Williamson's sapsucker (Sphyrapicus thyroideus) COSEWIC assessment and status report: chapter 6

• Previous page
• Table of contents
• Next page

Biology

A comprehensive review of recent sources of the biology of Williamson’s Sapsucker was undertaken by Dobbs et al. (1997). Prior to 1997, there was one comprehensive review of Williamson's Sapsucker biology (Crockett 1975) that used many earlier sources which were not cited directly in Dobbs et al. (1997). The information presented below relies mainly on those reviews, but where original, more recent, or information not cited in those reviews has been used, the source is cited here. There are no known biological differences between the two subspecies, and they will be treated jointly here for most aspects of their biology. There is no information on life span, diseases, parasites or physiology of the species.

Life cycle and reproduction

The males establish territories when they arrive on the breeding grounds in the spring; females arrive 1-2 weeks later (Crockett and Hansley 1977). Excavation of a nest cavity in a standing tree or snag begins shortly after pair formation and lasts 3-4 weeks. They may on occasion re-use an old cavity rather than excavating a new one. There is only one brood per season. Back calculation of egg dates from dates when nestlings were found indicates eggs could be found from 23 April to 15 June (Campbell et al. 1990). Incubation period is 12-14 days in Colorado (Crockett and Hansley 1977) and Arizona (Dobbs et al.1997) with no information available for Canada. Clutch size is usually 4-6, occasionally 3-7 (Dobbs et al. 1997) with 4 and 5 eggs found in the only two nests for which there are records in Canada (BCNRS). Nestlings fledge after 26-33 days in the nest. After fledging, the adults and young may stay in the vicinity of the nest for a few days or weeks, or may disperse widely (Dobbs et al. 1997; Gyug personal observations).

Nest trees

Cavities are excavated in nesting trees or snags, although occasionally an old nesting hole may be reused. Crockett (1975) found 15% of 40 nests and Conway and Martin (1993) 21% of 28 nests, to be in reused holes. This has also been seen in British Columbia at one nest site in a small Trembling Aspen grove where only three nest holes in two trees have been used, and each of the holes reused at least once, from 1998-2004 (Jerry Herzig pers. comm. 2000; Gyug unpublished data).

Trembling Aspen is the most commonly used nest tree of nataliae in the southern Rocky Mountains but not in the northern Rocky Mountains. In Arizona and Colorado Trembling Aspen is the primary nesting tree (95% of the 257 published records in Crockett and Hadow 1975, Conway and Martin 1993, Dobbs et al. 1997) with Ponderosa Pine used in stands without suitable Trembling Aspen for nesting (Crockett and Hadow 1975). Trembling Aspen were not selected randomly for nesting as they form a minority (12-14%) of the available trees in the stands studied (Conway and Martin 1993). Individual aspens were selected for nesting based on being larger than average (Conway and Martin 1993), and if infected with heart rots or otherwise softer than average (Crockett 1975, Schepps et al. 1999), or if they were already dead (Conway and Martin 1993). Even when aspen was used for nesting, the adults would still forage primarily in adjacent coniferous stands (Dobbs et al. 1997).

In contrast, in the northern Rocky Mountains of Montana, there have been fewer nesting records reported but generally nataliae seems to nest in Western Larch (McClelland et al. 1979 give a nest tree size range of 43 – 94 cm DBH for the 4 Williamson's Sapsucker nests found out of 308 total cavity nests – these were more likely to have been Western Larch than Trembling Aspen given that size range). The only nesting records of nataliae in British Columbia are three from Western Larches (Johnstone 1949, EKOOTENAYBIRD 2004) and one from a Black Cottonwood (BCNRS). In the subspecies contact zone in northeastern Oregon, all 86 nests were in conifers, 41% in Western Larch, 40% in Ponderosa Pine, 10% in Douglas-fir and 9% in Grand Fir (Abies grandis) (Bull et al. 1986). The majority (74%) of Western Larches used for nesting were live. More than half of the Ponderosa Pines used for nesting were dead.

The nesting trees of thyroideus are primarily coniferous. In California, these are primarily Ponderosa Pine and Lodgepole Pine (citations from Crockett 1975). Lodgepole Pine in California are the non-serotinous Sierra-Cascade subspecies, and more similar in growth form to the Ponderosa Pine than to the serotinous Rocky Mountain Lodgepole Pine which occurs in the range of Williamson's Sapsucker in British Columbia. In the Cascades of western Oregon and Washington, nest trees were primarily Ponderosa Pine (citations from Crockett 1975).

