Study area

Our study area was located in the Arctic Coastal Plain near the village of Barrow (71°18′N, 156°40′W) and encompassed c. 200 km². The soil in this region is frozen during winter and underlain with permafrost at depths of 360 to 650 m below the active soil layer (Brown et al. 1980, Davies 2001). During summer, the thaw zone is approximately 30–40 cm and generally wet (Brown et al. 1980). The Barrow region is characterized by low relief, with elevations ranging from 5–10 m, and ice-wedge created polygonal terrain, and shallow lakes (Sellmann et al. 1972, Brown et al. 1980). Much of our study area is covered by polygonal terrain (Fig. 1), and approximately 65% of the landscape above 71°54′N is polygonal terrain (Brown et al. 1980).

Winters are long, dry, and cold. Temperatures remain below freezing for almost nine months per year, but can also fall below freezing at anytime during the three summer months (Brown et al. 1980). Snow melt begins in early June, and there is 24 h of light from 10 May to 2 August. Barrow's short summers (June to August) are cool (0.6–3.7°C), moist (8.9–26.4 mm precipitation) and windy (5.1–5.5 m/s) (Bunnell et al. 1975, Brown et al. 1980). However, Arctic temperatures have been increasing over the past 150 years, and are predicted to rise for the next 50 years (Mitchell et al. 1990, Overpeck et al. 1997).

DWH has been studying Snowy Owls in Barrow from 1992 to present. During this period Snowy Owls have nested only in 1993, 1995, 1996, 1999, 2000, 2002, 2003, 2005, 2006 and 2008. Although we did not monitor breeding activity in 1991, many nests were located by local researchers. We measured 114 nest mounds and 2280 random mounds for the years: 1991, 1993, 1995, 1996, and 2000. Our experience with definitions, recognition, and polygonal terrain ecology increased after the first two seasons. Thus, polygon recognition, and mound measuring precision improved with field experience.

Nest mounds

We used binoculars and spotting scopes to locate Snowy Owl nests. Owls observed in a horizontal (incubating/brooding) posture were investigated for nest sites. Nests were mapped and their GPS coordinates recorded. We measured the nest bowl length, width, and depth to the nearest cm. Nests enlarge and fill with debris during the nesting season therefore they were measured during incubation or early brooding. We visually estimated percent vegetation cover on top of each mound and recorded the four most abundant plant species. Vegetation was identified using Hulten (1974). We also recorded other cover types such as exposed soil (Brown et al. 1980), lichen and mosses (Thomson 1979, See 1981, Vitt et al. 1988).

Most Snowy Owl nests were on the tops of mounds within a polygon that appeared as a distinctive feature of the landscape. For these sites, measurements were taken in the following way. Length was measured as the furthest distances between two points defining the mound and dropping-off where the inflection of the mound changed from convex to concave. Width was measured perpendicular to length at the widest point of the mound and dropping-off at its inflection point. Height was measured at the highest point of the mound and the lowest level of its inflection point, although they tended to be uniformly level (Fig. 2). We did not include the man-made sites that the owls nested on such as dredge plies and gravel mounds, for this analysis.

Random points

We randomly selected 20 mounds within 100 m of each nest to make comparisons of used and unused mounds. To choose a random point we used a random number table to choose numbers between one and eight, to correspond to the cardinal compass directions; N, NE, E, SE, S, SW, W, NW We then chose a random number between 1–100 m and proceeded along the compass line until a mound fell on the line or within 10 m of the line at the selected distance. That mound was then measured. If a mound was questionable in its definition, we replayed the random selections. We only did this one time, as not to bias our picking of one mound over another. Above all, a mound had to be a definable unit.

Figure 2.

Variables measured at mounds within polygons at Barrow, Alaska (width, length, height). Both mounds with Snowy Owl nests and a random selection of non-used mounds were sampled.

A few nest mounds were not on polygonal terrain (e.g. on ridges or relatively flat tundra) and had zero or few adjacent mounds. In these cases we defined the nest mound as an independent unit defined by its inflection points from concave to convex. If possible, random sites were measured.

Analysis

We compared the height, length and width of nest mounds to those of random mounds within each year by calculating the percent of nest mounds that fell above a certain percentile of the distribution of random mounds. Additionally, we used a one-sample t-test to explore if the average difference between nest mounds and random mounds were significantly different from zero. Where large outliers in the distribution of differences occurred, a Wilcoxon sign rank test was performed. Alpha levels were set at <0.05.

