Breeding ecology of sympatric greater and lesser scaup (Aythya marila and Aythya affinis) in the Subarctic Northwest territories.
James E. Hines (1)
ABSTRACT. We studied the breeding ecology of greater and lesser scaup on islands of the North Arm of Great Slave Lake, Northwest Territories, and on the nearby mainland during 1990-98. The occurrence of nests of both species on the North Arm islands was determined primarily by the distribution of nesting gulls and terns and secondarily by habitat features. Nest parasitism was frequent on the islands, but not on the mainland. Average clutch size was 8.99 [+ or -] 0.12 (n = 169) for greater scaup and 9.20 [+ or -] 0.17 (n = 93) for lesser scaup on the North Arm, and 8.71 [+ or -] 0.18 (n = 55) for lesser scaup on the mainland. No greater scaup nests were found on the mainland. Apparent nest success on the islands was higher (greater scaup 75%, n = 271; lesser scaup 77%, n = 158) than on the mainland (lesser scaup 37%, n = 59). Apparent egg success was 63% (n = 1485) for greater scaup and 67% (n = 934) for lesser scaup on the islands, and 40% (n = 435) for lesser scaup on the mainland. Hatchability of eggs was 9 8% (n = 556) for greater scaup and 94% (n = 416) for lesser scaup on islands, and 98% (n = 435) for lesser scaup on the mainland. Our findings, when compared to those of previous studies, do not indicate that either clutch size or egg hatchability has declined in recent years. Therefore, it seems unlikely that broad changes in these reproductive parameters are responsible for local or continental declines in lesser scaup populations. However, nest success on our mainland study area may have been too low to maintain the local population.
Key words: Aythya affinis, Aythya marila, boreal forest, egg success, greater scaup, lesser scaup, hatchability, nest success, Northwest Territories, Subarctic
RESUME. De 1990 a 1998, on a etudie l'ecologie de reproduction du fuligule milouinan et du petit fuligule sur des iles situees dans le Bras-Nord du Grand Lac des Esclaves (Territoires du Nord-Ouest) ainsi que sur la terre ferme avoisinante. La presence, dans les iles du Bras-Nord, de nids appartenant aux
deux especes etait surtout determinee par la distribution des mouettes et des sternes, et en second lieu par les caracteristiques de l'habitat. Le piratage des nids etait frequent sur les iles, mais pas sur la terre ferme. La taille moyenne des couvees etait de 8,99 [+ or -] 0,12 (n = 169) pour le fuligule milouinan et de 9,20 [+ or -] 0,17 (n = 93) pour le petit fuligule dans le Bras-Nord, et de 8,71 [+ or -] 0,18 (n = 55) pour le petit fuligule sur la terre ferme. On n'a pas trouve de nids de fuligule milouinan sur la terre ferme. Le succes apparent de la nidification sur les iles etait plus grand (fuligule milouinan: 75 p. cent, n = 271; petit fuligule: 77 p. cent, n = 158) que sur la terre ferme (petit fu ligule: 37 p. cent, n = 59). Le succes apparent de la ponte etait de 63 p. cent (n = 1485) pour le fuligule milouinan et de 67 p. cent (n = 934) pour le petit fuligule sur les iles, et de 40 p. cent (n = 435) pour le petit fuligule sur la terre ferme. L'eclosabilite etait de 98 p. cent (n = 556) pour le fuligule milouinan et de 94 p. cent (n = 416) pour le petit fuligule sur les iles, et de 98 p. cent (n = 435) pour le petit fuligule sur la terre ferme. Quand on les compare a ceux d'etudes anterieures, nos resultats ne revelent aucun declin au cours des dernieres annees quant a la taille de la couvee ou l'eclosabilite. Il semble donc peu probable que des changements majeurs dans ces parametres de reproduction soient responsables des declins au niveau local ou continental dans les populations du petit fuligule. Le succes de la nidification dans la zone couverte par notre etude situee sur la terre ferme peut toutefois avoir ete trop faible pour maintenir le niveau de population locale.
Mots cles: Aythya affinis, Aythya marila, foret boreale, succes de la ponte, fuligule milouinan, petit fuligule, eclosabilite, succes de la nidification, Territoires du Nord-Ouest, Subarctique
Traduit pour la revue Arctic par Nesida Loyer.
Greater scaup (Aythya marila) and lesser scaup (Aythya affinis) breed sympatrically throughout much of northwestern North America (Palmer, 1976; Bellrose, 1980) including the boreal forest of Canada's Northwest Territories (Godfrey, 1986). With the exception of Trauger's (1971) study of lesser scaup, information on the ecology of scaup in the important Subarctic breeding areas of the Northwest Territories is limited to a few general investigations of waterfowl and other migratory birds (Murdy, 1965; Weller et al., 1969; Trauger and Bromley, 1976; Toft et al., 1982; Nudds and Cole, 1991). None of these studies involved the simultaneous investigation of both species of scaup, and information from most studies is now decades old. In light of recent declines in populations of lesser scaup in boreal Canada and elsewhere and the uncertain status of greater scaup, more current information and a better understanding of the ecology of scaup in the region are required for management purposes (Austin et al., 2000).
During 1990-95, we studied sympatrically breeding greater and lesser scaup on islands of the North Arm of Great Slave Lake near Yellowknife, Northwest Territories. In addition, we investigated the breeding ecology of lesser scaup on the mainland near Yellowknife during 1994-98. Our objectives were to (1) document aspects of the breeding ecology of lesser and greater scaup in the region; (2) compare nesting biology of the two species; and (3) evaluate factors potentially limiting reproductive success of local scaup populations.
Our study area is located within the Taiga Shield Ecozone (Wiken, 1986), a region of open boreal forest. The climate is Subarctic-continental, characterized by long cold winters, short cool summers, and low annual precipitation (Atmospheric Environment Service, 1993).
North Arm Study Area
The North Arm study area (hereafter North Arm) includes a 384 [km.sup.2] section of the North Arm of Great Slave Lake, bordered by Yellowknife Bay on the southeast (62[degrees]22'N, 114[degrees]20'W) and Frank Channel on the northwest (62[degrees]48'N, 115[degrees]58'W) (Fig.1). The northern boundary of the study area is the northern shore of the North Arm, and the southern boundary occurs approximately 200 m off the outermost islands, a distance of 210-9700 m from the mainland.
