much as on the thicker trunks.
The Downy Woodpecker has had a remarkable career in San Diego County. Around the turn of the last century, Clarence Sharp considered it "rather rare" at San Pasqual. The early egg collectors found only three Downy Woodpecker nests, at Bonsall, San Pasqual, and the head of Lake Hodges in the 1920s. Paul Dehnel collected two specimens along the San Diego River near Santee in 1949. This was virtually all that was known of the species in the county until the 1960s, and from then till the mid 1970s there were only a few reports of single individuals, apparently wandering. Possibly by that point the range of resident Downy Woodpeckers had retracted north entirely out of San Diego County.
In 1976 the species was "rediscovered" along the San Luis Rey River in Oceanside, and searches of the San Luis Rey and Santa Margarita rivers in the late 1970s, in part incidental to the search for Bell's Vireos, revealed the Downy Woodpecker as a scarce resident in northwestern San Diego County's most extensive riparian woodlands. In the 1980s, the species spread rapidly down the coast, reaching the Tijuana River Valley by 1990. Since then, it has continued to fill in and spread east, to higher elevations.
Our atlas data demonstrate this spread, with new locations for the species being reported every year. To the south, just a fraction of a mile from the Mexican border along the Tijuana River (W11), I had a female feeding a fledgling on 19 June 1999. To the southeast, Peter Beck noted four equally close to the border in Marron Valley (V17) on 12 June 2000. In northern San Diego County, we now have several records, both breeding season and winter, from the Lake Henshaw/Warner Springs region (squares E18, F19, G16, and G17). A single bird reported by Mary Beth Stowe near Julian (J20) on 17 August 2000 may have been a postbreeding disperser. Finally, on 3 December 2000, Lori Hargrove reported the first instance of a Downy Woodpecker going over the top, over the crest of the mountains onto the desert slope, with a single bird in San Felipe Valley (I21).
We recently added our first specimen of the Downy Woodpecker from San Diego County to the museum's collection. A recently fledged juvenile, it exemplifies the species' continuing dispersal over unsuitable habitat to new locations: Chris Tratnyek found it freshly dead in the driveway of her mother's home in El Cajon on 14 June 1998.
Why should a species restricted to riparian woodland, with a more northern distribution, be spreading south in San Diego after much of this riparian woodland has been eliminated, at the onset of global warning? The many conflicting factors governing bird populations are often mysterious. In the case of the Downy Woodpecker, I can only speculate. The damming of most of our rivers and creeks has stabilized the riparian environment in a way it never experienced previously. Under natural conditions winter floods would knock over trees and perhaps prevent many from growing to maturity and developing enough dead snags for really suitable Downy Woodpecker nest sites. Now such flooding has been greatly reduced, allowing more trees to live to maturity and senility. Changing land use has allowed riparian woodland to regenerate in some areas, such as the Tijuana River valley, where it was absent 25 years ago.
Some other subtle factors may be at work, too. The southward spread of the Downy Woodpecker parallels that of two other species, the Western Flycatcher and Orange-crowned Warbler. Though these too live in riparian woodland, their ecology is quite different. The spread of the flycatcher and warbler as breeding species has been masked somewhat because, unlike the Downy Woodpecker, they are common as migrants. I can't offer an explanation why the spread of a ground-nester like the Orange-crowned Warbler and a species that nests in knotholes and on snags like the Western Flycatcher should coincide with the spread of a cavity-excavator like the Downy Woodpecker. But all three species are thriving at the southern tips of their breeding ranges and pushing even farther south.
The Downy Woodpecker was unknown anywhere in Mexico until recently. Kurt Radamaker made the first believable sighting in 1996 in Parque Morelos in Tijuana, and three more records, two supported by photographs or videotape, have followed since, from La Misión as well as Parque Morelos. How far south into arid Baja California can this species of humid riparian woodland spread?