Sensitivity of Black Turnstone to Coastal Threats Anne L. Schaefer
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Sensitivity of Black Turnstone to Coastal Threats Anne L. Schaefer Prince William Sound Science Center 1. Background The Black Turnstone (Arenaria melanocephala) is a stocky, black and white shorebird species strongly associated with the rocky shorelines of western North America (Fig. 1). The most recent global population estimate for Black Turnstone is approximately 95,000 (Handel and Gill 1992). This species breeds only within Alaska (Handel and Gill 1992). During the nonbreeding season, Black Turnstones are bound to the rocky intertidal habitats that extend from southern Alaska to Mexico (Handel and Gill 2001; Fig. 2). As a consequence of its year-long dependence on low-lying coastal habitats, the Black Turnstone is particularly vulnerable to habitat loss or degradation resulting from coastal development, pollution, and climate change. Due to these threats and its relatively small population size, the U.S. and Alaska Shorebird Conservation Plans both list the Black Turnstone as a "Species of High Concern" (Brown et al. 2001, USFWS 2004, Alaska Shorebird Group 2008). This species also was included on the 2010 Audubon Alaska WatchList (Kirchhoff and Padula 2010), which highlights the avian species in Alaska most in need of immediate conservation effort. The Black Turnstone is a coastal obligate throughout all stages of the year (Fig. 2) and demonstrates high site-fidelity on both the breeding and wintering grounds (Handel and Gill 2001). During the breeding season, this species nests exclusively within the coastal grasslands of western Alaska, with 85% of the population concentrated within the central Yukon-Kuskokwim Delta. In this area, individuals nest in the highest densities (1.11 ± 0.16 birds/ha) in low-lying salt grass meadows and in sparsely vegetated habitats that are within 2 km of the coast (Handel and Gill 1992). Turnstones also nest in graminoid and dwarf shrub meadows in intermediate densities and in dwarf shrub mat tundra in low densities (0.04 ± 0.04 birds/ha; Handel and Gill 1992). During winter, the Black Turnstone is found along the entire Pacific coast of North America. Fine-scale distribution within this broader range is determined by the presence of their preferred coastal rocky habitat, although turnstones will also use sandy coastal beaches where algae and detritus concentrate onshore (Connors 1977, Handel and Gill 2001). Black Turnstones are found on human-created features, such as jetties, piers, and riprap, as well (Connors 1977, Handel and Gill 2001). The diet of the Black Turnstone is diverse and shifts seasonally, presumably in response to spatial and temporal changes in the relative availability of forage throughout the year. On the breeding grounds, Black Turnstones forage extensively over coastal sedge meadows and intertidal mudflats for small aquatic and terrestrial invertebrates, seeds, eggs of small nesting birds, and carrion (Handel and Gill 2001). Outside of the breeding season, turnstones commonly forage in the wave splash zone of open rocky shores (Connors 1977) where they feed on marine- based prey, such as small mobile crustaceans, hard-shelled invertebrates, mussels, small barnacles, herring roe, snails, worms, and limpets (Connors 1977, Bishop and Green 2001, Handel and Gill 2001). 2. Migratory Connectivity Information regarding the migratory patterns of Black Turnstone remains limited. Data suggest that this species is a short- to intermediate-distance migrant, flying in flocks of a just a few individuals up to several hundred. Black Turnstones depart the breeding grounds in western Alaska during early- to mid-summer depending on breeding success (Gill et al. 1983, Handel 2002, Bishop and Taylor 2015). Turnstones then move rapidly to its wintering areas along the North American Pacific coast, arriving in late July or early August. On average, the southbound migration lasted 5.3 days (range = 0 –15 days) for 23 turnstones fitted with geolocators in 2011 and 2013 (Bishop and Taylor 2015). In the spring, turnstones depart for the breeding grounds in late March or early April and arrive in early May (Connors 1977, Handel 2002, Bishop and Taylor 2015). The spring migration lasted 10.9 days on average (range = 0 – 23 days; Bishop and Taylor 2015), almost double that of fall migration. While migrating, turnstones appear to hop along the coast, typically remaining at each stop for 1-5 days (mean = 3.9, range = 1.5 – 15). Previous survey efforts identified northern Montague Island in Prince William Sound, Alaska as an important staging area for migrating turnstones for a 3-week period during spring (Norton et al. 1990, Bishop and Green 2001). Turnstones begin arriving to Montague Island during the fourth week of April, and their numbers peak during the first week of May. In this area, Black Turnstones forage most frequently at or just above the tideline (Bishop and Green 2001). Pacific herring (Clupea palassii) eggs attached to substrates, such as kelp, dominate their diet during this period (found in 69-75% of gut analyses; Martin 