Williamson's sapsucker (Sphyrapicus thyroideus) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

Williamson's Sapsucker was first recorded from Canada in the Similkameen Valley of the southern interior of British Columbia in 1882 (Fannin 1891 cited in Cannings et al. 1987). There are two subspecies of Williamson’s Sapsucker, Sphyrapicus thyroideus (Cassin, 1852). The subspecies occurring in the western part of the range in the Cascade Mountains, Sierra Nevada and the mountains of southern California and northern Baja California is the nominate race, Sphyrapicus thyroideus thyroideus (Cassin). The subspecies occurring in the eastern part of the range in the Rocky Mountains is Sphyrapicus thyroideus nataliae (Malherbe). Intergradation probably occurs in the only area of contact of the two subspecies in eastern Oregon and Idaho (Cowan 1938; Dobbs et al. 1997). The French name for the species is Pic de Williamson (Godfrey 1986).

The plumages of the male and the female are strikingly different (see Morphological Description below). The female was originally described in 1851 by Cassin as the Black-breasted Woodpecker (Picus thyroideus) based on specimens from northern California but he had changed the name to Melanerpes thyroideus by 1854. The male was originally described as Williamson’s Woodpecker (Picus williamsonii) by Newberry in 1857 based on a specimen collected in southern Oregon. By 1858 Baird had recognized both as sapsuckers and renamed them to the genus Sphyrapicus as S. thyroideus and S. williamsonii. The male and female were not recognized as the same species (S. thyroideus) until 1873 by Henry Henshaw (taxonomic history summarized from Ridgway 1914 and Bent 1939).

The two subspecies are weakly defined with the only quantifiable difference based on bill length and width. Mearns (1890) noticed plumage differences in Arizona Williamson’s Sapsuckers that he thought would probably warrant a difference in subspecies designation compared to Pacific Coast specimens. A difference in bill length was first noticed by Ridgway (1914) on 20 specimens from the “Rocky Mountain district” compared to 17 specimens from California, Nevada and Oregon. Swarth (1917) concluded that the difference in bill size was consistent enough to separate the “Pacific Coast” (California to British Columbia) form (thyroideus) as a subspecies from the Rocky Mountain (Arizona, Colorado and New Mexico) form (nataliae) based on examination of 123 total specimens. While he did describe the coloration of the adult male abdominal patch as “a trifle darker shade of greenish yellow” in the Rocky Mountain specimens, there were no other distinctive plumage characteristics of the two subspecies.

Cowan (1938) quantified the difference in bill length and widths for the two subspecies. Using Cowan’s (1938) data (which were presented graphically but not statistically analyzed), there were significant differences in mean bill length among the five regions from which he examined specimens (ANOVA F(4,48) =10.4, P<0.0001) and in mean bill width (ANOVA F(4,52) =11.8, P<0.0001). Using Tukey’s multiple means test at the P = 0.05 level of significance, mean bill width and length of Arizona-Colorado nataliae were not significantly different from Eastern British Columbia nataliae, and neither were significantly different from the intergrade area in eastern Oregon and Idaho; mean bill width and length of specimens from the above three regions were significantly different from the California thyroideus and the western British Columbia thyroideus, which were not significantly different from each other.

However, looking at the individual specimen measurements reported by Cowan (1938) all nataliae had bill widths ≤7.0 mm, and all thyroideus had bill widths ≥6.6 mm, not including specimens from the intergrade region in eastern Oregon and Idaho. The zone of overlap in bill widths between the subspecies (6.6-7.0 mm) contained 4 of 14 nataliae (29%) and 10 of 32 thyroideus (31%). On individual specimens, bill widths would appear unable to reliably distinguish about 30% of the specimens to subspecies. The overlap in bill length was even higher, with 75% of the specimens (34 of 45) in the zone of bill length overlap between 18.9 and 21.6 mm. There have been no further published studies on subspecific status since Cowan (1938).

