San Diego Natural History Museum--Your Nature Connection[San Diego County Bird Atlas Project]

Focus On...

Nuttall's and Ladder-backed Woodpeckers

The Nuttall's and Ladder-backed Woodpeckers are two closely related similar species that are both residents of San Diego County and occasionally confused. Our atlas effort has already revealed unexpected new aspects of their distribution and ecology, making them an ideal pair to "focus on" in Wrenderings. Over much of their ranges, including San Diego County, their ranges fit together almost like puzzle pieces, distributions called "parapatric" by biogeographers.

Both of these woodpeckers are readily distinguished from others by their black-and-white barred "ladder" backs, black-and-white patterned heads, and white or whitish underparts. Adult males sport red patches on the head; adult females lack red, having the crown largely or entirely black. Juveniles of both sexes, however, have red on the crown and are the principal culprits in birders' confusing the two species.

As happens so often with closely similar species, the voices are the best clue to the birds' identification. Both species sometimes give a single-noted call that could be confusing in isolation, but usually the notes are run together in a pattern characteristic of each species. Nuttall's makes a crisp, slightly metallic-sounding rattle, "trrrrrrt." The pitch of the call ascends slightly toward the end of the rattle, and the call ends abruptly with a definite "t" sound. The call of the Ladder-backed, however, descends in pitch, sounding more like a whinny than a rattle. The whinny trails off at the end with no crisp conclusion. Interestingly, the calls of the Ladder-backed and Downy Woodpeckers are almost identical.

The species can be identified by plumage as well, with careful study. The underparts of the Nuttall's are pure white, with scattered, variable black spots and bars on the sides and flanks. In the Ladder-backed the underparts are lightly tinged smoky brown, though seeing the difference from white in the field requires a good view. The upperparts of the Ladder-backed have more white: the white bars on the back of the Ladder-backed are broader than the black bars (equal to or narrower than in Nuttall's), and the white spots on the wings are larger. On the Ladder-backed the white bars on the back extend to the base of the neck, while on Nuttall's the upper back is black with little or no white barring. This difference is most obvious in the males, where Nuttall's shows a distinct black patch between the red nape and the barring of the back, the Ladder-backed little or none.

Understanding the differences in head pattern between the two species requires determining whether the bird is adult or juvenile. The juvenile plumage in these woodpeckers is seen only from fledging (midsummer) to about the first of September; later in the fall the young birds have finished their molt and have assumed adult plumage. In the adult male Nuttall's the hindcrown is red, the forecrown black with some white streaking. In the adult male Ladder-backed the red (mottled with black and white) extends forward almost to the base of the bill. In the juveniles of both the Nuttall's and Ladder-backed, however, the red extends to the forehead--when birds in juvenile plumage are about, the pattern of red on the crown cannot be used to distinguish the two species.

The black on the sides of the head and neck is more extensive in the Nuttall's than the Ladder-backed, though this can often be difficult to assess in the field. A distinct and contrasting white patch in the lores and nasal tufts at the base of the bill is a diagnostic feature of the Nuttall's; in the Ladder-backed this area is black or just obscurely grizzled with white, not forming a distinct patch. This difference is not well illustrated in the National Geographic field guide but Ridgway used it as the key difference distinguishing the species in his classic "Birds of North and Middle America."

Nuttall's Woodpecker is very widespread on the coastal slope of San Diego County, being fairly common in riparian, oak, and mixed conifer woodland. An interesting event illustrated by our atlas results is the species' recent spread into residential areas in central San Diego. Before 1990 Nuttall's Woodpecker was absent from this area, occurring even in well-wooded Balboa Park as a rare visitor only. In the last decade the species has become widespread in urban areas, being recorded during the breeding season as well as winter in such squares as R10, R11, S9, S10, and S11 where it was previously absent. Perhaps with the aging of the central city and the maturing of "urban forest" in what was once treeless sage scrub the stature and density of trees finally achieved a threshold Nuttall's Woodpeckers consider suitable.

