Eric M. Anderson

Questionable multivariate statistical inference in wildlife habitat and community studies (a reply)

We analyzed a data set constructed from functionally unrelated, easily collected observations (e.g., meat, stock, and liquor prices) around Fort Collins, Colorado, using principal components analysis (PCA), canonical correlation analysis (CC), and discriminant function analysis (DFA). Each produced seemingly significant results and suggested strong relationships between the variables measured. We suggest that multivariate techniques can provide invalid inferences when used with data containing no relationships. We question the use of these techniques in studies of wildlife habitat.

Citizen Science Reveals an Extensive Shift in the Winter Distribution of Migratory Western Grebes

Marine waterbirds have shown variable trends in abundance over the past four decades with some species displaying steep declines along the Pacific coast from British Columbia through California. One of the most dramatic changes has been that of western grebes (Aechmophorus occidentalis) in the Salish Sea. This region was a former core of the species wintering distribution but they have become increasingly rare prompting calls for conservation action. A more thorough understanding of this situation requires the analysis of trends at broader geographic scales as well as a consideration of mechanisms that might have led to a change in abundance. We used hierarchical modeling with a Bayesian framework applied to 36 years of Audubon Christmas Bird Count data to assess continent-wide and regional population trends in western and Clark’s grebes (A. clarkii) from 1975 to 2010. Our results show that the North American wintering population of Aechmophorus grebes decreased by ∼52% after 1975, but also that western grebes displayed strongly opposing regional patterns. Abundance decreased by about 95% over 36 years in the Salish Sea but increased by over 300% along coastal California. As a result, the mean centre of the species distribution shifted south by an estimated 895 km between 1980 and 2010. Mechanisms underlying this shift require further study but we hypothesize that it may be related to a change in the abundance and availability of their forage fish prey base. Since the mid-1980s, the Pacific sardine stock off the California coast increased from a few thousand metric tonnes to over two million. At the same time both the abundance and availability of Pacific herring declined in the Salish Sea. Studies are needed to examine this hypothesis further and additional consideration should be directed at other changes in the marine environment that may have contributed to a range shift.

Reevaluating Marine Diets of Surf and White-Winged Scoters: Interspecific Differences and the Importance of Soft-Bodied Prey

Abstract Past studies of the marine diets of Surf Scoters (Melanitta perspicillata) and White-winged Scoters (M. fusca) have reported that they eat mostly bivalves, but deviations from well-established methods by most studies suggest that the importance of soft-bodied prey has been underestimated for both species. Methods needed to reduce bias in diet estimates include collecting birds that are feeding, immediately preserving gut contents, excluding gizzard contents, averaging food-item percentages across birds versus pooling gut contents for all birds, and using energy or ash-free dry mass versus wet mass values of foods. For Surf Scoters collected in northern Puget Sound, Washington during 2005–2006, adherence to the latter three methods alone resulted in the assessed bivalve component of diet declining by over half and a near doubling of soft-bodied prey (i.e., crustaceans, polychaetes). Diets of Surf Scoters differed among three heavily used bays with distinct benthic habitats, yet 67%–86% of the ash-free dry mass of esophagus contents from each bay was nonbivalve prey. A synthesis of previous and new diet data revealed differences between scoter species: relative to White-winged Scoters, Surf Scoters consume smaller bivalves, a smaller and more variable percentage of mollusk prey (including bivalves and gastropods), and a declining percentage of bivalves as winter progresses. Past diet studies for scoters may provide misleading guidelines to conservation efforts by implying that only standing stocks of bivalves require consideration when prioritizing critical foraging sites.

Variable wind, pack ice, and prey dispersion affect the long‐term adequacy of protected areas for an Arctic sea duck

With changing climate, delineation of protected areas for sensitive species must account for long-term variability and geographic shifts of key habitat elements. Projecting the future adequacy of protected areas requires knowing major factors that drive such changes, and how readily the animals adjust to altered resources. In the Arctic, the viability of habitats for marine birds and mammals often depends on sea ice to dissipate storm waves and provide platforms for resting. However, some wind conditions (including weak winds during extreme cold) can consolidate pack ice into cover so dense that air-breathing divers are excluded from the better feeding areas. Spectacled Eiders (Somateria fischeri) winter among leads (openings) in pack ice in areas where densities of their bivalve prey are quite high. During winter 2009, however, prevailing winds created a large region of continuous ice with inadequate leads to allow access to areas of dense preferred prey. Stable isotope and fatty acid biomarkers indicated that, under these conditions, the eiders did not diversify their diet to include abundant non-bivalve taxa but did add a smaller, less preferred, bivalve species. Consistent with a computer model of eider energy balance, the body fat of adult eiders in 2009 was 33-35% lower than on the same date (19 March) in 2001 when ice conditions allowed access to higher bivalve densities. Ice cover data suggest that the eiders were mostly excluded from areas of high bivalve density from January to March in about 30% of 14 winters from 1998 to 2011. Thus, even without change in total extent of ice, shifts in prevailing winds can alter the areal density of ice to reduce access to important habitats. Because changes in wind-driven currents can also rearrange the dispersion of prey, the potential for altered wind patterns should be an important concern in projecting effects of climate change on the adequacy of marine protected areas for diving endotherms in the Arctic.

