Overwintering Habitat of American Dipper, Cinclus mexicanus, Observed in an Arctic Groundwater Spring Feeding on Dolly Varden, Salvelinus malma

Perennial groundwater springs along the Alaska and Yukon North Slope provide overwintering habitat for various organisms, including birds and fishes. We observed an American Dipper, Cinclus mexicanus, in the open water of a perennial spring situated in Fish Creek, Yukon, in Ivvavik National Park on 8 March 2018. The observation at Fish Creek was among the most northern documented sightings of an American Dipper during the winter in North America. Moreover, the observation was approximately 650 km farther north than where American Dippers have been documented overwintering in Yukon, making this the most northern Canadian observation documented for this species in any season. Additionally, the American Dipper was photographed feeding on a juvenile Dolly Varden, Salvelinus malma. Although American Dippers are known to feed on small fish, our observation was a novel documentation of a trophic interaction between both species during winter. The open-water habitat in Fish Creek, which is important for both species and has not been previously described, was short (~730 m long), shallow (mean = 20 cm deep), narrow (mean = 2.8 m wide), and cold (mean water temperature = 0.34oC). While there is little information regarding the ecological interactions of American Dipper overwintering in the Arctic, we note that all observations in the North Slope area during winter occurred in river systems also used by Dolly Varden, which indicates that juvenile Dolly Varden could be an important source of food for American Dipper in winter.

Perennial groundwater springs along the Alaska and Yukon North Slope can produce areas of open water throughout winter that create an aufeis field (a mass of layered ice) downstream as water freezes (Craig and McCart, 1975;Clark and Lauriol, 1997;Kane et al., 2013). Groundwater is known to influence the distribution, reproductive success, productivity, and movement of various organisms, including birds and fishes (Power et al., 1999;Huryn et al., 2005;Parker and Huryn, 2006). Although perennial springs account for a small proportion of stream types and amount of habitat area along the North Slope, these habitats have important consequences for biodiversity and food-web dynamics (Huryn et al., 2005). The springs along the North Slope support distinct macroinvertebrate assemblages and provide critical habitat for cold-tolerant riverine fishes including Arctic grayling (Thymallus arcticus) and Dolly Varden (Salvelinus malma) (Craig and McCart, 1975;Mochnacz et al., 2010;Kendrick and Huryn, 2014). During winter, the warmer water temperatures of the perennial springs protect Dolly Varden eggs that cannot tolerate freezing, while juveniles and adults (> ~300 mm fork length; anadromous life history) take cover under the ice of the upper aufeis field (Sandstrom et al., 2001). Additionally, these areas of open water can provide overwintering habitat for American Dipper (Cinclus mexicanus) (Clough et al., 1987;Kendrick and Huryn, 2014), a semiaquatic songbird 14 -20 cm long that nests near streams and walks, dives, and swims underwater to feed (Kingery, 1996).
American Dipper is found in western North America from northern Alaska down to the Pacific coast between British Columbia and California, and south from northwestern Mexico to western Panama (Willson and Kingery, 2011). The eastward distribution includes Yukon and locations mainly between Alberta and New Mexico (Willson and Kingery, 2011). American Dipper inhabits clear, fast-flowing mountain and cold coastal streams that include cascades, riffles, or waterfalls with a stream bank structure (e.g., cliffs, large rocks, or overhanging ledges), and substrates consisting of cobble and coarse gravel (Willson and Kingery, 2011). Migration is characterized as generally short-distance, with many birds remaining on territories year-round and a winter range determined partly by the presence of open water for foraging (Willson and Kingery, 2011). American Dipper feeds on aquatic insects and their larvae, small fish typically 100 mm or less in length (trout and salmon fry, Arctic grayling, sculpin [Cottus sp.]), fish eggs, and tadpoles (Brent, 1948;Kingery, 1996;Obermeyer et al., 1999;Morrissey and Olenick, 2004;. Little documentation exists on American Dipper overwintering in the Arctic outside its year-round range (see Willson and Kingery, 2011), particularly on their aquatic habitats and trophic interactions.
On 8 March 2018 we observed one American Dipper in the open water of a perennial spring situated in Fish Creek, Yukon, Canada (Figs. 1, 2), presumably overwintering at this location (i.e., having remained in Fish Creek since freeze-up). The two nearest known perennial springs are at a distance of ~22 km (unnamed creek in the Firth River Delta; Craig and McCart, 1974) and ~63 km (Kongakut River, see Kane et al., 2013; or Joe Creek). The observation was made during a day trip in the area in collaboration with the Parks Canada Agency to investigate the distribution of muskox (Ovibos moschatus) along the Canadian North Slope and collect data on winter habitat for Dolly Varden in Ivvavik National Park. During the three-hour site visit at Fish Creek, the bird was frequently seen walking in the water and flying among the small pools of open water. The observation at Fish Creek was among the most northern documented sightings of an American Dipper during winter in North America (Table 1). Moreover, the observation was approximately 650 km farther north than where American Dipper has been documented overwintering in the Yukon (approximately < 64˚ N between December and February; Sinclair et al., 2003;Schonewille, 2010), which makes ours the most northern Canadian observation for this species, not only for winter but for all seasons. Additionally, the American Dipper was photographed feeding on a juvenile northern form Dolly Varden (S. m. malma) (approximately < 100 mm and swallowed whole) (Fig. 3). Although American Dippers have been documented feeding on juvenile Dolly Varden in the North Slope during the summer (Ivishak River; Parker and Huryn, 2006), our photograph is the first record of this trophic interaction occurring during winter. On the same day, one American Dipper was observed in the open water of both Firth River and Joe Creek, which are systems also inhabited by Dolly Varden (Table 1). Previous winter sightings for American Dipper have occurred at both locations (Sinclair et al., 2003).  (Fig. 2). The tundra vegetation consisted of snow-covered willow shrubs (< 2 m height) growing along the banks of the creek. Measurements of stream depth and width (i.e., distance between snowbanks) and water temperature were collected from 10 pools (one, two, three, four measurements made in five, two, two, one pools, respectively; total of 19 readings). Depth was measured in the centre of the channel and at the left and right snowbanks of the pools. Pools of open water in Fish Creek were shallow, as depths in the middle of the channel averaged (± SD) 20.7 ± 7.8 cm, while those on the banks averaged 14.1 ± 6.8 cm. Stream width averaged 2.8 ± 0.9 m and the mean water temperature was 0.34 ± 0.74˚C (range = −0.7˚C to 1.3˚C). The substrate was composed of gravel and cobble. The height of the snowbanks was between approximately 1 and 2 m and tended to be higher in the lower reach where the aufeis had formed. Craig and McCart (1974) in October 1972 and by the authors in mid-September 2016 and 2017 (unpubl. data) in Fish Creek at the location of the spring have only documented the presence of a relatively high concentration of Dolly Varden, although two vagrant adult sockeye salmon (Oncorhynchus nerka) (see Dunmall et al., 2013) were captured in the same habitat occupied by Dolly Varden in 2017. Both the stream resident and anadromous types of Dolly Varden, which have distinct phenotypes (for detailed descriptions see Armstrong and Morrow, 1980;COSEWIC, 2010), inhabit Fish Creek. In addition to these two types of Dolly Varden, we have captured numerous small juveniles (pre-smolt fish that have yet to exhibit the resident or anadromous phenotype) that also have a distinct appearance characterized by brown colouration, parr marks, and a white belly matching that of the fish observed in the bird's beak (Fig. 3).

