Nest Site Selection by Williamson and Red-Naped Sapsuckers

Abstract

Two congeneric woodpeckers, the Williamson Sapsucker (Sphyrapicus thyroideus) and the Red-naped Sapsucker (S. [varius] nuchalis) are sympatric throughout much of the Rocky Mountain region of the western United States. Although these two picids are closely enough related that two probable hybrids have been reported (Short and Morony 1970), there is generally effective reproductive isolation between them. This reproductive isolation apparently is not due to different nest site preferences. Bent (1939), Packard (1945), Bailey and Niedrach (1965), and Burleigh (1972) reported that Red-naped Sapsuckers prefer deciduous trees, especially aspen (Populus tremuloides), while Williamson Sapsuckers favor conifers. However, Rasmussen (1941), Hubbard (1965), and Tatschl (1967) noted that thyroideus nest in aspen, which Ligon (1961) suggested was their preferred nest tree in northern New Mexico. These differences in the literature led us to investigate the nesting habitat preferences of the two sapsuckers in Colorado and Wyoming. We use the name Rednaped for the Rocky Mountain form of the Yellowbellied Sapsucker (S. varius) complex, as suggested by Short (1969) and Short and Morony (1970). We examined a total of 57 thyroideus and 46 nuchalis nests at two locations in Colorado and one location in Wyoming. Both species nested at each location and were provided a choice of nest trees by the close proximity of conifers and aspen (fig. 1 and 2). At each nest site, we recorded the species and condition of the nest tree, its diameter at breast height (dbh, measured at 4.5 feet above the ground), the height of the nest hole, and the compass direction that the hole faced. We also measured various nest stand parameters, including stand density and stand size. These data were examined for significance using t-test and Chi-square statistical analyses. The primary study area, in northern Colorado, consisted of six sites within the eastern half of Rocky Mountain National Park, Larimer County. Elevations at which nests were found ranged from 2440 to 2800 m, roughly corresponding to the upper montane region and the upper montane/subalpine ecotone region of Marr (1967). The area was predominantly an open forest of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii), with lodgepole pine (P. contorta) on cooler north-facing slopes and with limber pine (P. flexilis) in exposed areas above 2700 m. Aspen occurred in discrete stands along permanent to ephemeral drainages within the coniferous forest (fig. 1). The understory was highly variable due to topographic differences but was always rather sparse and low. The second Colorado study area was located on Missionary Ridge, about 20 mi NE of Durango, La Plata County. Elevations here ranged from 2500 m at Wallace Lake to 2900 m atop surrounding hillsides. Mature aspen ringed the ephemeral lake and extended up the adjacent slopes along drainages (fig. 2). Drier areas were covered by a coniferous forest of ponderosa pine, Douglas-fir, white fir (Abies concolor), and blue spruce (Picea pungens). Open areas on hillsides were covered by a dense growth of scrub oak (Quercus gambelii) and snowberry (Symphoricarpos oreophilus). Grasses, sedges, and forbs grew in a lush understory beneath the aspen, with kinnikinnik (Arctostaphylos uva-ursi) under the denser spruce/fir forest. Ground cover was sparse under the Douglas-fir/pine forest. The third study area was located in the Bridger Wilderness on the west slope of the Wind River Mountains, 40 mi NE of Pinedale, Sublette County, Wyoming. Nests were found from 2440 to 3200 m. Areas at lower elevations were covered by a forest of aspen interspersed with Englemann spruce (P. englemannii), Douglas-fir, and lodgepole pine. Areas at higher elevations had a forest of mixed spruce and subalpine fir (A. lasiocarpa) with patches of lodgepole pine and small stands of aspen, the latter occurring as late seral stages and being replaced by spruce and fir. At all three study locations, the two sapsucker species showed a strong preference for aspen (table 1), in which 85 percent of thyroideus nests and 100 percent of nuchalis nests were located. The close proximity of aspen and conifers at the three sites