Bobcat Diurnal Loafing Sites in Southeastern Colorado

I measured 11 habitat variables at bobcat (Felis rufus) diurnal loafing sites (n=22) and compared them to random sites (n=22) in southeastern Colorado during 1983-85. Univariate analysis of variables indicated bobcats selected steep-sloped, rocky areas with dense vertical cover and little herbaceous ground cover. Dense vertical cover (>70%) appeared to be the most significant feature of loafing sites, permitting correct classification of 88% of sites

Contrasts in Energy Status and Marine Foraging Strategies of White-Winged Scoters (Melanitta fusca) and Surf Scoters (M. Perspicillata)

ABSTRACT. White-winged Scoters (Melanitta fusca) and Surf Scoters (M. perspicillata) are often assumed to rely on similar marine resources. To evaluate the accuracy of this assumption, we contrast seasonal distributions, foraging effort, and indicators of energy status (body mass and composition, plasma metabolites) in three major foraging sites in Puget Sound, Washington, for these rapidly declining sea duck congeners. For Surf Scoters, distributions and energy status indicated that a mussel-dominated site was relatively important in early winter, but that importance shifted during late winter and spring to seagrass sites that provided either herring spawn or epifaunal invertebrates. As winter progressed, movements among foraging sites and increased foraging effort by Surf Scoters were accompanied by greater variability in their energy status compared with White-winged Scoters; body mass declined over winter by >9% in about one-third of past studies for Surf Scoters, well above the range of losses observed in White-winged Scoters. For White-winged Scoters, lower variability in energy status, foraging effort, and distributions throughout winter suggests that they are better able to regulate energy balance regardless of changing foraging conditions. Greater resistance to seasonal environmental changes in White-winged Scoters may be related to their >50% larger body size, which confers lower mass-specific energy costs and access to a wider size range of bivalve prey. Perhaps because of their greater sensitivity to winter foraging conditions, Surf Scoters appear to rely on a broader range of foraging sites than White-winged Scoters.

Variation in Physiology and Energy Management of Wintering White-Winged Scoters in Relation to Local Habitat Conditions

Abstract. Along the Pacific coast of North America, White-winged Scoters (Melanitta fusca) winter in sites that vary widely in habitat conditions. This habitat variation likely alters the relative costs and benefits of using specific sites in terms of factors such as foraging conditions, degree of predation danger, and thermoregulatory costs. To assess how White-winged Scoters adjust their physiology and energy management in response to variation in habitat conditions, we contrasted overwinter dynamics in several physiological indicators across five sites in British Columbia and Washington. We tested the relative support for various hypotheses that considered exposure to wind and waves, water depth, predation danger, diet composition, and collection period as possible underlying causes of variation in physiological indicators. Total body mass and lipid mass were highest at an exposed offshore site, which may reflect an adaptive response to buffer against unpredictable foraging conditions and increased thermoregulatory costs. At nearshore sites where exposure was lower, scoters maintained lower lipid masses throughout the winter. Total lipid mass declined through the winter in all sites, a result consistent with findings for many waterfowl species. However, levels of plasma metabolites (triglycerides, &bgr;-hydroxybutyrate) varied little by site, suggesting that, irrespective of body composition, birds at all sites maintained physiological homeostasis. Digestive morphology was strongly related to diet, with smaller gizzards and longer intestines associated with a greater fraction of soft-bodied foods in the diet. Our results suggest that the physiology and energy management of wintering White-winged Scoters is related to local habitat conditions.

Baseline Corticosterone in Wintering Marine Birds: Methodological Considerations and Ecological Patterns

Previous studies have related levels of plasma corticosterone (CORT) of seabirds to variation in foraging conditions during the breeding period, but it is unclear whether similar relationships between foraging conditions and baseline CORT exist during other life stages. We validated methods for identifying baseline CORT of lethally sampled birds and assessed variation in baseline CORT relative to winter habitat conditions. We collected free-living white-winged scoters (Melanitta fusca) at four wintering sites during December and February. We found increasing CORT values beyond 3 min after time since flush (the duration between initial flush and death), presumably reflecting acute stress responses. Our results demonstrate that it is possible to obtain baseline CORT from lethally sampled birds if the time from initial flush until death is measured. Our study sites varied appreciably in exposure to wind and waves, predation danger, diving depths, and the fraction of preferred foods in scoter diets. Despite these habitat differences, baseline CORT did not vary across sites or winter periods. We interpret this lack of variation as evidence that birds select wintering areas where they can successfully manage site-specific costs and maintain physiological homeostasis.

Geographic and Temporal Variation in Diet of Wintering White-winged Scoters

Abstract. Quantifying variation in diet over time and space is important for understanding patterns of habitat use in marine birds. Diet composition of adult male White-winged Scoters (Melanitta fusca) was quantified at five study sites in coastal British Columbia and Washington during mid-winter (December) and late winter (February-— March). At four sites where White-winged Scoters fed in nearshore areas, diet varied little between winter periods and birds fed almost exclusively on large infaunal bivalves (≥85% of mean ash-free dry mass of esophagus contents for each season × site combination). The main prey of White-winged Scoters in intertidal foraging areas (N = 3 of 5 study sites) were Varnish clams (Nuttalia obscurata), which were introduced to the region within the last 25 years. At an offshore site, diet consisted mainly of bivalves except during one period when White-winged Scoters had consumed mainly fish, crustaceans, polychaetes and echinoderms. Greater temporal variation in diet at the offshore site may have been an effect of reduced time available to locate preferred prey items and lower predictability of prey distributions owing to this sites greater exposure to wind and waves. However, neither exposure nor water depth received appreciable support in models of the dietary fraction of bivalves across sites and periods. Our results underscore the importance of marine areas with high densities of infaunal bivalves to White-winged Scoters, but also show that White-winged Scoters exhibit flexibility to adjust diet in response to differences in prey composition across habitats.