Nest bowls

Female Snowy Owls dug a somewhat circular nest bowl at or near the top and centre of the mound (Fig. 1). In this depression the owls laid eggs on bare soil. No nest material was added. The length and width from 91 nests averaged 47.5 cm (SD 11.8, range 24–78) × 44.0 cm (SD 10.1, range 22.0–68) cm and the depth of 68 nests averaged 9.6 cm (SD 2.6, range 4.5–17.0).

Mound height

Mean nest height for 114 mounds was about 1 m (range 95–106 cm). In all years nest mounds were significantly higher than random mounds (Table 1). Sixty-three percent of nest site mounds were taller than all of the randomly selected mounds. In fact, almost all nest site mounds were >50th percentile of the distribution of random mounds (Table 2).

Table 1.

Parameters of nest mounds used by Snowy Owls at Barrow, Alaska, n = number of nests measured; P = significance level (t-test).

Mound length and width

While there were significant differences in mound length and width in 1991 and 1993, no significant differences occurred in 1995, 1996, and 2000 (Table 1). Only 67.8% (Table 2) of the 114 nest mounds were in the top 50 percentile of the distribution of random mound length at that site. About 80% of the nest mounds were in the top 50 percentile of the distribution of random mound width (Table 2). Defining mound length and width was sometimes problematic, especially in the early years of the study.

Nest mound vegetation

Vegetation from nest mounds was quantified in 1993 (n = 19) and 1995 (n = 19). All mounds had sparse vegetation <10 cm tall. Incubating Snowy Owls were not concealed by vegetation and were clearly visible from long distances. Higher and drier nest mounds had >50% Poa spp. grass cover, while lower and wetter random mounds had <20% Poa spp. cover. Differences in vegetation at nest and random mounds were associated with mound age, height and moisture levels.

Table 2.

Percent of nest mounds (n = 114) that fell above and between the 50th and 100th percentile of the distribution of randomly selected mounds. For example, for the height variable, 99% of the nest mounds fell above the 50th percentile of the random mounds.

Nest mounds

The polygonal mounds used by nesting Snowy Owls in Barrow are important to the management and conservation of this species. The formation of high-centred polygonal mounds by the freezing and thawing of the ground has been underway for c. 14 000 years in the Barrow region (Brown et al. 1980, Davies 2001). The process starts when the ground rapidly cools and begins a contraction and cracking process at about -15°C (Davies 2001). Snow, ice, and water are then able to penetrate the space during the freeze-thaw period. Ice wedges form in the cracks and over centuries of repeated thawing, freezing, and cracking, the wedge grows in depth and width (Davies 2001). As the volume of ice increases, land above it is heaved upwards on both sides of the wedge. Coupled with melting of the top of the wedge, parallel rims and a lower trough are formed (Fig. 3). Troughs are interconnected and further define the polygonal appearance. Over time, the upheaved terrain goes through many landform stages of growth and deformation from low-centred to high-centred polygons, to decay. It is the older high-centred polygon mounds where most Snowy Owl nests are located in the Barrow region. The high-centred mounds used by Snowy Owl can be 10 000 or more years old (Brown et al. 1980, Pielou 1994, Davies 2001).

Snowy Owl habitat conservation

Barrow was once called Ukpiagvik, meaning a place where owls are hunted, met or found. Native peoples throughout the Arctic and the Snowy Owl have had a long history. Snowy Owls have been a food source, kept as pets, honoured in artwork, and its body parts used for various cultural functions (Brower 1942, Bailey 1948, Vaughan 1992). Currently the logo of the Alaska United Eskimo Corporation is the Snowy Owl.

During this study, we recorded the loss of Snowy Owl nesting areas to development of the growing Barrow community. The drier upland sites where Snowy Owls prefer to nest are also the sites most appealing to housing development. On the other hand land has been set aside (Barrow Environmental Observatory Research Area) by the native people of Barrow as a conservation and research area benefiting all plants and animals. Development however, does increase access to Snowy Owl nest sites to residents, their pets, tourists, and researchers. This will push Snowy Owls to other areas for nesting. Snowy Owls are a forgiving species, and with management guidelines, both community development and Snowy Owl breeding can co-exist in Barrow. As Barrow grows and the surrounding land is used for expansion, city planners and mangers will need to conserve Snowy Owl habitat and nest sites. How human population growth will effect local lemming populations is unknown.

It is also unknown how climate change might affect the cryogenic process that creates the polygonal terrain surrounding the Barrow region. However, there is ample evidence of changes in Arctic climate (Mitchell et al. 1990, Chapin et al. 1992, Overpeck et al. 1997, Divoky 1998).