Prior fisheries investigations conducted by
The observations of American Dipper overwintering among several perennial springs underscore the importance of these habitats for this species to persist in Arctic environments at the northern extent of its known range. Similar to polynyas in the marine environment (Stirling, 1980), albeit on a vastly different geographic and ecological scale, perennial groundwater springs in the Arctic are small oases of open water used by multiple species that are important for maintaining biodiversity during winter. While there is little information regarding the ecological interactions of American Dipper overwintering in the Arctic, we note that all observations in the North Slope area during winter have occurred in river systems also used by Dolly Varden for rearing, spawning, and overwintering (Table 1, Fig. 1). Individual birds that remain year-round in a perennial groundwater spring may possibly rely heavily on lipid-rich juvenile Dolly Varden as a source of food, not only in winter but potentially throughout the year (see Parker and Huryn, 2006). Research on the seasonal diet of American Dippers utilizing perennial springs in the Arctic year-round, specifically on prey preference and availability, could increase our understanding of how diet influences energetic thresholds and affects the decision to overwinter or migrate to more southern overwintering areas. Future surveys to document additional locations where American Dipper overwinter in river systems draining the North Slope region should use the maps provided by Craig and McCart (1974) and Kane et al. (2013) to assist with site selection. Many of the specific locations mentioned by Craig and McCart (1974) are also inhabited by Dolly Varden.
In addition to documenting the northernmost observation of American Dipper in Canada, we characterized openwater habitat of a previously undescribed river system in Ivvavik National Park and provided novel photographic evidence demonstrating that American Dippers prey on juvenile Dolly Varden during winter. Documenting this small yet important habitat feature is relevant for conservation planning in Ivvavik National Park, as Dolly Varden in the western Canadian Arctic was listed as a species of "Special Concern" by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2010 and received the same designation under Canada's Species at Risk Act in 2017. Therefore, describing habitat and identifying specific sources of natural mortality for Dolly Varden are important for an ecosystem-based approach towards conservation. Our observation is an interesting example of an aquatic-terrestrial food-web linkage in an Arctic stream during winter, particularly given that little is known about the ecological relationship between American Dipper and Dolly Varden.   Clough et al., 1987 to Ivvavik National Park and for assisting with habitat sampling. We also acknowledge the assistance provided by Sasha Olekshy (Parks Canada Agency, Inuvik) and Frankie Dillon (Aklavik). Jay Frandsen (Parks Canada Agency, Inuvik) confirmed the identification of the photographed American Dipper. We also thank Christopher Lennie (Canadian Helicopters) for a safe and wonderful flight, Cameron Eckert for commenting on an earlier draft, and Adriana Rivas Ruiz (Fisheries and Oceans Canada) for creating the map.