Progressing from southeast to northwest, there is a transition from the deeper, relatively clear waters of Yellowknife Bay, approximately 60 m deep at the West Mirage Islands (Rawson, 1950), to the very shallow and turbid waters of Frank Channel, approximately 1 m deep (Rawson, 1950). The North Arm is on the margin of the Canadian Shield. It contains hundreds of rocky islands, which range in size from less than 0.01 ha to approximately 3.5 [km.sup.2]. A large number of bays, many with extensive shallow wetlands, are also present. The islands consist of glacially polished granitic or basaltic rock with reliefs of 3-7 m. Organic soil cover ranges from none to extensive, and both abundance and type of vegetation vary greatly from island to island.
The following vegetation descriptions are adapted in part from Porsild and Cody (1980) and Weller et al. (1969). Lichens and mosses occur on rock surfaces and in crevices. Most islands support grasses (mainly Calamagrostis spp.), sedges (Carex spp.), and forbs such as saxifrage (Saxifraga tricuspidata), wild onion (Allium schoenoprasum), and yarrow (Achillea sp.). Low shrubs are common and include sweet gale (Myrica gale), wild rose (Rosa acicularis), currant (Ribes spp.), raspberry (Rubus spp.), and Labrador tea (Ledum groenlandicum and L. decumbens). Tall shrubs, primarily willows (Salix spp.) and alders (Alnus spp.), occur in depressions and along some shorelines. Many islands lack tree cover, and offshore islands generally have fewer, smaller trees than do islands close to the mainland. Trees include white and black spruce (Picea glauca and P. mariana), jack pine (Pinus banksiana), and paper birch (Betula papyrifera). On the offshore islands, conifers frequently assume a low, prostrate growth form.
Shoreline marshes occur in bays and typically are sheltered from heavy waves by barrier islands. However fluctuations in water levels occur in these marshes because of wind seiches. Most marshes have influent creeks, and bottom substrates vary from clay to rich organic deposits. Common emergent plants include sedges and horsetails (Equisetum spp.), which often are interspersed with cattail (Typha atifolia) or bulrushes (Scirpus spp.). Submergent plants include pondweeds (Potamogeton spp.), bladderworts (Utricularia spp.), and milfoils (Myriophyllum spp.).
Located near the southeastern corner of the study area are the West Mirage Islands, an isolated archipelago of 97 islands lying about 6 km from the mainland. The phenology on this archipelago is delayed from that on the mainland, and the avifauna shows an affinity with Low Arctic areas more than 300 km north of Yellowknife ( Weller et al., 1969; LaRoi and Babb, 1974).
For further information on the physical features and avifauna of the North Arm of Great Slave Lake, see Sirois et al. (1995).
Yellowknife Study Area
The Yellowknife study area (YKSA) consists of a 38 [km.sup.2] roadside transect beginning 16 km west of Yellowknife, Northwest Territories, continuing 48 km west, and extending 0.4 km on each side of the road. Of the 575 ponds on the YKSA, 262 are natural in origin and 313 are man-made. Man-made ponds developed where surface materials were removed during road construction, circa 1960. Ponds range in size from less than 0.1 ha to 18.2 ha. Man-made ponds are generally smaller, shallower, and less permanent than natural ponds.
Additional details on the physical environment, wetlands, and vegetation of the YKSA were reported by Fournier and Hines (1999).
During the summers of 1990-95, we searched 485 islands for the presence of breeding waterfowl. Many islands were visited in more than one year. We used a combination of random and systematic sampling to select islands. We randomly selected 1 x 1 km blocks from 1:50000 maps of the study area and set out to search every island within each block. This often proved impractical because of time and weather constraints. In such situations, we searched as many islands per block as possible. In addition, we searched every island encountered on which we suspected there were breeding gulls or terns (larids). Effectively, this meant we visited most larid breeding islands on the North Arm each year. Overall, we believe that our sample reflected the range of physical and biotic features of the islands within the study area and adequately satisfied the assumptions of the statistical methods employed in data analysis.
We visually estimated the percentages of each island covered by different vegetation types (forbs, grasses/ sedges, low shrubs, tall shrubs, trees, and total cover). An index to the area of each island was obtained using a dot grid overlaid on 1:50000 scale topographic maps. Distance to the mainland was measured from these maps as the shortest distance from the edge of the island to the shore.
Thorough nest searches on each island were conducted in mid to late June by two people on foot. Vegetation on the islands was generally limited in extent and patchily distributed, so all of the available nesting cover could be searched effectively. Whenever possible, we attributed scaup nests to species by observing the wing stripes of females as they flushed from their nests (Russell, 1975; Fix, 1985; Wilson and Ankney, 1988). When the sex of the bird is known, the percentage of scaup that can be identified accurately by this method approaches 100% (Fix, 1985). However, Russell (1975) and Wilson and Ankney (1988) noted that a few greater scaup females with unusually dark wings could not be correctly identified by this method.
At each nest, we recorded clutch size, incubation status (incubating or laying), and type of plant cover present. As nests were visited only once prior to hatch (i.e., at the time of discovery), clutches were considered complete and included in the calculation of mean clutch size only if there was ample evidence that incubation had begun. Evidence of incubation included warmth of eggs (following Weller et al., 1969), amount of down present in the nest (both species of scaup add down to nests mainly during incubation; Bellrose, 1980), and behaviour of the nesting female (once incubation has begun, females become reluctant to leave the nest; Johnsgard, 1975). Each nest was marked with a numbered plastic disc approximately 4 cm in diameter, which was buried beneath the nest material. We revisited each nest in late July or early August to determine its fate (i.e., hatched, abandoned, or destroyed by a predator). Criteria used to identify hatched and abandoned nests follow Klett et al. (1986). Causes of nest losse s were determined from criteria outlined in Rearden (1951) and Sargeant et al. (1998).
During 1990-93, clutches for which no female was observed were recorded simply as scaup. In 1994 and 1995, all eggs were measured and those of uncertain origin were classified to species using a discriminant function analysis (Kleinbaum and Kupper, 1978; SAS Institute, 1990). The discriminant function, based on measurements of lengths and widths of 432 lesser and 471 greater scaup eggs, was 98% accurate for classifying individual eggs (Fournier and Hines, unpubl. data).
The discriminant function was used to classify some clutches to species and also to identify possible instances of inter-specific nest parasitism involving both lesser and greater scaup. We defined intra-specific parasitism as occurring when the clutch exceeded 12 eggs, following Weller et al. (1969).
Apparent nest success is the percentage of nests found in which one or more egg hatches. In waterfowl studies, it typically overestimates true nesting success (Miller and Johnson, 1978). To overcome this possible bias and make our estimates more directly comparable to some other values reported in the literature, we converted apparent nest success to Mayfield (1961) nest success, following Green (1989). We refer to these conversions as Mayfield-Green estimates throughout the paper. The reliability of the Green (1989) estimator was evaluated on both real and simulated data by Johnson (1991), who found that it produced results similar to those produced by the Mayfield method.