In British Columbia, nesting trees used in the Okanagan-Greenwood population of thyroideus were primarily Western Larch (77%; Table 4) with 80% of these Western Larch live nesting trees being >69 cm DBH (Figure 4). Western Larch trees >69 cm DBH would typically be at least 200 years old. An 80-cm DBH live veteran Western Larch nest tree cut down for firewood in 2003 was 543 years old. Since the Western Larch nesting trees are typically still alive, scar tissue forms over the nesting holes and many of them eventually close over. Up to 98 nesting holes were seen in one Western Larch indicating the nesting trees will be used over many years. Western Larch is unique among the dry-belt and mid-elevation conifers of British Columbia in that heartwood decay can persist for many years with the very hard sapwood maintaining the tree (see McClelland and McClelland 1999).

Figure 4. Frequency of live Western Larch in 10-cm Diameter-at-breast-height (DBH) classes used for nesting by the thyroideus subspecies of Williamson's Sapsuckers in the Okanagan-Greenwood population, 1996-2004, based on Gyug (unpublished data).

Western Larch is widespread in the southern interior of British Columbia from Okanagan Lake eastward to the Alberta border, and as far northward as Salmon Arm (see Figure 2), so it would seem to be an anomaly that a species that seems to associate so highly with Western Larch as Williamson's Sapsucker would be so limited in distribution in British Columbia. However, only at the drier southern edges of the range of Western Larch in British Columbia is there an abundance of veteran Western Larches--trees that have survived multiple fires.

Outside of the Okanagan-Greenwood population in British Columbia, nesting trees of thyroideus (n = 36) were primarily Trembling Aspen (64%) followed by Ponderosa Pine (28%). This trend is similar to the trends for nesting trees found in the southern Rocky Mountains for nataliae where Trembling Aspen is the primary nesting tree followed by Ponderosa Pine. The choice of nesting trees is clearly not related to subspecies of Williamson's Sapsucker, but is related to the available and suitable trees in a given area. The other three reported nesting trees were a Black Cottonwood, a Douglas-fir, and (apparently) a Lodgepole Pine. Three BCNRS cards were submitted by separate observers on this last nest: one not identifying the species of nest tree, one identifying it as an Engelmann Spruce, and one (on the most meticulously filled-in card) identifying the tree as a Lodgepole Pine.

Height of nesting trees varied from as low as 5 m for a broken-trunk spruce snag to as high as 49 m for a Western Larch (Table 5). Nest heights covered a wide range as well, from as low as 1 m above ground to as high as 42 m above ground (Table 5). General heights of mature Douglas-fir canopy trees in the Okanagan-Greenwood Area of Occupancy are in the 24 to 28-m range. The mean height of Western Larches used for nesting was 29 m (Table 5), even though many of these had broken tops. The veteran Western Larches typically used for nesting could often be spotted from hundreds of meters away because they stood out above the main tree canopy layer.

¹ Ponderosa Pine, Douglas-fir, spruce
² Trembling Aspen, birch, Black Cottonwood

Diet and foraging

Sapsuckers are specialized for feeding on the sap and phloem fibres of trees with brush-like tufted tongue tips, rather than the barbed tongue-tips of most woodpeckers (Winkler et al.1995). Williamson's Sapsucker feeds exclusively on conifer sap and phloem during the pre-nestling period, shifting to mainly carpenter ants after hatching of the young.

Williamson's Sapsuckers were observed feeding on 12 Douglas-fir sap trees during nest searches in 2004 in British Columbia (Gyug unpublished data). Ten of these sap trees were between 15 and 39 cm DBH, one was 55 cm DBH, and one was 62 cm DBH. For nine of the sap trees, the location of the nest of the bird or pair feeding on the tree was located. Distance of the sap trees to nest ranged from 6 m to 258 m with a mean of 88 m (SE = 26.2 m). The majority of the sap trees (6 of 9) were between 32 and 91 m from the nest.

During the nestling period, carpenter ants form the majority (75-99%) of the diet of Williamson's Sapsucker based on the four studies that have examined detailed food habits (Beal 1911; Otvos and Stark 1985; Stanford and Knowlton 1942; Crockett 1975). Carpenter ants only nest in downed trees, stumps and snags >30-cm DBH, and in live trees >20-cm DBH (Sanders 1970).

Other insects besides ants are taken opportunistically during the breeding season (Crockett 1975; Dobbs et al. 1997; Gyug personal observations) but the main adult food items, and the main food items fed to nestlings, are carpenter ants. These are primarily gleaned from tree trunks (84% of the time, Crockett 1975; 97% of the time, Stallcup 1968). Sap and phloem fibres constitute the primary diet during the non-breeding season although berries may provide a substantial part of the diet during winter.

