Williamson's sapsucker (Sphyrapicus thyroideus) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

Search effort

Quantitative studies of the distribution and abundance for the thyroideus subspecies have been undertaken from 1996 to 2004 (Gyug unpublished data). This included 1400 call playback points with 400 to 500-m interpoint spacing completed in 1997 (206), 1998 (46), 2003 (161), and 2004 (987) in areas between Grand Forks and Merritt, British Columbia (Gyug unpublished data). Most points with call playback responses of Williamson's Sapsuckers were searched later for nests, the only exception being the call playback points near Merritt in 2004, where there were three responses to the call playbacks but no later nest searches. An additional 37 areas (of 100 to 200 ha each) were searched intensively for nests between 1996 and 2004, even though no call playbacks had been done in these areas previously.  Complete census was attempted in five areas (see Table 1). The nest site data at Wallace Creek from Gyug and Bennett (1995) and Manning and Cooper (1996) were used as additional census areas, even though they were not initially attempted as censuses.

Using a 250-m radius (the mean distance from the call playback points where Williamson's Sapsuckers were initially detected to the discovered nest) as the sampled area for each call playback point, and the actual areas of search areas and census areas, 162 km² (27%) of the 594 km² of the Area of Occupancy of the main locality of the Okanagan Greenwood population was sampled. An additional 182 km² were searched outside of this Area of Occupancy, but these searches had negative results and were used to refine the boundaries of the Area of Occupancy. Given that only 20% or less of the Area of Occupancy of this locality is estimated to be high quality habitat for Williamson's Sapsucker (see Abundance estimate below), and that most of this high quality habitat in this Area of Occupancy was searched, there remain very few areas of suitable habitat that have not been searched at all within that area.

The report writer is fairly confident that there are no large populations (i.e., certainly not >3 or more pairs) of Williamson's Sapsucker outside the known Areas of Occupancy. Extensive records of other species of sapsuckers and woodpeckers throughout southern British Columbia (as summarized in Campbell et al. 1990) indicate that the lack of records of Williamson's Sapsucker outside the Areas of Occupancy is not because of a lack of searching, but is a true reflection of the absence or rarity of Williamson's Sapsuckers outside these areas.

Prior to these fairly intensive and extensive searches for Williamson's Sapsuckers, there had been no quantitative basis on which to make any population estimate of Williamson's Sapsuckers in British Columbia. Gyug and Peatt (2000) published an estimate in 2000 which should no longer be viewed as authoritative since the density estimates were based on only one census area (Johnstone Creek 1997-1999) which we later found to be lower than the density in other areas (see Table 1). 

Search effort for nataliae in 1996 was 52 call playback points and 14 intensive search areas (Cannings 1997). No nataliae were found on these searches. All current locations and knowledge of the nataliae subspecies come from incidental observations from a wide range of sources.

Abundance

The Area of Occupancy of the main group of locations of the Okanagan-Greenwood population was 594 km² in size in 2003-2004. The current total is about 89 km² (15.0%) of older forests within this Area of Occupancy (see Habitat Trends section). The mean Williamsons Sapsucker breeding pair densities of each of the three census areas (Okanagan Falls 3.11; Gregoire Creek 1.96; Johnstone Creek 0.75; see Table 1) were averaged to estimate the mean density found in older Western Larch/Douglas-fir forest in the area. The mean density estimate was 1.94 breeding pairs/km². Multiplying this estimate by the area estimated to be in older forest yields an estimate of 173 breeding pairs in this locality. Additional localities in the Okanagan-Greenwood population north of Shuttleworth Creek would add nine breeding pairs to this estimate. There were therefore 182 breeding pairs estimated in the Okanagan-Greenwood population in 2004.

Additional localities in the other thyroideus populations must be added to this estimate. There would be up to six breeding pairs in the Hat Creek population based on three nests found in one year, and others probably missed (Wayne Campbell pers. comm. 2004). The report writer estimates that there would be up to eight breeding pairs in the Merritt population based on estimates of two in the Sunshine Valley locality, four at Midday Creek and two at Kane Valley/Aspen Grove. I would estimate up to ten breeding pairs currently in the Princeton population based primarily on 3 known nests, one breeding pair and two other adult males in 2004 in the Copper Mountain Road/August Lake locality, and possibly a few others in unsurveyed locations. There would be a few nesting pairs not included in any of the above populations. I would estimate these at four breeding pairs in 2004 based on the one known from Trout Creek in 2004, and possibly up to three northwards to the area near Kamloops that may remain undiscovered.