For 1994-95 only, we estimated apparent egg success (percentage of all eggs that eventually hatched) from the presence or absence of egg caps or membranes, the number of unhatched eggs remaining in the nest, and the number of eggs found outside the nest. If no egg caps or membranes were found, we assumed that all eggs in a clutch had been destroyed, whereas the presence of one or more egg caps or membranes was considered to indicate that all eggs in a clutch had hatched, unless there was evidence to the contrary.
A map showing the number of nests found on different parts of the North Arm was generated with the Spatial Analysis extension of ArcView 3.1 (Environmental Systems Research Institute Inc., 1996). We used a kernel method of density calculation, a resolution (cell size) of 100 m, and a search area of 10 [km.sup.2] in map production.
Yellowknife Study Area
Ground surveys of breeding pairs on the YKSA, conducted in May and June of 1994-98, helped us identify areas in which to concentrate nest searches (Fournier and Hines, 1998). However, we searched not only the ponds where pairs were observed, but a number of nearby ponds and other ponds on the study area as well. The proportion of the YKSA searched each year was approximately 10 - 15% and was similar to the percentage of the North Arm islands searched. We concentrated nest searching during June and early July to coincide with egg-laying and early incubation. A team of 2-3 people searched for nests by wading or canoeing through the emergent vegetation of ponds and by walking the nearby snoreline. Areas arouna individual ponds were searched only once, as were the islands on the North Arm. Nests were found during laying and incubation on both study areas. On the YKSA, incomplete clutches were revisited every few days (typically twice) until the clutch size remained constant; however, this could not be done on the North Arm islands.
We used similar nest study methods on both areas, except that the YKSA nests were not marked, but were located again by the use of 1:7500 maps (coarse scale) of the study area and hand-drawn maps (fine scale) in a field notebook.
We used analysis of variance (PROC ANOVA) followed by Duncan's multiple range test (if the overall F-value was significant) and Wilcoxon or Kruska1-Wallis tests (PROC NPAR1WAY) to compare means or medians of two or more samples (SAS Institute, 1990). To further evaluate the importance of different habitat features for selection of islands by nesting scaup and for nesting success, we followed these tests with both simple direct and multiple direct logistic regression (PROCLOGISTIC) and Wald's [chi square] test (Tabachnick and Fidell, 1996). Variables were considered in the multiple logistic regression analyses if they demonstrated a significant (p [less than or equal to]0.15) relationship in the simple regressions (Hosmer and Lemeshow, 1989). To deal with the potential problem of multicolinearity, we eliminated a variable from the multiple logistic regression analysis if its variance inflation factor exceeded 10 (Rawlings et al., 1998). Potential differences in nesting success among study areas, species, and years were evaluated using [[chi].sup.2] tests based on two-dimensional contingency tables (PROC FREQ) or a logit analysis of a three-dimensional contingency table (PROC CATMOD; Allison, 1999).
We found 605 scaup nests on the islands of the North Arm: 300 nests of greater scaup, 170 of lesser scaup, and 135 for which the species was not determined. On the YKSA, we found 64 lesser scaup nests but no greater scaup nests.
Nesting Habitat on the North Arm
We found greater scaup nests on 89 islands and lesser scaup nests on 55 islands. Both species used 38 of these islands, either simultaneously or in different years. Nesting scaup of unidentified species used an additional 9 islands. Thus, the total number of islands used by scaup was 115.
The distribution of lesser and greater scaup nests on the North Arm is indicated in Figure 1. The two species were not equally distributed throughout the study area, and the proportion of greater scaup in the sample of nests found decreased as we moved from east to west. At the West Mirage Islands, near the southeastern corner of the study area and closest to the main body of Great Slave Lake, 99 of 100 nests (99%) belonged to greater scaup, and only one belonged to a lesser scaup. In the area between Baker Island and Enodah (Fig. 1), 15 of 18 nests found (83%) were of greater scaup, and 3 (17%) were of lesser scaup. On the portion of the North Arm between Enodah and Old Fort Rae, 175 of 316 nests found (55%) belonged to greater scaup and 141 (45%) belonged to lesser scaup. In the relatively shallow northwestern corner of the study area, from Old Fort Rae to Frank Channel, 11 of 36 nests (31%) belonged to greater scaup and 25 nests (69%) belonged to lesser scaup.
Habitat characteristics of greater and lesser scaup nesting islands and of the entire sample of islands visited are presented in Table 1. The results of the analysis of variance and Duncan's multiple range test suggested that selection of islands by scaup was influenced by distance of an island from shore and by several vegetation characteristics (Table 1).
Logistic regression analysis and the resulting values of Wald's [[chi].sup.2] test demonstrated the over-riding and positive influence of the presence of breeding gulls and terns (family Laridae, hereafter lands) on island use by nesting scaup of both species (Table 2). Only 12 (5%) of the 262 islands without nesting lands supported nesting scaup, whereas 103 (46%) of 223 islands with nesting lands had nesting scaup. Overall, 98% of the scaup nests that we found were on islands with nesting lands. This relationship held for greater scaup (98%), lesser scaup (97%), and unidentified scaup (99%) nests.
In addition to the presence/absence of lands, the logistic regression analysis indicated other factors that may have influenced island choice. For greater scaup, these were the distance of an island from shore (islands farther from shore had more nests) and the presence of tall shrub cover (islands with abundant tall shrubs had fewer nests), although these results were of borderline statistical significance. Lesser scaup were apparently influenced by island size (larger islands had more nests), distance from shore (nearshore islands had more nests), and tree cover (islands with fewer trees had more nests) (Table 2).
Islands used by nesting greater scaup were located, on average, farther from shore than those used by lesser scaup (Table 1). After we eliminated data from the West Mirage Islands, where nesting greater scaup were predominant, the difference between average distances to shore for the two species decreased (from 1443 m to 559 in), but the means (1991 [+ or -] [SE] 142 in for greater scaup and 1432 [+ or -] 116 m for lesser scaup) remained significantly different (Kruskal-Wallis Test, 1 df, p < 0.01).
In all other aspects (area and vegetation characteristics), the islands used for nesting by the two species were similar (Tables 1 and 3).
Grass/sedge was most frequently used as nesting cover by both species of scaup. Low shrubs (such as currant, wild rose, raspberry, and Labrador tea) were also commonly used (Table 4).
Nesting Habitat on the Yellowknife Study Area
Lesser scaup nests were found in the YKSA on the peripheries of 48 different ponds, including 25 natural and 23 man-made ponds. Mean size of ponds used by lesser scaup was 1.89 [+ or -] 0.59 ha (median 0.80 ha) for natural ponds, 0.22 [+ or -] 0.03 ha (median 0.20 ha) for man-made ponds, and 1.09 [+ or -] 0.33 ha (median 0.35 ha) overall.