1993, Bishop and Green 2001). Migrating turnstones also forage on Bay mussels (Mytilus trossulus), Balanas sp., amphipods, gastropods, and insects (Norton et al. 1990, Martin 1993, Bishop and Green 2001). Pacific herring eggs, in particular, appear to be an important and energy-rich source of food for turnstones prior to the breeding season, enabling them to accumulate enough energy stores to complete migration. Moreover, arrival on the breeding grounds with greater fat reserves may enable individuals to better withstand uncertain feeding conditions or adverse weather events. In May of 1989, ~10,000 turnstones, in addition to thousands of Surfbirds (Aphriza virgata) and tens of thousands of Glaucous-winged Gulls (Larus glaucenscens), were observed feeding on herring roe in bays on northern Montague Island (Rocky Bay, Stockdale Harbor, and Port Chalmers; Norton et al. 1990). Similar abundances were again recorded in 1990 (Norton et al. 1990) and during surveys conducted from 1994–1997 (Bishop 2011). Because of its apparent importance as a spring stopover location for a substantial proportion of the global Black Turnstone population, the National Audubon Society and Birdlife International designated Montague Island as an Important Bird Area in 2006. More recent surveys have documented a decline in the use of northern Montague Island by turnstones during the spring migration. In 2010, only a total of 3,535 Black Turnstones were recorded over a 19-day period and 2,504 km of survey effort (Bishop 2011). In 2015, 872 turnstones were recorded during 9 days of surveys with 676 km of survey effort (Bishop, unpublished data). It remains unclear if these results reflect a true population decline or a shift in the migratory route of this species. To better understand the importance of Montague Island as a spring staging area for migrating Black Turnstones, light-level geolocators were deployed on 30 turnstones at their breeding grounds on the Yukon-Kuskokwim Delta, on 5 birds at Cape Krusenstern in northwestern Alaska (a breeding site) and on 7 birds wintering near Oak Harbor, Washington (Bishop and Taylor 2015). Twenty-two individuals were recaptured in the subsequent year and one individual after two years. Of the 51 total stopovers detected during migration (Fig. 3), only five were recorded in the vicinity of Prince William Sound (Fig. 4). These findings suggest that Montague Island is not an obligate stopover location for Black Turnstones during migration and that turnstones may be plastic in their migratory routes. Despite the reduced use of Prince William Sound during migration, the geolocator data identified two other potentially important stopover regions for turnstones: coastal southeastern Alaska/western British Columbia and Lower Cook Inlet (Fig. 3). Both areas were frequented by turnstones consistently during both spring and fall. In southeastern Alaska alone, Black Turnstones made 12 (of 31) and 5 (of 20) migratory stops. Interestingly, 14 of the 23 tagged birds also overwintered in southeastern Alaska/western British Columbia (n = 9 in southeastern Alaska, n = 5 in western British Columbia), although wintering areas appear to be located farther inland than stopover locations (Fig. 3). 3. Threats Climate change Due to its year-long dependence on low-lying, coastal habitats, the Black Turnstone is particularly susceptible to the impacts of climate change, including habitat loss and degradation resulting from sea level rise and salt water intrusion into freshwater habitats. Further, the Black Turnstone is vulnerable to climate-mediated changes in weather, such as increased frequencies and intensities of coastal storms. Black Turnstones may be even more sensitive to the impacts of climate change because they breed at high latitudes, where the effects of climate change are expected to be most pronounced (IPCC 2007). Over the past century, global sea levels have risen 0.10–0.25 m primarily due to thermal expansion of the ocean and the input of water from melting ice (Hopkinson et al. 2008, Rahmstorf et al. 2007). Climate models predict that sea level will continue to increase approximately 0.30–1.00 m over the 21st century (Meehl et al. 2007). On the Yukon-Kuskokwim Delta breeding grounds, it is predicted that a sea level increase of 0.50 m would increase the frequency of overbank flooding from 1-4 times/year to a monthly occurrence (Terenzi et al. 2014). Increased overbank flooding combined with rising seas would lead to enhanced coastal erosion, increased sedimentation and salinization of freshwater wetland habitats, and flooding of turnstone nests and existing turnstone habitat. Results from study areas where both habitat change and shorebird abundance were monitored concurrently indicated that shorebirds do not compensate reductions in foraging habitat by aggregating in denser groups. Instead, less total available habitat led to declines in the number of shorebirds using the habitat (Galbraith et al. 2002, Meire 1991, Goss-Custard and Moser 1988). Therefore, decreased quantity and quality of available habitat resulting from rising seas may force some turnstones into new or less optimal areas.