There is insufficient information to provide a reliable estimate of the number of breeding pairs in the nataliae Williamson's Sapsucker population in British Columbia. Based on the rarity of nest records, or any other sightings, of this subspecies, a best estimate would be that there are fewer than five breeding pairs, or 10 breeding adults. This population would be similar in size and extent to the Princeton population of the thyroideus subspecies, although there is no known nucleus or single location where nataliae is relatively abundant as is the case in Princeton.

The estimated breeding pairs in the Williamson's Sapsucker Total Population in British Columbia in 2004 is 215. This is based on 182 in the Okanagan-Greenwood population (85% of the Total Population), and 33 in four other populations, and in locations outside any population centres. The estimated number of breeding adults in the Total Population is 430, i.e., double the estimate of breeding pairs.

Only 95.6 km² (3.4%) of the nataliae breeding range was estimated to be in forests older than 140 years (see Habitat Trends section). There is insufficient density information from which to extrapolate population numbers from this estimate of potential habitat size. However, given that only 3.4% of the breeding range might be considered currently potentially suitable habitat, this may indicate that significant loss of older forests may have already occurred in the breeding range of the nataliae subspecies, and this could be one of the reasons it is very rare.

Fluctuations and trends

No long-term population numbers are known for Williamson's Sapsucker in Canada. However, based on decreases in suitable habitat, the population is probably declining. There are sketchy long-term population records for only two areas of the Okanagan-Greenwood population of thyroideus. At Schoonover Mountain (east of Okanagan Falls), they were first collected in 1913 (Munro and Cowan 1947), and are still common there--probably because the stand of veteran Western Larch has remained. However, this stand is much smaller now than it once was, is now isolated from similar stands, and a substantial portion is proposed to be timber harvested (see Habitat Trends section).

The only other area for which there is a long-term record is Anarchist Mountain which was the “centre of abundance” of this subspecies in the province (Guiguet 1970), and where the species has been “Common in all the heavy timber from Midway to Osoyoos Lake, B.C., at an altitude of about 4000 ft [1200 m]” (Spreadborough, cited in Macoun and Macoun 1909). That area is no longer the centre of abundance because of extensive land clearing and logging in the past 50 years. The area formerly colloquially known as “Sapsucker Woods” on Anarchist Mountain (Dick Cannings pers. comm. 2004) has been entirely cleared and no longer has any breeding Williamson's Sapsuckers (Gyug unpublished data). A few veteran Western Larch that appear to be former Williamson’s Sapsucker nesting trees were left standing after land clearing, but the habitat around them is completely unsuitable. Overall numbers appear to be decreasing because of habitat loss in this population, but there are no prior estimates of population numbers to quantify this numerical loss.

Williamson's Sapsuckers are detected too infrequently on Breeding Bird Survey (BBS) routes to develop reliable overall population trends (Sauer et al. 2004), but in the only place where there is a reliable trend (Oregon), the species is declining at an annual rate of 3.3%.

Rescue effect

There are populations of thyroideus Williamson's Sapsuckers in adjacent areas of Washington State. However, while there is a possibility of rescue effect from immigration should populations in British Columbia decline to unsustainable levels, the principal population limit for the Okanagan-Greenwood population is currently habitat, and the rescue effect is not relevant. At any rate, even if the rescue effect should be required to repopulate suitable habitat, there is probably relatively little long-distance dispersal of young in Williamson's Sapsucker based on the only banding study of six nestlings (Crockett 1975) where half the young returned to his study area to breed within two years. Given that the overwinter mortality of young is unknown, but could easily be equal to half the fledglings, most or all of the young that survived to their first year may have returned to near the natal site, and there may be almost no long-distance dispersal.

Populations of nataliae Williamson's Sapsucker are relatively low south of the Canadian border in Montana (Sauer et al. 2004; Hutto and Young 1999; Weydemann and Weydemann 1928; McClelland et al. 1979). Even if suitable habitat was abundant, or more was created, in Canada, there may not be large enough populations in Montana with much long-distance dispersal to provide a rescue effect to this population.