Like the scaup nesting on islands of the North Arm, scaup on the YKSA most frequently nested in grass/sedge cover (Table 4). In contrast to island-nesting scaup, lesser scaup on the YKSA made little use of low shrubs for nesting cover (one nest in sweet gale), and they occasionally used einergents such as cattail, horsetail, marsh cinquefoil (Potentillapalustris) and water-arum (Calla palustris), species not widely available to island-nesting scaup. None of the lesser scaup nests found on the YKSA were associated with the small number of breeding lands there.
Clutch Size and Nest Parasitism
Incubated scaup clutches ranged in size from 3 to 33 eggs (neither extreme could be attributed to a species) on the North Arm and from 6 to 11 eggs for lesser scaup on the YKSA (Table 5). Overall, mean clutch sizes of lesser scaup (10.52 [plus or minus]0.30 eggs) and greater scaup (10.13 [plus or minus]0.21 eggs) nesting on islands were higher (one-way ANOVA; F = 6.70; df = 2,409; Duncan's Multiple Range Test, p < 0.01) than the mean for lesser scaup (8.71 [plus or minus]0.18 eggs) nesting on the YKSA. However, the larger average clutch sizes on the North Arm were apparently due to egg parasitism. After controlling for the effects of parasitism (i.e., eliminating clutches with more than 12 eggs from the calculations), we found no significant differences (one-way ANOVA: F = 1.76, df = 2,3l4, p = 0.17) in clutch size between island-nesting and mainland scaup (Table 5).
The results of our discriminant function analysis of all eggs measured in 1994 and 1995 and the assumption that clutches with more than 12 eggs were the product of more than one female led us to conclude that may scaup clutches on the North Arm were parasitized, either inter-or intraspecifically. In 1994 and 1995, 23 (35%) of 65 lesser scaup nests were parasitized by other waterfowl: 17 (26%) by greater scaup, one (2%) by a northern pintail (Anas acuta), and 10 (15%) by other lesser scaup. Inclued in the above total are five (8%) nests parasitized by bot a greater scaup and another lesser scaup (i.e., at least three females were laying in the same nest). Similarly, 44 (38%) of 116 greater scaup nests were parasitized by other ducks: 31 (27%) by lesser scaup, 5 (4%) by other species (see below), and 14 (12%) by other greater scaup. Five (4%) of these nests were parasitized by both a lessers scaup and another greater scaup, and one nest was parasitized by both a lesser scaup and a red-breasted merganser (Mergus errator). One such incident led to the concurrent incubtion of a mixed clutch by both a lesser and a greater scaup (Fournier and Hines, 1996).
During 1990-95, species that laid eggs in scaup nests on the North Arm included northern pintail (n = 2 nests), gadwall (Anas strepera) (n = 3), northern shoveler (A. clypeata) (n = 2), and red-breasted merganser (a = 2). On the North Arm, scaup eggs were found in the nests of a number of other species, including northern pintail (n = 4 nests), gadwall (n = 2), northern shoveler (n =1), mallard (Anas plalyrhynchos) (n = 1), canvasback (Aythya valisineria) (n = 1), and red-breasted merganser (n = 2). In addition to cases of parasitism involving other species of waterfowl, we found a scaup egg in the nest of a ring-billed gull (Larus delawarensis) and a ring-billed gull egg in a scaup nest (Fournier, 2000).
On the YKSA, no clutches exceeded 11 eggs, so there was no evidence of intraspecific parasitism based on the criterion of Weller et al. (1969). However, the results of our discriminant function analysis suggested that five of 51 lesser scaup clutches contained greater scaup eggs. The eight parasitic eggs represented less than 2% of all eggs measured (n = 432). No greater scaup nests were found on the YKSA, and greater scaup sightings there have been infrequent. Therefore, it seems possible that the discriminant function analysis incorrectly classified some of these eggs to species.
We determined apparent nest success and derived Mayfield-Green estimates for 271 greater scaup nests and 158 lesser scaup nests on the North Arm and 59 lesser scaup nests on the YKSA. A three-way contingency table indicated that apparent nest success differed significantly between study areas ([chi square] = 13.9, 1 df, p < 0.01), but not between species ([chi square] = 0.20, 1 df, p = 0.65) or years ([chi square] = 12.6,7 df, p = 0.08) (Table 6). Overall, the success of nests on the North Arm was virtually the same for greater scaup (75%) and lesser scaup (77%), and was much higher than for lesser scaup on the YKSA (37%). As expected, the Mayfield-Green estimates for these three samples were much lower than the apparent nest success estimates, and averaged 58%, 61%, and 18%, respectively.
We examined the influence on scaup nesting success of habitat characteristics of different islands (macrohabitat) and the presence of larids, using both univariate comparisons (Table 7) and logistic regression (Table 8). Because of the small number of islands (n = 12) with scaup nests but without lands, we pooled data from both scaup species for the logistic regression and any other comparisons involving this particular variable.
Only one variable had a clear effect on nest success: successful nests of scaup were most frequently located on islands that supported nesting gulls and terns. Overall, nest success on islands with nesting lands was higher (75%, n = 525) ([chi square] = 6.96, df = 1, p <0.01) than on islands without lands (42%, n = 12). In addition, there was some evidence (of borderline statistical significance) that nests located closer to shore were more successful than those located farther offshore (Table 8).
Where possible, we examined nesting success in relation to the type of vegetation in which nests were concealed (microhabitat). Sample sizes for each species were too small for meaningful comparisons involving forb, tall shrub, and tree cover (Table 4). As there were no differences ([chi square] tests, p [greater than or equal to] 0.59) in nest success between greater and lesser scaup in grass/sedge and low shrub cover, we pooled data for both species and included data from unidentified scaup for comparisons of cover types. There were no significant differences ([chi square] = 3.22, 4 df, p = 0.52) among cover types. Apparent nest success was 73% in grass/sedge (n = 406), 81% in forbs (n = 27), 78% in low shrubs (n = 85), 88% in tall shrub cover (n = 8), and 57% for nests under prostrate branches of trees (n = 7).
On the YKSA, there was no difference ([chi square] = 0.09, 1 df, p = 0.76) in apparent nest success between lesser scaup nests located on natural ponds (35%, n = 31) and those located on man-made ponds (39%, n = 28). Sample sizes were too small to test for differences in nest success among most vegetation types (Table 4). There was no difference ([chi square] = 1.50, 1 df, p = 0.22) in the success of nests located in grass/sedge and tall shrub, the two most frequently used cover types.
