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Declines of Greater and Lesser Scaup Populations: Issues, Hypotheses, and Research Directions

Review Of Current Knowledge

Breeding Ecology


Distribution of Studies

Of the 2 scaup species, lesser scaup have received the most intensive study during the breeding season. Whereas lesser scaup often have been the principal focus of studies (Rogers 1962, Trauger 1971, Hammell 1973, Hines 1977, Afton 1983, Austin 1983), mostly in the Prairie-Parkland biome, few studies (Munro 1941) have focused on breeding biology of greater scaup, likely because of their low densities and remote breeding locations. Both species were part of studies of multiple waterfowl species. Austin et al. (1998) reviewed breeding biology information for lesser scaup, including comparisons of nest success and clutch size among grassland, parkland, and boreal forest regions. Most knowledge of breeding biology of lesser scaup came from 2 parkland areas: near Erickson in southeastern Manitoba (Rogers 1962, 1964; Hammell 1973; Afton 1983, 1984, 1985, 1993; Afton and Ankney 1991; Afton and Hier 1991; Austin et al. 1998) and St. Denis National Wildlife Area (NWA) in central Saskatchewan (Dawson and Clark 1996, R. Clark, unpubl. data). Knowledge of lesser scaup breeding ecology in boreal forests is far less extensive, and comes largely from 2 areas in the Northwest Territories (Yellowknife Study Area [YKSA]; and Great Slave Lake; Trauger 1971, Nudds and Cole 1991, Fournier and Hines 1996, M. Fournier and J. Hines, unpubl. data) and from 3 areas in Alaska (Yukon Flats, Yukon- -Kuskokwim Delta, and Copper River Delta; Grand 1995; D. Esler, P. Flint, and B. Grand, unpubl. data). Low breeding densities, access, and other logistics make extensive research in the boreal forest difficult and expensive.

Studies of Basic Breeding Ecology and Recruitment

Prairie-Parkland.--Breeding ecology of lesser scaup has been intensively studied at Erickson during 1958-61 (J. Rogers), 1970-71 (G. Hammell), and 1977-80 (A. Afton). J. Austin studied postbreeding ecology of female lesser scaup there in 1981-82. R. Clark (unpubl. data; Dawson and Clark 1996) studied breeding populations and reproductive biology of lesser scaup at St. Denis NWA, Saskatchewan, during 1982-98. He found no trend in annual median breeding population size (pairs plus lone females) during that period. Timing of nesting (upland nests) did not vary annually, and median initiation dates were similar to that in other parkland areas (Austin et al. 1998). Nest success, estimated in 1980 and during 1989-98, ranged from 3.5% to 28%, and was typically <15%, which is lower than average scaup nest success for the Prairie-Parkland biome (Austin et al. 1998).

Female lesser scaup have a lower reproductive rate than most other ducks, and reproductive success is age-related in both the Prairie-Parkland and Boreal Forest biomes (Trauger 1971; Afton 1983, 1984); this also is likely true for greater scaup. Scaup recruitment in prairie-parkland declines during dry years due to low nest success and increased proportions of nonbreeding 1- and 2-year-old females (Rogers 1964; Afton 1983, 1984).

Boreal forest.--Number and productivity of lesser scaup on the YKSA, a 38-km2 study area in the Northwest Territories, were monitored in 1962-65 (R. Murdy), 1967-70 (D. Trauger), and 1985-98 (M. Fournier and J. Hines). M. Fournier and J. Hines conducted parallel studies of greater and lesser scaup on nearby Great Slave Lake during 1991-98. These studies show significant annual variations in local numbers and reproductive success of scaup. During 1991-98, nest success averaged 37% for lesser scaup at YKSA and 72% for both species at Great Slave Lake (sample sizes there were small). Nesting studies are difficult in this region as it takes >1 person-day to find 1 nest. Egg hatchability was high (76-89% on Greater Slave Lake; 94-97% on YKSA), but hen success, as measured by brood:pair ratios, was highly variable and averaged 22.7% during 1985-98. Long-term pair and brood counts on the YKSA indicate that both declined between 1962-65 and 1985-98, but the brood:pair ratios have not changed. Nudds and Cole (1991) reported similar results when comparing data from the 1960s and 1980s in the YKSA.