Morphological description

Williamson’s Sapsucker is a medium-sized woodpecker with total length averaging 23 cm (range 21-25 cm; Winkler et al. 1995; Godfrey 1986), and weighing 44-64 g (Short 1982). The male and female plumages exhibit distinct sexual dimorphism to a degree that is unique among all woodpeckers (Figure 1). The female is predominantly brown and black reminiscent of the coloration of the Gila Woodpecker (Melanerpes uropygialis) or the Northern Flicker (Colaptes auratus). The male is predominantly black and white, reminiscent of many of the Picoides woodpeckers.

Figure 1. Male (left) and female (right) Williamson's Sapsuckers at a nest hole in a Trembling Aspen, Copper Mountain Road, Princeton, British Columbia, June 2001. Photos by Les W. Gyug.

Figure 1. Male (left) and female (right) Williamson's Sapsuckers at a nest hole in a Trembling Aspen, Copper Mountain Road, Princeton, British Columbia, June 2001. Photos by Les W. Gyug.

The male has a glossy black head, breast and underparts with white supercilium, moustache, rump and wing panel, small red throat patch and yellow belly with flanks heavily striped and barred black and white (Winkler et al. 1995). The female has a brownish head with obscure moustachial stripes, heavily barred underparts and wings, a white rump, and a blackish breast with yellow belly and heavily barred flanks (Winkler et al.1995). The juveniles resemble the adults but the juvenile male lacks the red throat of the adult male and the juvenile female lacks the black breast of the adult female.

Genetic description

There are four species in the sapsucker genus Sphyrapicus, which is limited in distribution to North America. Williamson’s Sapsucker is the most distinctive of these genetically with the other three species forming the Sphyrapicus varius superspecies: the Yellow-bellied Sapsucker, S. varius; the Red-breasted Sapsucker, S. ruber; and the Red-naped Sapsucker, S. nuchalis (Johnson and Zink 1983; Cicero and Johnson 1995). S. thyroideus appeared to have diverged genetically from the S. varius superspecies about 3.7-5.2 million years ago, is closer to ancestral stock, and most closely related to Melanerpes woodpeckers (Cicero and Johnson 1995). S. thyroideus probably evolved in western North America while the ancestral S. varius probably evolved in eastern North America and then spread westward, evolving more recently into the currently recognized three species (Short and Morony 1970; Johnson and Zink 1983, Cicero and Johnson 1995). The range and distribution of Williamson’s Sapsucker overlaps broadly with the Red-naped Sapsucker with which it occasionally hybridizes (Short and Morony 1970).

Johnson and Zink (1983) in their genetic review of North American sapsucker taxonomy at 39 loci (17 of which showed variation within the genus), could show no genetic differences between the two subspecies of S. thyroideus and lumped all S. thyroideus together for their analyses although their sample size was very limited. They based their study on 15 specimens from California (presumed subspecies thyroideus), 1 from Oregon (location not specified) and 2 from Montana (presumed nataliae). Cicero and Johnson (1995) looked at mitochondrial DNA of sapsuckers, but only had specimens of subspecies thyroideus from the Cascade Mountains of Oregon and Washington, and therefore could not compare the two subspecies. There have been no genetic studies that have included specimens from the entire range of the species.

Williamson's Sapsucker was found to have a low index of genetic heterogeneity (H = 0.016) using 39 loci. Values for other sapsuckers were all higher, up to H = 0.043, which was the mean value reported for other birds in general (Johnson and Zink 1983). This low index of genetic heterogeneity may indicate that Williamson's Sapsucker has a relatively low ability to adapt to a variety of habitats or to changing habitats.

Designatable units

Although two subspecies of Sphyrapicus thryroideus--S. t. thyroideus and S. t. nataliae--occur in Canada, they are only weakly defined and the species is in need of a modern taxonomic study to confirm that the differences between Pacific Coast and Rocky Mountain forms are sufficient to recognize actual subspecies. The species will be therefore be assessed as one unit. However, throughout this report, the use of the subspecies designations of nataliae have been retained to refer to the population of Williamson's Sapsucker occupying the Rocky Mountain Trench area of British Columbia near Cranbrook (see Figure 3) and of thyroideus to refer to populations of Williamson's Sapsucker west of Greenwood.

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