Apparent egg success of greater scaup (63%) and lesser scaup (67%) on the North Arm was similar and considerably higher than that of lesser scaup on the YKSA (40%) (Table 9). The difference in egg success between areas was attributable to the much higher rate of egg predation on the YKSA (57%) than on the North Arm (14%). The observed difference would have been even greater had the rates of nest abandonment and the number of unhatched eggs left in successful nests been similar between study areas. Rates of abandonment were higher on the North Arm (6% for greater scaup and 12% for lesser scaup) than on the YKSA (< 2%), as were the numbers of unhatched eggs left in successful nests (8% for greater scaup and 9% for lesser scaup on the North Arm and 1% for lesser scaup on the mainland), probably as a result of parasitism.
Hatchability of eggs (the percentage of incubated eggs in successful clutches that eventually hatched) was 89% (n = 1485 eggs) for greater scaup and 88% (n = 934) for lesser scaup on the North Arm, and 98% (n = 435) for lesser scaup on the YKSA. When parasitized clutches were excluded from the analysis, hatchability of greater and lesser scaup eggs on the North Arm increased to 98% (n = 556 eggs) and 94% (n = 416 eggs), respectively.
Our study areas provided a variety of habitat types for nesting scaup. Lesser scaup were the more widely distributed of the two species and used a greater variety of cover types for nesting. The only area that did not support many lesser scaup was the West Mirage Islands (Fig. 1). Despite the abundance of nesting greater scaup on the North Arm of Great Slave Lake, particularly on the West Mirage Islands, we found no nests of this species on the mainland. Thus, the overall distribution pattern of breeding scaup in the region was lesser scaup on the mainland, greater scaup on the West Mirage Islands, and a zone of overlap on the remainder of the North Arm islands.
Islands used by greater scaup tended to be farther from shore and have smaller amounts of tall shrub cover than all available islands. Islands used by lesser scaup were, on average, closer to shore, and had less tree cover than available islands. Islands with few tall shrubs or trees offer fewer perching sites for ravens, probably the most frequent predator of duck eggs in both study areas (see below), and were possibly selected for this reason. The tendency of greater scaup to nest farther offshore than lesser scaup (and thus near deeper and clearer water) appeared to hold throughout the North Arm. It is unclear why greater scaup, in general, nest farther offshore and why the West Mirage Islands, in particular, are used almost exclusively by greater scaup. Given the similar preferences in nesting habitat of the two species in terms of vegetation and associations with larids, we suspect that the difference in distribution reflects some ecological difference between species other than selection of nesting habi tat. Perhaps the difference in distribution reflects differences in feeding behaviour and food habits of the two species during the pre-nesting, nesting, or brood-rearing periods, but further study is required to test these hypotheses.
The most important factor affecting island selection by both species was the presence or absence of larids. Scaup were frequently found nesting in land colonies. Ducks and other aquatic birds (e.g., grebes) apparently benefit from such associations because lands are effective at defending the nesting area against some types of predators (Koskimies, 1957; Hilden, 1964; Kistchinski and Flint, 1974; Newton and Campbell, 1975; Nuechterlein, 1981; Burger, 1984; Young and Titman, 1986; Burger and Gochfeld, 1995). However, such advantages might be offset by high duckling mortality when ducks nest among highly predatory larids, such as California gulls (Larus californicus) and herring gulls (Larus argentatus) (Vermeer, 1968; Dwernychuk and Boag, 1972).
Overall, apparent nest success on islands with breeding larids (75%) was higher than either nest success of scaup on islands without larids (42%) or nest success of lesser scaup on the YKSA (37%), as has been reported for other waterfowl (Kistchinski and Flint, 1974; Newton and Campbell, 1975; Genell, 1985; Gotmark and Ahlund, 1988; Gotmark, 1989). The similar nest success of scaup nesting on islands without larids and on the mainland is of interest because many other studies have reported higher waterfowl nest success on islands, even in the absence of larids (Hammond and Mann, 1956; Duebbert, 1966, 1982; Vermeer, 1968; Young, 1968; Long, 1970; Lokemoen, 1991; Lokemoen and Woodward, 1992). We believe this similarity likely occurred because ravens were the dominant predator in both study areas. Predation of duck and grebe eggs by ravens was reported previously for the YKSA (Murdy, 1963, 1965; Fournier and Hines, 1999) and has been reported for other waterfowl breeding areas (Sargeant et al., 1998). During the course of fieldwork on the North Arm, we visited two islands with active raven nests and found numerous destroyed eggs of scaup and other ducks. Eggs destroyed in a similar fashion were observed on many other islands as well, and were frequently found near typical raven perching sites (tall shrubs or trees), indicating the widespread nature of egg predation by ravens on the North Arm.
Austin et al. (1998) summarized average nest success of lesser scaup by different geographic regions and habitat types, as follows: prairie parklands 30%, prairie islands 31%, prairie grasslands 37%, and northern boreal forest 57%. Our Mayfield-Green estimate for nest success of lesser scaup on the North Arm (61%) was similar to Austin's estimate for the boreal forest in general, but our estimate for the YKSA (18%), which is probably more typical of lesser scaup in the region, was lower than Austin's estimates for all other areas.
The few reported estimates of greater scaup nest success (Shepherd, 1955: 25%, Mayfield-Green estimate 10%, n = 16; Kirkpatrick and Buckley, 1954, cited in Bellrose, 1980: 45%, Mayfield-Green estimate 24%, n = 20) are much lower than what we observed on the islands of the North Arm (75%, Mayfield-Green estimate 58%, n = 284).
Clutch Size and Hatchability
Clutch sizes of scaup in our study areas were similar to values reported previously. Weller et al. (1969) reported an overall mean clutch of 9.3 eggs and, after controlling for parasitism, a mean clutch of 8.5 eggs for greater scaup nesting at the West Mirage and several nearby islands. In the same area, Trauger and Bromley (1976) observed an average clutch of 9.0 eggs overall and 8.5 eggs after parasitic eggs were eliminated (Bellrose, 1980). Thus, our averages of 10.1 eggs per clutch overall and 9.0 eggs per unparasitized clutch indicate clutch sizes of greater scaup have not declined in recent decades on the North Arm.
Although there are no published data on clutch sizes of lesser scaup from the Great Slave Lake region, two accounts provide data from other northern breeding areas. Nelson (1953) gave an average clutch of 8.6 eggs for Alaska, and Townsend (1966) reported an average clutch of 9.0 eggs at the Saskatchewan River Delta. Our average estimate of 8.7--9.2 eggs suggests no long-term change in average clutch size of lesser scaup.
