Scientific Investigations Report 2007–5278
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5278
Glacier Bay is an important breeding area for several species of ground-nesting marine birds in southeastern Alaska. This section discusses the relevance of distribution, habitat, and behavior, and the potential for human disturbance at nest sites for common and notable species of ground-nesting birds. It also presents a summary of findings on the areas of concern, wildlife closures, and historical native egg collection sites. Finally, it discusses data limitations and management applications associated with this study.
The majority of Arctic Tern nesting concentrations were found in areas open to overnight visitation within the upper arms of Glacier Bay. The preferred nesting habitat for Arctic Terns—cobble outwash areas and rocky outcrops on small islands (Hatch, 2002)—is found mostly up-bay in early successional habitats. Nelson and Lehnhausen (1983) noted that Arctic Terns were more common in Glacier Bay than elsewhere in southeastern Alaska, and their survey found nesting terns in the upper bay at six of nine sites. The aggressive behavior exhibited by nesting Arctic Terns when humans approach may make their nesting areas less vulnerable to human disturbance; however, individual Arctic Tern nests are inconspicuous and therefore are more likely to be trampled. On several occasions, Arctic Terns were observed eating and holding small schooling fish, such as capelin (Mallotus villosus) and pink salmon (Oncorhynchus gorbuscha), and feeding these fish to their young. On several occasions, hundreds of Arctic Terns were observed feeding nearshore close to colony sites on glacial outwashes. The colony sites in Glacier Bay may be important to Arctic Terns because of the availability of nesting habitat and the relative proximity to surface schooling fish (Arimitsu and others, 2007). However, Arctic Tern nest counts were variable among years and within years at sites that were revisited in this study (table 4 and fig. 16) and the counts represent only a snapshot in time. A better understanding of factors influencing the spatial and temporal variability in Arctic Tern nesting requirements would improve assessment of visitor disturbance risks.
Black Oystercatcher pairs were found nesting throughout Glacier Bay in areas within wildlife closures and areas open to public visitation. They prefer to nest on gravel beaches with low-sloping substrates and usually nest near the high-tide line (Andres and Falxa, 1995), and these areas may be more readily available in Glacier Bay owing to newly emergent reefs formed by glacial rebound (Lentfer and Meier, 1995). Oystercatcher nests may be particularly susceptible to disturbance because kayakers tend to use the same beaches for camping. Nests are made of gravel and may be trampled when boats are hauled above the high-tide line.
Glacier Bay appears to be an important range-wide breeding and post-breeding area for Black Oystercatchers. Nelson and Lehnhausen (1983) found unusually high concentrations of Black Oystercatchers in Glacier Bay compared to the rest of the species’ range, including a count of 67 adults at the closed area at Flapjack Reef. A non-breeding aggregation with 43 Black Oystercatchers was observed at the same reef in June 2003 during this study. Large aggregations of Black Oystercatchers during the post-breeding season, including one sighting of an estimated 600 adult and hatch-year birds in September 1992 (Gus van Vliet, State of Alaska Department of Environmental Conservation, oral commun., 2003), have been documented in Geikie Inlet and date back at least to the 1960s (Wik, 1967). The proximity to mammalian predator-free roosting sites at the closed Geikie Rock and Lone Island, location within relatively protected waters, and the freshwater runoff from small streams (which may enhance intertidal productivity, or increase bathing opportunity for post-breeding plumage maintenance) may make this area particularly suitable for these birds (Gus van Vliet, State of Alaska Department of Environmental Conservation, written commun., 2004).
With the exception of a small number of nests in Berg and Fingers Bays, Mew Gull nests were found only in the upper reaches of Glacier Bay during this study. Mew Gulls prefer nesting in areas with little or no vegetation and on gravel banks or beaches, often near freshwater streams (Moskoff and Bevier, 2002). Nelson and Lehnhausen (1983) found that Mew Gulls made up about 2.9 percent of the birds in Glacier Bay and documented five colonies at the islet at Shag Cove, Hugh Miller Complex, White Thunder Mountain, the head of Rendu Inlet, and the islet east of Russell Island. During this study, nesting activity was found at all those sites except the islet at Shag Cove, which was overgrown with vegetation, and White Thunder Mountain. Mew Gull chicks may be susceptible to human disturbance because they usually entered the water when humans approached them during this study. This behavior might make them more accessible to predators, such as Bald Eagles.