In Alaska, B. Grand (Grand 1995, unpubl. data), D. Esler, P. Flint, and T. Fondell (unpubl. data) collected data on nesting biology, nutrient-reserve dynamics, and exposure to lead for greater and lesser scaup during 1990-98 on the Yukon-Kuskokwim Delta, Yukon Flats, and Copper River Delta. Clutch sizes were similar among 3 Alaska study sites, and nest success was moderate to high relative to that of other waterfowl. Scaup were the last ducks to initiate nests at all 3 locations. In the Yukon-Kuskokwim Delta, greater scaup were among the last ducks to begin nesting each year and frequently renested. Clutch volumes of first nests were similar or larger than those of sympatric nesting geese. Nutrients for lesser scaup clutch formation in Alaska are derived from reserves stored before arrival, reserves stored on the breeding area, and foods consumed during egg formation (B. Grand, D. Esler, P. Flint, and T. Fondell, unpubl. data). Nesting scaup rarely ingested lead shot, unlike eiders (Somateria spp.) and oldsquaw (Clangula hyemalis) nesting there (Flint et al. 1997).

Duckling Survival

Duckling survival has been estimated only in Erickson in 1977-80 (Afton 1984) and St. Denis in 1992 (R. Clark, unpubl. data), using multiple counts of broods attended by marked hens. In Erickson, average survival rate during the first 20 days posthatch was 0.675 (SE=0.049, n=39 broods; Afton 1984). In St. Denis NWA, duckling survival rate was 0.38 during the first 48 days post-hatch and was lower for young ducklings (first 15 days posthatch; 0.401) than for older ducklings (16-48 days; 0.955) (n=11 broods; Dawson and Clark 1996). Ducklings from larger eggs had higher survival rates than those from smaller eggs, as did those hatching later in the breeding season (Dawson and Clark 1996). However, preliminary analyses (R. Clark, unpubl. data) suggest that female recruitment (i.e., females entering the breeding population the following spring) is higher for early-hatched ducklings.

Duckling survival has not been studied in boreal forests or tundra. Nudds and Cole (1991) suggested that duckling survival, based on rate of within-year decline in brood sizes, did not change between the 1960s and 1980s in the YKSA.

No duckling or brood survival data exist for greater scaup.

Annual Survival and Philopatry

L. Reynolds (unpubl. data) assessed band recovery and survival rates for lesser scaup. Most data sets provide recovery and survival estimates only for the 1950s and 1960s; there have been insufficient lesser scaup bandings to estimate survival since 1978. Seven reference areas were established by combining degree blocks with similar distributions of recovered lesser scaup: 3 preseason areas (north Alaska, south Alaska, and Alberta), 3 postseason areas (North Atlantic Coast, South Atlantic coast, and Louisiana), and 1 spring-migration area (Michigan). Recovery rates (0.90-5.80%) generally are lower for lesser scaup than for other ducks. Survival rates ranged from 0.427 to 0.866 for females and 0.524 to 0.817 for males, with most estimates falling between 0.57 and 0.71. Reynolds found no effect of very restrictive hunting regulations during the 1961 and 1962 seasons on male survival in 2 reference areas. He cautioned that the preponderance of nonsignificant tests for geographic, temporal, and sex differences in survival should be interpreted cautiously because of large standard errors for many estimates and consequently low statistical power of those tests. He recommended that low recovery rates and poor model fit from past band-recovery data be considered in planning future banding efforts of lesser scaup. Assuming a 1.65% recovery rate for both sexes, 70% male survival, 60% female survival and a coefficient of variation of 0.05, then total bandings needed for a 5-year study are 3,300 males and 4,100 females.

Lesser scaup are highly philopatric to natal and breeding areas (Afton, pers. comm. in Johnson and Grier 1988), and likely this is true of greater scaup. R. Clark, A. Afton, and J. Rotella (unpubl. data) analyzed mark-recapture data for lesser scaup marked at Erickson (1977-81) and St. Denis NWA (1989-98) using Program MARK to estimate philopatry and annual survival of lesser scaup females. At Erickson, models indicated that all surviving adult females returned the following year because resighting estimates were 1.0, and annual survival rate estimates were as high or greater than those derived from independent analyses (0.46-0.57) based on banded birds (above). Annual survival of marked females averaged 0.56 for yearlings and 0.57 for adult females, implying that natal philopatry also was strong. On St. Denis NWA, annual survival of adult females was ~0.68, with much annual variation, and breeding philopatry appeared virtually complete.