Hatchability of eggs in our study areas (88--98%) was similar to that previously reported for both lesser scaup (Miller and Collins, 1954: 88%; Rienecker and Anderson, 1960: 93%; Keith, 1961: 83%; Vermeer, 1968: 93%) and greater scaup (Hilden, 1964: 96%). As well, data collected in Alberta in 1998 indicated a high rate of hatchability (95%) for lesser scaup eggs there (D. Duncan, pers. comm. 1999). Despite the comparatively high rate of hatchability observed both in the YKSA and on the North Arm, we did observe a difference in hatchability of eggs between our study areas. Lesser scaup in the YKSA averaged approximately 10% higher hatchability than did either species on the North Arm. Several previous studies also reported higher rates of hatchability of eggs from mainland versus island nests (Hammond and Mann, 1956; Duebbert et al., 1983; Hines and Mitchell, 1983). High densities of waterfowl nesting on islands have frequently led to increased levels of egg parasitism (Hilden, 1964; Vermeer, 1968; Giroux, 198 1; Hines and Mitchell, 1984; Robertson et al., 1992), and lower hatchability of eggs (Newton and Campbell, 1975; Duebbert et al., 1983), which is probably due to age differences between host eggs and parasitically laid eggs. Once parasitized clutches were eliminated from our analyses, the hatchability of eggs in both study areas was similar.
Status of Breeding Populations & Management Implications
The lesser scaup population has declined throughout its range (Austin et al., 2000), and the status of the greater scaup population is uncertain. We have no survey data on long-term population changes of greater scaup on the North Arm of Great Slave Lake, although we can use the results of nesting studies at the West Mirage Islands as an index of population change. On this archipelago, Weller et al. (1969) found 40 greater scaup nests on 72 islands searched in 1968, an average of 0.6 nests per island. During 1993-95, we searched an average of 22 islands per year and found an average of 28 greater scaup nests per year (or 1.3 per island), and we are confident that 40 or more nests would have been found each year had more islands been searched. These data suggest that no decline has occurred in this breeding population since the 1960s. However, numbers of lesser scaup on the nearby mainland have decreased by approximately 30% since 1962-65 (Hines and Fournier, unpubl. data), a rate of decline similar to or slig htly higher than that observed for the continental population of this species over the same time interval (see Austin et al., 2000).
A number of hypotheses have been put forward to explain the continental decline of lesser scaup (and the possible decline of greater scaup) (Austin et al., 2000). Our data bear on some of these hypotheses, particularly those pertaining to low reproductive success as a cause of population reduction. Negative effects of contaminants on reproductive parameters such as clutch size, egg hatchability, and nest success have been invoked as possible explanations for decreasing scaup numbers (Austin et al., 2000). However, average clutch sizes on our study areas (from data collected mainly during the period when lesser scaup were declining) were not lower than values previously reported for either species. Similarly, the hatchability of eggs in our study areas was as high as that found in earlier studies, and the success of scaup nests on islands of the North Arm was as good or better than that reported in most studies prior to the population decline. Therefore, if contaminants are impairing reproductive output of sca up, we believe their effect would have to be manifested through reduced breeding propensity of adults or lower survival of ducklings, and not through their influence on nest or egg success.
Given that greater and lesser scaup have somewhat different wintering areas and migration routes (Palmer, 1976; Bellrose, 1980), the similarly high nest success of the two species on the North Arm and the low success of lesser scaup on the nearby mainland are of interest. This pattern suggests that local conditions on breeding areas are more apt to limit nest success (and perhaps overall population productivity) than are conditions distant from the breeding grounds.
The main cause of nest failure in both of our study areas, and in most previous studies of waterfowl nesting, was clutch predation. Our study areas provided a steep gradient of habitat quality in terms of accessibility to predators: mainland (accessible to both avian and mammalian predators); islands without nesting lands (mammalian predators reduced, but avian predators still common); and islands with nesting lands (both mammalian and avian predators reduced). Nest success was high only under the latter condition, suggesting that avian nest predators such as ravens had a substantial impact on nest success.
The 18% average nest success in our mainland study area was low compared to that found elsewhere in the boreal or other regions, but was similar to that needed to maintain populations of dabbling ducks (15-20%) (Cowardin et al., 1985; Klett et al., 1988). However, many female scaup (particularly young birds 1-2 years of age) do not breed in some years (Afton, 1984), and scaup are a late-season nesting species with presumably very limited opportunity to renest in the Subarctic. The overall productivity of lesser scaup on the YKSA was lower (on average, 23% of the paired females produced broods during 1985-98; Hines and Fournier, unpubl. data) than the level required to maintain other populations of ducks (> 30%) (Cowardin et al., 1985). Thus, the nest success observed on the YKSA may not have been sufficient to maintain the local lesser scaup population without exceptionally high survival of adult females and their ducklings. The possible role of low nest success and other demographic parameters (such as survi val of adult females and their broods) in limiting the growth of lesser scaup populations warrants further investigation.
We thank all of the field assistants who participated in the various aspects of this study. Foremost among these in years of participation were J. Roosdahl, R. Brook, and V. Chisholm. Dave Duncan generously provided us with his unpublished data on lesser scaup egg hatchability. As well, we thank D. Duncan, K. Dickson, J. Austin, and two anonymous reviewers for their helpful comments on the manuscript and M. Wiebe for producing the map. Funding for this study was provided by the Canadian Wildlife Service.