Although Glaucous-winged and Herring Gulls are known to use a wide variety of habitat types for nesting (Verbeek, 1993), in Glacier Bay they were found nesting mostly in flat areas with low vegetation or on rocky cliffs. They are conspicuous birds who place their eggs in large nest bowls. Therefore, trampling of these nests by humans is less likely than for other ground-nesting bird species.
The occurrence of breeding Herring Gulls in Glacier Bay was documented by Wik (1968) and Patten (1974). Further accounts of Herring Gulls are lacking, which may be due to the difficulty of distinguishing between the relatively rare Herring Gull and the rather ubiquitous Glaucous-winged Gull (Wik, 1968). Hybridization between these species occurs in Glacier Bay (Patten and Weisbrod, 1974) and is common in other areas where their breeding ranges overlap (Grant, 1987).
Glacier Bay represents the southeastern edge of the breeding range for Aleutian Terns (North, 1997). Aleutian Terns nest very rarely in Glacier Bay; low concentrations of nesting birds have been observed on Sealers Island, North Marble Island, and Johns Hopkins Inlet in the past (Bruce Paige, retired, National Park Service, oral commun., 2005). Breeding Aleutian Terns are highly sensitive to human disturbance and may seasonally or permanently abandon colonies when disturbed (Haney and others, 1991; North, 1997). However, they may benefit from nesting in mixed colonies with Arctic Terns, who generally are more aggressive in their nest defense (Hanley and others, 1991).
Although nesting bird concentrations generally were low in most areas open to visitor use, the potential for disturbance to nesting birds exists at several high visitor use areas, including the north spit at McBride Glacier, the west entrance to Reid Inlet, and Sealers Island. These areas have historically supported nesting colonies of Arctic Terns and other bird species (Wik, 1968; Greg Streveler, retired, National Park Service, oral commun., 2003; Bruce Paige, retired, National Park Service, oral commun., 2005). Owing to their close proximity to tidewater glaciers, they are among the more heavily used areas in the bay (National Park Service, unpub. data, 2004). There also was concentrated nesting activity in several low visitor use areas including the north shore of Muir Inlet, the Adams Glacier outwash, the moraine of the Grand Pacific Glacier, an unnamed islet northwest of Eider Island, the islet at the entrance to Scidmore Bay, and the islet at Tlingit Point. These areas represent the largest unprotected nesting concentrations in the bay.
Arctic Tern nesting concentrations were highly variable at several areas of concern. Terns are known to relocate colony sites in response to disturbance (Roby and others, 2002). In a study of the related Caspian Tern (Sterna caspia), Cuthbert (1988) found that the majority of terns with failed nests deserted the colony, and 39 percent of those renested at a different location. Although the number of Arctic Tern nests and adults decreased at Sealers Island, numbers of nests at McBride Glacier and the islet at Tlingit Point increased over the course of the 2005 breeding season (fig. 16). Given the proximity of McBride and the islet at Tlingit Point to this colony, the possibility that the increase in nesting activity at adjacent sites was a result of nest failure at Sealers Island in 2005 cannot be ruled out.
Nesting activity was documented at all wildlife closures surveyed. Geikie Rock, Boulder, Flapjack, and Lone Islands are some of the more important Glaucous-winged Gull nesting areas in the bay. The four unnamed islands east of Russell Island are small islands that have evidence of concentrated nesting activity. In addition, camping potential is relatively poor on these islands. Leland Islands, including the southern portion, are important for nesting and roosting birds.
Although our census of South Marble Island was conducted with considerably less effort than the 1999 census (1 day compared to a summer’s worth of effort in 1999), a comparison of count data suggests that Glaucous-winged Gull and Pelagic Cormorant (Phalacrocorax pelagicus) populations have not changed and that the Black-legged Kittiwake population has increased since 1999 (table 12, at back of report; Zador and Piatt, 1999). A total of 285 active Glaucous-winged Gull nests were counted in mid-June 2005, and 200 nests containing eggs and another 90 nests with incubating adults were found in 2005 (tables 5 and 12). This suggests that the number of nesting Glaucous-winged Gulls has remained stable since 1999, although the maximum count of adults was higher in 2005 than was previously documented (table 12). We also found that the number of Pelagic Cormorants roosting on the island was comparable, with 201 and 195 individuals in 1999 and 2005, respectively. Although the number of Black-legged Kittiwakes in incubation posture remained about the same (Zador and Piatt, 1999), the number of adults observed on the island increased from 159 in 1999 to 647 in 2005. It was not possible to examine trends for other species because the counts were too small (for example, Common Murres, Uria aalge, and Tufted Puffins) or were potentially affected by the time of observation (for example Pigeon Guillemots).
