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Article Excerpt
ABSTRACT. Population estimates for long-tailed ducks in North America have declined by nearly 50% over the past 30 years. Life history and population dynamics of this species are difficult to ascertain, because the birds nest at low densities across a broad range of habitat types. Between 1991 and 2004, we collected information on productivity and survival of long-tailed ducks at three locations on the Yukon-Kuskokwim Delta. Clutch size averaged 7.1 eggs, and nesting success averaged 30%. Duckling survival to 30 days old averaged 10% but was highly variable among years, ranging from 0% to 25%. Apparent annual survival of adult females based on mark-re capture of nesting females was estimated at 74%. We combined these estimates of survival and productivity into a matrix-based population model, which predicted an annual population decline of 19%. Elasticities indicated that population growth rate ([ambda]) sensitive to changes in adult female survival. Further, the relatively high sensitivity of [lambda] to duckling survival suggests that low duckling survival may be a bottleneck to productivity in some years. These data represent the first attempt to synthesize a population model for this species. Although our analyses were hampered by the small sample sizes inherent in studying a dispersed nesting species, our model provides a basis for management actions and can be enhanced as additional data become available.
Key words: Alaska, Clangula hyemalis, elasticity, long-tailed duck, population dynamics, sensitivity, sea duck, vital rates, Yukon-Kuskokwim Delta
RESUME. Les estimations de populations d'hareldes kakawis en Amerique du Nord ont chute de pres de 50 pour cent ces 30 dernieres annees. Le cycle biologique et la dynamique des populations de cette espece sont difficiles a etablir car ces oiseaux nichent moyennant de faibles densites dans une vaste gamme d'habitats. De 1991 a 2004, nous avons recueilli des donnees sur la productivite et la survie des hareldes kakawis a trois emplacements du delta Yukon-Kuskokwim. Les couvees atteignaient 7,1 oeufs en moyenne, tandis que le succes de reproduction s'etablissait generalement a 30 pour cent. En moyenne, 10 pour cent des jeunes canards survivaient jusqu'a 1'age de 30 jours, mais ce taux variait beaucoup d'une annee a l'autre, allant de pour cent a 25 pour cent. Annuellement, d'apres la methode par marquage et recapture des femelles nidificatrices, la survie apparente des femelles adultes etait evaluee a 74 pour cent. Nous avons combine ces estimations de survie et de productivite dans un modele de population matriciel, ce qui a permis de predire un declin de population annuel de 19 pour cent. Selon les elasticites, le taux de croissance de la population ([lambda]) etait plus sensible aux changements dans le cas de la survie des femelles adultes. Parailleurs, la sensibilite relativement elevee du [lambda] par rapport a la survie des jeunes canards laisse croire que le faible taux de survie des jeunes canards pourrait presenter une embuche en matiere de productivite d'ici quelques annees. Ces donnees representent la premiere tentative de synthese d'un modele de population pour cette espece. Bien que nos analyses aient ete genees par la petite taille des echantillons inherente a Tetude d'especes de nidification dispersees, notre modele fournit un fondement permettant d'aboutir a des mesures de gestion en plus de presenter la possibilite d'etre ameliore au fur et a mesure que des donnees supplementaires sont disponibles.
Mots cles: Alaska, Clangula hyemalis, elasticite, harelde kakawi, dynamique des populations, sensibilite, canard de mer, taux vitaux. delta Yukon-Kuskokwim
Traduit pour la revue Arctic par Nicole Giguere.
INTRODUCTION
The long-tailed duck (Clangula hyemalis), a sea duck (Mergini) species with a circumpolar distribution, nests in tundra habitats and winters along sub-Arctic and temperate coastlines. In North America, the ducks' breeding range includes western Alaska, the Arctic coastal plain of Alaska, and Arctic Canada; their wintering areas are located primarily along the Pacific and Atlantic coastlines, but also in the Great Lakes region (Robertson and Savard, 2002). Like many species of sea ducks in North America, long-tailed ducks have declined dramatically in number in recent decades without apparent cause. Population size estimates based on aerial counts of breeding birds in both Alaska and Canada's Yukon Territory are nearly 50% lower than estimates from the mid-1970s (Conant and Mallek, 2006). Difficulty identifying causal factors for this decline was largely attributed to the paucity of available biological information for the species. Despite the widespread distribution of long-tailed ducks, few studies have focused on them, principally because they often winter offshore and nest at very low densities in regions that seldom are accessible or surveyed (Robertson and Savard, 2002). Consequently, life-history attributes of long-tailed ducks are poorly understood because few estimates for basic vital rates (e.g., annual adult survival, nest success, duckling survival) are available (Alison, 1975; SDJV Management Board, 2001). In response to these data gaps, the SDJV (Sea Duck Joint Venture) Management Board (2001) has listed study of long-tailed duck population dynamics as a high priority.
Current management of long-tailed ducks is based largely on analyses of long-term aerial survey data of breeding pairs, from which estimates of population growth rate ([lambda],) are derived. However, traditional coverage of sea duck breeding areas during the breeding pair survey was incomplete and mistimed relative to sea duck breeding chronology. Thus, reliable data are available only for portions of their range where intense aerial surveys have been conducted, such as the Yukon-Kuskokwim Delta (YKD), Alaska (Platte and Stehn, 2006). Although these survey data are useful for determining population trends, in the absence of a demographic model, managers are unable to distinguish among the intrinsic processes underlying apparent trends. Demographic models integrate probabilities of vital rates into an estimate of [lambda], thereby providing a functional link between life history and population dynamics. This link is important because managers often can manipulate [lambda] in accordance with management goals by altering extrinsic factors linked to vital rates. Thus, demographic models enable managers to identify critical vital rates and develop focal management strategies to meet specific goals--in this case, the potential reversal of the apparent long-tailed duck population decline.
We developed a heuristic demographic model for a dispersed population of long-tailed ducks breeding on the YKD, Alaska. The YKD supports an estimated 30% of the breeding population in Alaska (Bellrose, 1980; Hodges et at., 1996; Mallek et al., 2006). We estimated variability in clutch size, nesting success, duckling survival, and apparent adult female survival from three disjunct study areas on the YKD and then integrated these estimates into a stage-based matrix model to examine the relative importance of each vital rate to [lambda].
METHODS
Study Areas
We studied long-tailed ducks at two coastal sites and one inland location on the YKD, Alaska (Fig. 1): the lower Kashunuk River (61[dregrees]20' N, 165[degrees]35' W), Kigigak Island (60[degrees]50' N, 165[degrees]50' W), and Aropuk Lake (61[degrees]07'N, 163[degrees]53' W). The Kashunuk River (KR) study site, located approximately 5 km from the Bering Sea coast, was described in detail by Grand et al. (1997). The Kigigak Island (KI) study area, located approximately 60 km south of the KR site, encompassed nearly the entire island, as described in detail by Moran (2000). At both sites, long-tailed ducks nested primarily in sedge meadow habitats that contained countless small, high-saline ponds and lakes. Specific nest sites included grass flats, slough banks, and pond edges. In contrast, the Aropuk Lake (AL) study site was located nearly 70 km inland from the Bering Sea coastline. Habitat surrounding AL consisted predominantly of dry lichen tundra interspersed with numerous freshwater lakes...
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