(1.) Canadian Wildlife Service, Environment Canada, 5204-50th Avenue, Suite 301, Yellowknife, Northwest Territories, X1A 1E2
(2.) Corresponding author: email@example.com
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TABLE 1 Characteristics of islands used by nesting greater scaup (GRSC) and lesser scaup (LESC) and all islands surveyed on the North Arm of Great Slave Lake, Northwest Territories, 1990-95. Greater Scaup Lesser Scaup (n = 89) (n = 55) Mean SE Mean SE Grass/sedge (%) 4.5 0.4 3.8 0.4 Forbs (%) 2.2 0.3 1.9 0.2 Low shrub (%) 3.4 0.5 2.6 0.4 Tall shrub (%) 1.8 0.3 2.2 0.4 Tree (%) 2.5 0.5 2.0 0.4 Total cover (%) 14.4 1.6 12.4 1.3 Distance to shore (in) 2959 216 1516 141 Island area (ha) 0.6 0.1 0.4 0.1 Available Islands ANOVA Duncan's multiple range test (1) (n = 485) Mean SE p GRSC Grass/sedge (%) 4.4 0.2 0.45 - Forbs (%) 1.9 0.1 0.61 - Low shrub (%) 5.0 0.3 <0.01 B Tall shrub (%) 4.5 0.3 <0.01 B Tree (%) 6.3 0.4 <0.01 B Total cover (%) 22.1 0.9 <0.01 B Distance to shore (in) 2058 83 <0.01 A Island area (ha) 0.5 0.0 0.17 - Duncan's multiple range test (1) LESC Available Grass/sedge (%) - - Forbs (%) - - Low shrub (%) B A Tall shrub (%) B A Tree (%) B A Total cover (%) B A Distance to shore (in) C B Island area (ha) - - (1)Similar Duncan groupings indicate no significant difference between means (p > 0.05). TABLE 2 Logistic regression analysis indicating the influence of macrohabitat features on use of islands by nesting greater and lesser scaup on the North Arm of Great Slave Lake, Northwest Territories, 1990-95. Simple logistic regression Variable Slope SE Wald's [chi square] Greater Scaup Grass/sedge (%) 0.0175 0.0291 0.3624 Forbs (%) 0.0426 0.0349 1.4942 Low shrub (%) -0.0714 0.0253 7.9490 Tall shrub (%) -0.2110 0.0447 22.2554 Tree (%) -0.1115 0.0253 19.3429 Total cover (%) (1) - - - Distance to shore (km) 0.0003 0.0001 25.4497 Island area (ha) 0.2007 0.1544 1.6892 Presence of Larids 2.9506 0.3857 58.5220 Lesser Scaup Grass/sedge (%) -0.0624 0.0492 1.6130 Forbs (%) 0.0094 0.0458 0.0421 Low shrub (%) -0.1153 0.0376 9.3800 Tall shrub (%) -0.1450 0.0467 9.6227 Tree (%) -0.1392 0.0374 13.8547 Total cover (%) - - - Distance to shore (km) -0.0002 0.0001 4.9506 Island area (ha) -0.4497 0.3102 2.1012 Presence of larids 2.7418 0.4795 32.6898 Simple Multiple logistic logistic regression regression Variable p Slope SE Greater Scaup Grass/sedge (%) 0.5472 (2) - - Forbs (%) 0.2216 (2) - - Low shrub (%) 0.0048 0.0516 0.0382 Tall shrub (%) <0.0001 -0.1174 0.0659 Tree (%) <0.0001 -0.0182 0.0407 Total cover (%) (1) - - - Distance to shore (km) <0.0001 0.0001 0.0001 Island area (ha) 0.1937 (2) - - Presence of Larids <0.0001 2.6905 0.4119 Lesser Scaup Grass/sedge (%) 0.2041 (2) - - Forbs (%) 0.8374 (2) - - Low shrub (%) 0.0022 0.0283 0.0584 Tall shrub (%) 0.0019 -0.0548 0.0660 Tree (%) 0.0002 -0.1350 0.0656 Total cover (%) - - - Distance to shore (km) 0.0261 -0.0006 0.0001 Island area (ha) 0.1472 0.8416 0.3833 Presence of larids 0.0001 2.7865 0.5160 Multiple logistic regression Variable Wald's [chi square] p Greater Scaup Grass/sedge (%) - - Forbs (%) - - Low shrub (%) 1.8247 0.1768 Tall shrub (%) 3.0681 0.0751 Tree (%) 0.2012 0.6537 Total cover (%) (1) - - Distance to shore (km) 3.6473 0.0562 Island area (ha) - - Presence of Larids 42.6605 <0.0001 Lesser Scaup Grass/sedge (%) - - Forbs (%) - - Low shrub (%) 0.2353 0.6276 Tall shrub (%) 0.6881 0.4068 Tree (%) 4.2266 0.0398 Total cover (%) - - Distance to shore (km) 23.4501 <0.0001 Island area (ha) 4.8221 0.0281 Presence of larids 29.1596 <0.0001 (1)Variable eliminated from regression analysis (Variance Inflation Factor> 10). (2)Variable eliminated from multiple regression analysis (p > 0.15). TABLE 3 Logistic regression analysis indicating the differences in macrohabitat features of nesting islands of greater and lesser scaup on the North Arm of Great Slave Lake, Northwest Territories, 1900-95. Simple logistic regression Variable Slope SE Grass/sedge (%) 0.1443 0.1065 Forbs (%) 0.0700 0.1210 Low shrub (%) 0.0197 0.0555 Tall shrub (%) -0.1717 0.0876 Tree (%) -0.0060 0.0514 Total cover(%) (1) - - Distance to shore (km) -0.0011 0.0003 Island area (ha) 0.7377 0.5458 Presence of larids 0.4747 0.7705 Simple logistic regression Variable Wald's [chi square] p Grass/sedge (%) 1.8350 0.1755 (2) Forbs (%) 0.3347 0.5629 (2) Low shrub (%) 0.1256 0.7230 (2) Tall shrub (%) 3.8425 0.0500 Tree (%) 0.0135 0.9077 (2) Total cover(%) (1) - - Distance to shore (km) 10.9906 0.0009 Island area (ha) 1.8269 0.1765 (2) Presence of larids 0.3796 0.5378 (2) Multiple logistic regression Variable Slope SE Grass/sedge (%) - - Forbs (%) - - Low shrub (%) - - Tall shrub (%) -0.1869 0.1126 Tree (%) - - Total cover(%) (1) - - Distance to shore (km) -0.0012 0.0004 Island area (ha) - - Presence of larids - - Multiple logistic regression Variable Wald's [chi square] p Grass/sedge (%) - - Forbs (%) - Low shrub (%) - - Tall shrub (%) 2.7532 0.0971 Tree (%) - - Total cover(%) (1) - - Distance to shore (km) 9.7491 0.0018 Island area (ha) - - Presence of larids - - (1)Variable eliminated from regression analysis (Variance Inflation Factor > 10). (2)Variable eliminated from multiple regression analysis (p > 0.15). TABLE 4 Dominant vegetative cover at greater and lesser scaup nests on islands of the North Arm of Great Slave Lake and