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Article Excerpt
Study of avian migratory strategies has been of intense interest to ornithologists, especially when bird species exhibit broad variation in seasonal habitat use (Alerstam 1990). Many species of passerines make seasonal altitudinal migrations to high elevation habitats during spring and summer to exploit resources when the climate is mild and productivity is high (Dixon and Gilbert 1964, Rabenold and Rabenold 1985, Hahn et al. 2004, Hollenbeck and Ripple 2007). Birds that use this migratory strategy can take advantage of the early spring bounty available at low elevations, move up in the mountains for the summer to exploit delayed seasonal resources after snow has melted and temperatures increase, and move down in fall before the onset of harsh winter conditions.
The American Dipper (Cinclus mexicanus), is an example of a highly aquatic passerine that commonly migrates upstream from low elevation wintering areas along rivers to higher elevation habitats (Dixon 1943, Packard 1945, Price and Bock 1983, Morrissey et al. 2004, Willson and Hocker 2008). Morrissey et al. (2004) found the majority (79-90%) of a marked American Dipper (hereafter 'dipper') population in British Columbia completed seasonal migrations from a low elevation fiver to completely segregated higher elevation tributaries in spring. Most migrants returned to low elevation rivers during fall prior to the onset of winter. Several studies report migrant dippers readily bred at higher elevation tributaries (Morrissey 2004, Middleton et al. 2006, Willson and Hocker 2008). Studies by Packard (1945) and Price and Bock (1983) in Colorado identified ice formation at higher elevations as the ultimate cause for dippers to exhibit downstream migration in fall.
Characteristic habitat for dippers has been described as fast-moving, clear, unpolluted streams with cascades, riffles, and waterfalls (Price and Bock 1983, Kingery 1996). Dippers dive for a variety of aquatic prey in these habitats, including invertebrates, fish (salmonid eggs and juveniles, cottids), and amphibians (Ehinger 1930, Bakus 1959, Sullivan 1973, Kingery 1996, Morrissey and Olenick 2004). Nest locations vary, but are usually on cliffs, banks, boulders, woody debris, and bridges in direct association with the stream channel (Hann 1950, Morrissey 2004, Loegering and Anthony 2006). Thus, dippers are primarily considered an obligate stream species (Price and Bock 1983, Middleton et al. 2007).
Dippers have been recorded foraging in both low elevation ponds and lakes during winter (Willson and Hocker 2008), and high elevation lakes during summer (Palmer 1927, McEneaney 2002). We predicted lentic waterbodies provide a significant seasonal habitat for migrant dippers. Breeding in California occurs between March and June depending on elevation (Wheelock 1904, Dawson 1923, Beedy and Granholm 1985, Harris 1991) and birds usually fledge by late June in higher elevation regions (Cogswell 1977). We predicted that both adult and juvenile dippers forage at high elevation lakes but that breeding does not occur since lakes are often frozen into late May or early June. No study has assessed the potential importance of these stillwater habitats for migrant dippers.
Our objectives were to: (1) describe the prevalence of American Dippers in lakes and correlate dipper occurrence with lake resources in the Trinity Alps Wilderness, California, USA; (2) describe foraging behavior on prey unique to lentic waters in the region; and (3) summarize published and confirmed observations of dippers using lakes throughout western North America. We focused on characteristics of lakes where dippers were found, frequency of occurrences, duration, and specific foraging behaviors that may indicate which high elevation lakes could be important to this species.
METHODS
Study Area.--The Trinity Alps Wilderness (212,713 ha) is in a glacially characterized portion (Sharp 1960) of the Klamath Mountains of northern California (Fig. 1) with many mountains in this sub-alpine region exceeding 2,300 m. Lakes, ponds, and meadow systems are common aquatic features and are predominantly in mixed conifer to sub-alpine habitats at terminal positions in drainages. High gradient outlet streams of most lentic waterbodies provide aquatic corridors to lower elevation streams. Deep winter snow accumulation in this region causes lentic waterbodies to remain completely frozen from late-November until June during average precipitation years.
We surveyed 16 headwater lakes throughout the eastern half of the wilderness. We chose lakes that had low recreation use, occurred at similar elevations, and were relatively small. Lakes were between 1,896 and 2,210 m in elevation, ranged from 0.3 to 1.98 ha in size, and were between 2.4 and 11.3 m deep. We also conducted a repeated basin-wide survey for dippers to examine high elevation lotic and lentic habitat use. Deep Creek Basin is a medium size (342 ha) glacial cirque in the southeast portion of the wilderness (Fig. 1). Aquatic habitats included a lake, several permanent ponds, a matrix of 12 wet meadows, and 5.8 linear km of perennial streams. Aquatic habitats ranged in elevation from 1,960 to 2,279 m.
Bird Counts.--Each of the 16 lakes was surveyed every 2 weeks between June and September 2003-2006 with six survey periods in summers 2004 and 2005, and five in 2003 and 2006. We conducted 5-min double-observer point counts (Nichols et al. 2000) around each lake during each survey period on two consecutive mornings starting within 15 rain of...
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