In Alaska, nest site fidelity and annual survival estimated from recapture of leg-banded greater scaup females was high (>70%) (B. Grand, P. Flint, D. Esler, and T. Fondell, unpubl. data).

Long-term declines in reproductive success or survival, combined with strong philopatry of breeding females, may have contributed to long-term population declines in some areas. It is unclear to what extent reproductive success and local recruitment are limited by factors on breeding areas (e.g., food resources, nesting habitat, nest or hen success, duckling survival), or on migration and wintering areas (e.g., contaminants). If scaup from a given breeding area share migration and wintering areas, limiting factors such as contaminants, food resources, or harvest pressure may be more important in long-term population trends for a breeding area than if a population has diverse migration and wintering areas. This highlights the need for better data on affiliation of scaup among breeding, migration, and wintering areas.

Habitat

Amphipods and pond characteristics in the Parkland.--Amphipods, primarily Hyalella and Gammarus spp., are a primary food of migrating and breeding scaup and for ducklings (reviewed by Austin et al. 1998). Hunters and biologists have long noted that scaup usually are found on wetlands with amphipods, but this relationship has never been quantified. D. Lindeman and R. Clark examined relationships among amphipod abundance, pond morphometry, and lesser scaup numbers in 12 sites in southern Saskatchewan (Lindeman and Clark in review). Because scaup frequently nest within wetland margins and are the only diving duck that nests in uplands (Austin et al. 1998), Lindeman and Clark also examined the role of wetland margins and upland habitats. Use of wetlands by breeding scaup was positively correlated with presence of amphipods, pond area, and absence of wetland margin impacts. Abundance of Hyalella, the more common of 2 main amphipod species in all but ephemeral wetlands, often was a key factor affecting abundance of breeding scaup and occurrence of ducklings. Because of the dominance of amphipods in scaup diets, particularly of ducklings, factors affecting amphipod abundance and relationships between amphipods and scaup also need to be examined in boreal forests.

Climate changes.--Productivity of lesser scaup in prairie-parklands is reduced during drought, largely due to low nest success and nonbreeding by younger females (Rogers 1964; Afton 1983, 1984). Afton and Anderson (in review) hypothesize that prairie drought may affect scaup breeding in boreal forest by reducing availability of food during spring migration and thus acquisition of nutrient reserves for breeding. If so, the recent long-term drought in the Prairie-Parkland biome may have contributed to the decline in the continental population. Examination of nutrient-reserve dynamics of lesser scaup breeding in Erickson, Manitoba, indicated that lipid reserves limit clutch size in lesser scaup (Afton and Ankney 1991). Afton and Ankney (1991) also concluded that food availability may limit lipid storage by breeding females and be an important factor affecting clutch size in lesser scaup. These conclusions were supported by an Alaskan study of lesser scaup nutrient-reserve dynamics (see above). Thus, changes in food resources on migration and breeding areas could reduce reproductive success. New research linking feeding ecology, nutrient dynamics, and breeding is needed to examine cross-seasonal effects, and to assess impacts of habitat alteration, including human- and climatic-driven changes.

Recent and anticipated long-term climate changes in the parkland and boreal forest also may affect scaup. Analyses by Larson (1994) and Sorenson et al. (1999) indicate that wetlands in the parklands are vulnerable to increased temperatures that are predicted in global climate models; wetland numbers and depths likely would decline with increased temperatures. The Boreal Forest ecosystem is expected to experience the greatest warming from global climate change (Environment Canada 1995, Rouse et al. 1997). Boreal areas of northwestern Canada already have experienced temperature increases of 2.3-2.4°C over the past 51 years (Environment Canada 1997). Given predictions of significant changes in hydrology, temperature, and precipitation patterns, effects on both biomes may be serious. Changes in wetland habitats and food resources used by scaup, due to climate change, could affect the continental scaup population. However, the magnitude and direction of changes of habitat or food resources in the Boreal Forest ecosystem, and the long-term effects to scaup populations, are unknown. Because amphipods are a key food of scaup, they may be a significant link in the relationship between scaup and climate change. To better understand the sensitivity and response of boreal wetlands to climate change, and to understand whether climate change will positively or negatively affect scaup, we need (1) more detailed examination of existing data, and (2) models that relate climate change to wetland characteristics and associated resources.

MacCluskie et al. are examining BGS data relative to climate data to assess correlations between past changes in temperature or precipitation on scaup populations in boreal forest strata.


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