Several traditional egg-collection sites no longer support Glaucous-winged Gull breeding populations, and this may be due to ecological succession following deglaciation. For example, Triangle Island (at the head of Queen Inlet) was once a treeless islet that supported a colony of several species of gull (Wik, 1968), but during our survey of the area, the island was overgrown with vegetation and surrounded by loose gravel. North Marble Island, which had an estimated 300 Glaucous-winged Gull nests in 1982 (Nelson and Lehnhausen, 1983), was also among the sites with fewer than 10 Glaucous-winged Gull nests in 2005 (table 13, at back of report). However, it is unknown whether succession played a role in the abandonment of the North Marble Island colony.
Zador and others (2006) found that a controlled harvest at South Marble Island restricted to the early incubation period would not impact the population size. Although the gull population at South Marble Island in 2003–05 had not changed since the study conducted in 1999–2000 (table 12), continued monitoring of the breeding population would be appropriate if harvesting is to occur.
Although it was not possible to determine how many nests were missed in the surveys, it is likely that most nesting birds were accounted for by recording territorial birds, especially for conspicuous species like Arctic Terns, Black Oystercatchers, Mew Gulls, and Glaucous-winged Gulls. The surveys attempted to locate nests during the incubation period in order to minimize disturbance and because chicks are mobile, inconspicuous, and therefore harder to count later in the season. In any case, the location and numbers of nests reported here must be considered as minimum estimates for Glacier Bay because of the variation in timing and detectability of different species. Data collected during this study provide a snapshot in time and are limited by seasonality, nest detectability, and differing levels of effort each year. Data from sites that were visited only once may underrepresent the actual nesting counts if they were collected too early or too late in the season. Variability in nest counts among years may be due to timing of surveys or to natural variability within the season and among years. An example of the latter is demonstrated from data collected at sites that were visited each year and more than once within a breeding season (see section, “Sealers Island”). The surveys were focused on the more abundant and conspicuous species to maximize nest detectability, and the distribution and abundance of the more cryptic nesters were undoubtedly underestimated. In addition, during the pilot year of the study (2003), the surveys were intended to focus only on high visitor use areas with the potential for disturbance. Because nesting counts overall were very low in these areas, comprehensive searches were conducted for nesting activities throughout the study area during the rest of the study. This shift in survey focus makes it harder to compare nesting densities among years.
Most of the largest seabird nesting areas in Glacier Bay are already closed to human use and therefore largely protected from disturbance by park visitors. In this study, surveys found additional concentrations of ground-nesting birds along shorelines that currently have high visitor use, and human disturbance could affect these breeding activities (fig. 13). Concentrations of ground-nesting birds also were found along shorelines that currently have low visitor use. Closing both types of concentration areas to human activities could reduce the potential for disruption of nesting. On the basis of our general understanding of breeding for these species, closures during incubation and chick rearing periods would likely be most effective. Further study on nest timing to identify key closure dates would facilitate use of this management and provide the most protection of nesting activities.
Many of the nest/territorial pairs found were solitary and dispersed along Glacier Bay shorelines. Closing all shorelines that have the potential to support nesting birds would be difficult. Perhaps the simplest way to minimize disturbance to nesting birds broadly in Glacier Bay is to educate visitors about where they may encounter nesting birds, nest identification (see fig. 17), nesting bird behavior, and appropriate responses (such as moving elsewhere) to encroachment upon nest sites or nesting behavior.
The baseline data gathered on the distribution and abundance of ground-nesting birds in the park can not only inform science-based visitor management but also can be used for monitoring changes in breeding-bird abundance and distribution over time, whether those changes are due to human disturbance or natural factors (such as climate change or landscape succession).
U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubs.usgs.gov/sir/2007/5278
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