Home ranges

Males establish breeding territories around the chosen nest site. Published breeding territory sizes have only been calculated for nataliae in the southern Rocky Mountains. Crockett (1975) reported territory sizes of 4 to 9 ha (mean 6.75 ha); Conway and Martin (1993) reported inter-nest distances of 175 to 375 m in Arizona which would be similar to the distances implied by Crockett. Young (1975, cited in Cooper 1995) found a minimum territory size of 0.8 ha; this seems impossibly small, and may have been based on too few observations to be valid. Six territorial radii in British Columbia ranged from 150-425 m (based on territorial challenges, Gyug unpublished data). Calculations based in inter-nest differences give a mean territory size of 17 ha for British Columbia pairs (Gyug unpublished data), suggesting that territories are larger at the northern end of the range. The only radio-tracking study of Williamson's Sapsucker ever undertaken was on one nesting male in British Columbia (Manning and Cooper 1996), so this range size of 54 ha should probably be given more credence than estimates based on indirect measures of territorial responses.

Density

Williamson's Sapsucker nest densities range up to 2.83/100 ha (100 ha = 1 km²) based on larger plots in the U.S.A (Table 6), and up to 2.71 based on larger plots in British Columbia (Table 1). Highest densities in British Columbia were just over 3 pairs/100 ha.

¹ Lemkuhl et al. 2004
² Crockett 1975
³ Vos and Yoder 2003
⁴ Audubon Colorado 2004
⁵ Bienz et al. 2004.

Dispersal/migration

Williamson’s Sapsucker is a migratory woodpecker that returns to British Columbia between mid-March and mid-April. Fall migrants generally depart by mid-September with occasional records from October (Campbell et al. 1990. There is very limited information on dispersal of young based on six young banded in nests by Crockett (1975, also see discussion under Survivorship below). Two of the six returned to his study areas in their first spring after migration, and a third was found returned in its second year.

Survivorship/mortality

Nesting success, i.e., percentage of nests fledging at least one young, averaged 96.1% in Arizona (Dobbs et al. 1997). Successful nests averaged 3.67 fledged young in Arizona (Dobbs et al. 1997) and 3.17 young per nest in Colorado (Crockett 1975). No information is available for Canadian Williamson's Sapsucker populations. Lifetime reproductive success is unknown because there is no information on life span for Williamson's Sapsuckers, and relatively little information on adult survivorship (see below). For the Red-naped Sapsucker, a similar-sized migratory woodpecker, the maximum age on record is at least six years (Walters et al. 2002).

One-year survivorship of Williamson's Sapsuckers banded as adults of unknown age was 54% (recalculated from Crockett 1975 using all his year-to-year data, not just the “first-year of data” subset he used to calculate survivorship of 48%). This one-year adult survivorship is similar to estimates for Red-naped Sapsucker of 43% in Nevada (Fleury 2000) and 56% in British Columbia (Walters et al. 2002). For the nine adults banded in the first-year of Crockett’s (1975) study, five were still on his study sites two years later. Some of the banded adults may have dispersed to other sites outside of his study areas, so this is a minimum survivorship estimate. Two of his banded pairs remated, four adults used the same nest hole, two adults the same nest tree, and six used the same territory.

The only information on survivorship of juvenile Williamson's Sapsuckers is from Crockett (1975). A total of six young were banded in nests of which three returned in following years to nest. Juvenile survivorship of fledged young was much lower for a much larger sample of Red-naped Sapsuckers with only 7 of 82 young returning to breed within 2 years in Nevada (Fleury 2000) and 8 of 141 young returning in British Columbia (Walters et al. 2002).

Williamson's Sapsucker is a prey species for all three of the Accipiter species of western North America – Sharp-shinned Hawk (A. striatus), Cooper’s Hawk (A. cooperii) and Northern Goshawk (A. gentilis)--with remains found in the pellets of each species in Oregon (Reynolds and Meslow 1983). Nest predators include Red Squirrels (Tamiasciurus hudsonicus; Dobbs et al. 1997), Long-tailed Weasels (Mustela frenata; Crockett and Hansley 1977) and Black Bears (Ursus americanus; Walters and Miller 2001), and probably include snakes when nests are fairly close to ground level (Crockett and Hansley 1977). Based on observations of nest predation on other species of woodpeckers in British Columbia in areas where Williamson's Sapsuckers were found, other possible nest predators may include Deer Mice (Peromyscus maniculatus) and House Wrens (Troglodytes aedon; Walters and Miller 2001).

Interspecific interactions

Pairs are territorial during the breeding season with little toleration of other conspecifics. Other woodpeckers and sapsuckers also occasionally experience hostility from territorial males (Dobbs et al. 1997) but closest inter-nest spacing can be much smaller than for conspecifics. When Williamson’s and Red-naped Sapsuckers interact, Young (1975 cited in Walters et al. 2002) considered the Williamson's to be aggressive toward, and behaviourally dominant to, the Red-naped.