on the mainland (YKSA) near Yellowknife, Northwest Territories, 1990-95. Cover Type Greater Seaup North Arm(n = 300) Lesser Scaup North Arm (n = 170) % of Notes (n) % of Notes Rock/moss <1 2 <1 Grass/sedge 79 238 71 Forbs 3 10 5 Low shrub 14 43 19 Tall shrub 1 3 3 Tree 1 4 1 Emergent - - - Cover Type Lesser Scaup North Arm Lesser Scaup YKSA (n = 64) (n = 170) (n) % of Nests (n) Rock/moss 1 0 - Grass/sedge 120 66 42 Forbs 9 0 - Low shrub 33 2 1 Tall shrub 5 22 14 Tree 2 2 1 Emergent - 9 6 TABLE 5 Clutch size of greater and lesser scaup nesting on islands of the North Arm of Great Slave Lake and on the mainland (YKSA) near Yellowknife, Northwest Territories, 1990-95. N Min Max Mean SE Greater Scaup North Arm All clutches 232 4 29 10.13 0.21 Clutches < 12 169 4 12 8.99 0.12 Lesser Scaup North Arm All Clutches 125 5 26 10.52 0.30 Clutches < 12 93 5 12 9.20 0.17 Lesser Scaup YKSA All Clutches 55 6 11 8.71 0.18 TABLE 6 Apparent and Mayfield-Green estimates of nest success of greater and lesser scaup on islands of the North Arm of Great Slave Lake and on the mainland (YKSA) near Yellowknife, Northwest Territories, 1991-98. Greater Scaup North Arm Lesser Scaup North Arm Year Apparent Mayfield-Green n Apparent Success (%) (%) Success (%) 1991 94 90 18 100 1992 79 64 29 67 1993 82 68 61 81 1994 72 52 78 71 1995 67 47 85 77 1996 No data -- -- No data 1997 No data -- -- No data 1998 No data -- -- No data Mean 75 58 271 77 Lesser Scaup North Arm Lesser Scaup YKSA Year Mayfield-Green n Apparent Mayfield-Green (%) Success (%) (%) 1991 100 7 No data -- 1992 47 12 No data -- 1993 67 42 No data -- 1994 51 49 33 15 1995 61 48 9 2 1996 -- -- 38 18 1997 -- -- 43 22 1998 -- -- 48 27 Mean 61 158 37 18 Lesser Scaup YKSA Year n 1991 -- 1992 -- 1993 -- 1994 6 1995 11 1996 8 1997 7 1998 27 Mean 59 TABLE 7 Macrohabitat characteristics of successful and unsuccessful greater and lesser scaup nests on the North Arm of Great Slave Lake, Northwest Territories, 1991-95. Greater Scaup Successful (n = 212) Unsuccessful (n = 72) Mean SE Mean Grass/sedge (%) 4.3 0.1 4.5 Forbs (%) 2.1 0.2 2.4 Low shrub (%) 3.3 0.3 4.3 Tall Shrub (%) 1.6 0.1 1.6 Tree (%) 1.9 0.3 2.9 Total cover (%) 13.2 0.9 15.6 Distance to shore (m) 3192 144 3759 Island area (ha) 0.5 0.0 0.7 Greater Lesser Scaup Scaup Unsuccessfu Successful (n = 118) l (n = 72) SE p (1) Mean SE Grass/sedge (%) 0.4 0.63 3.9 0.2 Forbs (%) 0.5 0.72 1.8 0.1 Low shrub (%) 0.6 0.22 2.6 0.2 Tall Shrub (%) 0.2 0.52 1.7 0.2 Tree (%) 0.6 0.09 1.1 0.2 Total cover (%) 1.9 0.26 11.1 0.6 Distance to shore (m) 260 0.05 1444 76 Island area (ha) 0.1 0.19 0.3 0.0 Lesser Scaup Unsuccessful (n = 38) Mean SE p (1) Grass/sedge (%) 3.5 0.3 0.25 Forbs (%) 2.0 0.2 0.46 Low shrub (%) 1.9 0.4 0.07 Tall Shrub (%) 2.5 0.5 0.35 Tree (%) 1.9 0.5 0.22 Total cover (%) 11.7 0.2 0.70 Distance to shore (m) 1624 136 0.16 Island area (ha) 0.34 0.1 0.97 (1)Kruskal-Wallis test results. TABLE 8 Logistic regression analysis indicating the influence of macrohabitat features on nest success of greater and lesser scaup (combined) on the North Arm of Great Slave Lake, North- west Territories, 1990-95. Simplelogistic regression Variable Slope SE Wald's [chi square] Grass/sedge (%) -0.0009 0.0418 0.0005 Forbs (%) -0.0320 0.0346 0.8520 Low shrub (%) -0.0229 0.0254 0.8161 Tall shrub (%) -0.0755 0.0525 2.0690 Tree (%) -0.0537 0.0261 4.2287 Total cover(%) (1) -- -- -- Distance to shore (km) -0.0001 0.0001 3.2591 Island area (ha) -0.1904 0.1847 1.0629 Presence of larids 1.5786 0.6557 5.7968 Simplelogisti Multiple logistic c regression regression Variable P Slope SE Grass/sedge (%) 0.9829 (2) -- -- Forbs (%) 0.3560 (2) -- -- Low shrub (%) 0.3663 (2) -- -- Tall shrub (%) 0.1503 (2) -- -- Tree (%) 0.0397 -0.0344 0.0275 Total cover(%) (1) -- -- -- Distance to shore (km) 0.0710 -0.0001 0.0001 Island area (ha) 0.3026 (2) -- -- Presence of larids 0.0161 1.5569 0.6757 Multiple logistic regression Variable Wald's [chi square] P Grass/sedge (%) -- -- Forbs (%) -- -- Low shrub (%) -- -- Tall shrub (%) -- -- Tree (%) 1.5648 0.2110 Total cover(%) (1) -- -- Distance to shore (km) 3.0127 0.0826 Island area (ha) -- -- Presence of larids 5.3093 0.0212 (1)Variable eliminated from regression analysis (Variance Inflation Factor > 10). (2)Variable eliminated from multiple regression analysis (p > 0.15). TABLE 9 Apparent egg success of greater and lesser scaup nesting on islands of the North Arm of Great Slave Lake in 1994-95 and on the mainland (YKSA) near Yellowknife, Northwest Territories, in 1995-98. Greater Scaup Lesser Scaup Lesser Scaup North Arm North Arm YKSA Hatched 940 (63%) 625 (67%) 173 (40%) Cracked 28 (2%) 7 (<1%) 0 - Displaced from nest 22 (2%) 0 - 1 (<1%) Predator destroyed 269 (18%) 97 (10%) 249 (57%) Abandoned 85 (6%) 110 (12%) 7 (2%) Failed to hatch (1) 113 (8%) 83 (9%) 4 (1%) Dead duckling (2) 21 (1%) 10 (1%) 1 (<1%) Miscellaneous (3) 7 (<1%) 2 (<1%) 0 - Total eggs 1485 934 435 (1)Left in nest after hatch--does not include abandoned clutches. (2)Dead ducklings or pipped eggs founds in nest after hatch. (3)Abnormal (dwarf) eggs and eggs buried under nest material.
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|Author:||Fournier, Michael A.; Hines, James E.|
|Article Type:||Statistical Data Included|
|Date:||Dec 1, 2001|
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