Nuttall's Woodpecker extends down the east side of the mountains only in wooded canyons: Middle Willows in Coyote Creek Canyon in C22, Borrego Palm Canyon in F23, and Sentenac Canyon in J23, where Bob Thériault found a nest in a mesquite. There were at least of a couple of records in the 1970s at Lower Willows on Coyote Creek (D23) but the species does not occur there now, as attested by both our atlas results and the surveys by Art Morley and Keith Smeltzer for the recent book "Birds of the Anza-Borrego Desert." In the southeastern corner of the county Nuttall's ranges east to Jacumba (U28). It hardly ever disperses even a short distance out of its breeding range in the winter. Our only records outside breeding habitat represent dispersal of less than 3 miles, in Earthquake Valley (K23), where Bob Thériault had a single bird in a cottonwood on 17 January 1998, and on the north slope of Grapevine Peak (I22), where Paul Jorgensen had one on 9 April 1998.

The Ladder-backed, conversely, is largely a desert species, uncommon even in its preferred habitat, favoring slopes vegetated with thorny shrubs of the genus Prunus and above all the desert agave. The birds feed eagerly in agave flowers while using the easily excavated flower stalks for nest sites. In the flatter areas of the Anza-Borrego Desert the Ladder-backed is very sparse, as suggested by how scattered our records are in these squares.

Interestingly, the Ladder-backed is evidently absent from the floor of the Borrego Valley; some of our best-covered desert squares, F24, G24, and G25, have no records for either winter or the breeding season. Apparently, the mesquite thicket in the Borrego Valley doesn't attract it, and it doesn't take to the urbanization of Borrego Springs the way Nuttall's has to that of San Diego.

The Ladder-backed Woodpecker ranges up the east slope of the mountains to at least 3200 feet elevation in places, being recorded west to Alder Canyon (C22), the east slope of San Felipe Valley (H21 and I21), Oriflamme Canyon (L22), and the upper end of Carrizo Creek (T28). Thus there is a narrow strip, perhaps no more than 5 miles wide, along the east slope where both the Ladder-backed and Nuttall's occur. In this strip, the two species are separated by habitat, Nuttall's in the riparian woodland in canyons, the Ladder-backed on dry slopes with catclaw, cacti, yuccas, and Prunus. Nowhere is the precise replacement of the two woodpeckers so dramatic as in San Felipe Valley, where Nuttall's comes down Volcan Mt. to the creek while the Ladder-backed lives just a 5-minute walk away up the opposite slope of the valley.

Among the more remarkable discoveries revealed by our atlas effort are two regions where the Ladder-backed occurs on the coastal slope. In the drainage basin of the Santa Margarita River, Ken Weaver has found the species in semidesert scrub in Dameron Valley in squares C15 and C16, confirming breeding in C15. During our blockbuster weekend in January, Mike Mathos and I found a pair in leafless elderberry trees among cholla thickets in the extreme northwest corner of square C17. As with the Black-throated Sparrows in the same areas, reported on in the fall 1998 issue of Wrenderings, this isolated population presumably extends into the Aguanga region of southern Riverside County, where the desert habitat is even better developed. Unfortunately, Ken reports that clearing of this unique habitat is in progress.

The other new site for the Ladder-backed Woodpecker on the coastal slope is in Miller Valley in square S24. Here Margaret and Bert McIntosh first discovered the species on our blockbuster weekend, 21 February 1998. Covering the area again in the breeding season on 2 May, they used a taped recording to search for the birds, finding a single female about three quarters of a mile away from the previous winter's site. As in Dameron Valley, the microclimate in Miller Valley promotes a desert fauna--both areas have Cactus Wrens and White-tailed Antelope Squirrels too.

--Philip Unitt, with sketches by Nicole Perretta, from the spring 1999 issue of WRENDERINGS

Sketch of Nuttall's male, by Nicole Perretta 1999Sketch of Nuttall's female, by Nicole Perretta 1999
Nuttall's male (left), female (right)

Sketch of Ladder-backed male, by Nicole Perretta 1999Sketch of Ladder-backed female, by Nicole Perretta 1999
Ladder-backed male (left), female (right)

Sketch of Nuttall's juvenile, by Nicole Perretta 1999
Nuttall's juvenile
Sketches by Nicole Perretta

 

Nuttall's
Nuttall's Woodpecker Breeding

red squareBreeding confirmed
yellow squareBreeding probable
green squareBreeding possible

 


Focus On ... | Spring 1999 Wrenderings | Bird Atlas Introduction