Robert A. Stehn

POPULATION SIZE AND TREND OF YELLOW-BILLED LOONS IN NORTHERN ALASKA

Abstract The Yellow-billed Loon (Gavia adamsii) is of conservation concern due to its restricted range, small population size, specific habitat requirements, and perceived threats to its breeding and wintering habitat. Within the U.S., this species breeds almost entirely within the National Petroleum Reserve-Alaska, nearly all of which is open, or proposed to be opened, for oil development. Rigorous estimates of Yellow-billed Loon population size and trend are lacking but essential for informed conservation. We used two annual aerial waterfowl surveys, conducted 1986–2003 and 1992–2003, to estimate population size and trend on northern Alaskan breeding grounds. In estimating population trend, we used mixed-effects regression models to reduce bias and sampling error associated with improvement in observer skill and annual effects of spring phenology. The estimated population trend on Alaskan breeding grounds since 1986 was near 0 with an estimated annual change of−0.9% (95% CI of−3.6% to +1.8%). The estimated population size, averaged over the past 12 years and adjusted by a correction factor based on an intensive, lake-circling, aerial survey method, was 2221 individuals (95% CI of 1206–3235) in early June and 3369 individuals (95% CI of 1910–4828) in late June. Based on estimates from other studies of the proportion of loons nesting in a given year, it is likely that <1000 nesting pairs inhabit northern Alaska in most years. The highest concentration of Yellow-billed Loons occurred between the Meade and Ikpikpuk Rivers; and across all of northern Alaska, 53% of recorded sightings occurred within 12% of the area. Tamaño y Tendencia Poblacional de Gavia adamsii en el Norte de Alaska Resumen. Gavia adamsii es una especie cuyo estado de conservación es preocupante debido a su rango de distribución restringido, su pequeño tamaño poblacional, sus requerimientos de hábitat específicos y a la inminente amenaza sobre sus hábitats reproductivos y de invernada. Dentro de los Estados Unidos, esta especie se reproduce casi enteramente dentro de la Reserva Nacional de Petróleo en Alaska, área que está disponible, o estará disponible para ser la explotación de petróleo. Actualmente, existe una carencia de estimadores rigurosos de los tamaños poblacionales y de sus tendencias, a pesar de que estos parámetros son esenciales para la conservación informada de esta especie. Utilizamos dos conteos aéreos anuales de aves acuáticas, uno realizado entre 1986–2003 y el otro entre 1992–2003, para estimar el tamaño poblacional y su tendencia en los territorios reproductivos del norte de Alaska. Para estimar la tendencia poblacional utilizamos modelos de regresión de efectos mixtos, de manera de reducir el sesgo y el error de muestreo asociados a la mejora de las aptitudes del observador y a los efectos anuales de la fenología de primavera. La tendencia poblacional estimada en los territorios reproductivos de Alaska desde 1986 fue cercana a 0, con un cambio anual estimado del−0.9% (95% IC de−3.6% a +1.8%). El tamaño poblacional estimado, promediado a lo largo de los 12 años y ajustado mediante un factor de corrección basado en un método de muestreo aéreo intensivo de vuelos circulares, fue de 2221 individuos (95% IC de 1206–3235 individuos) a inicios de junio y de 3369 individuos (95% IC de 1910–4828 individuos) a fines de junio. Con base en estimaciones de otros estudios sobre las proporciones de G. adamsii que se encontraban nidificando en un año determinado, es probable que hayan habitado <1000 parejas nidificantes en el norte de Alaska en la mayoría de los años. La mayor concentración de G. adamsii fue observada entre los ríos Meade y Ikpikpuk; y a través de todo el norte de Alaska, el 53% de los avistamientos fue registrado en un 12% del área.

Glaucous Gull predation of goslings on the Yukon-Kuskokwim Delta, Alaska

Abstract Glaucous Gulls (Larus hyperboreus) nesting on the Yukon-Kuskokwim (Y-K) Delta frequently prey on juvenile waterfowl. We collected 434 Glaucous Gulls from late June to early August 1994 to examine diet. Identification of undigested prey tissue, based on DNA microsatellite loci, showed three species of goslings in gull stomachs: Emperor Goose (Chen canagica), White-fronted Goose (Anser albifrons), and Cackling Canada Goose (Branta canadensis minima). Gulls that nested inland and were collected >1.6 km from the coast accounted for approximately 70% of the total gull predation on Emperor and Canada Geese, and 96% on White-fronted Geese. Our stratified sample of gull stomachs and aerial survey estimates of population size and distribution of gulls and juvenile geese enabled extrapolation of species-specific predation rates to the entire Y-K Delta. We estimated that a minimum of 21 000 Emperor Goose, 34 000 Canada Goose, and 16 000 White- fronted Goose goslings were consumed by 12 600 Glaucous Gulls during the brood-rearing period on the Y-K Delta in 1994. Minimum estimated take by gulls represented 33% of Cackling Canada Goose, 47% of Emperor Goose, and 39% of White-fronted Goose eggs estimated to have hatched in the same area as gull collections. Gulls selected the three species of geese approximately in proportion to their abundance. Although gull predation caused significant gosling mortality, its role in regulating goose populations on Y-K Delta remains unresolved. Depredación de Pichones de Gansos por Gaviotas Larus hyperboreus en el Delta del Yukon-Kuskokwim, Alaska Resumen. Las gaviotas Larus hyperboreus que nidifican en el delta del Yukon-Kuskokwim (Y-K) depredan aves acuáticas juveniles con frecuencia. Para examinar su dieta, colectamos 434 gaviotas de esta especie entre finales de junio y principios de agosto de 1994. Identificamos los tejidos de presas no digeridos con base en loci de ADN microsatelital y encontramos pichones de tres especies de gansos (Chen canagica, Anser albifrons y Branta canadensis minima) en los estómagos de las gaviotas. Las gaviotas que estaban nidificando tierra adentro y que fueron colectadas a más de 1.6 km de la costa representaron aproximadamente el 70% del total de las depredaciones de C. canagica y B. canadensis y el 96% de las de A. albifrons. Nuestra muestra estratificada de estómagos de gaviotas, junto con estimaciones del tamaño poblacional de las gaviotas y gansos juveniles hechas mediante censos desde el aire, permitieron hacer extrapolaciones de tasas de depredación especie- específicas para todo el delta Y-K. Los números estimados mínimos de pichones depredados por 12 600 gaviotas en el delta durante el período de cría de 1994 fueron 21 000 C. canagica, 34 000 B. canadensis y 16 000 A. albifrons. Estimamos que en lás áreas en que fueron colectadas, las gaviotas consumieron como mínimo el 33%, 47% y 39% del número estimado de huevos allí eclosionados de B. canadensis, C. canagica y A. albifrons, respectivamente. Las gaviotas seleccionaron a las tres especies de gansos aproximadamente en proporción a su abundancia. Aunque la depredación por gaviotas causó una mortalidad significativa de los pichones, aún debe determinarse su papel en la regulación de las poblaciones de gansos en el delta Y-K.

Variation in Age Ratio of Midcontinent Greater White-Fronted Geese During Fall Migration

Abstract Annual productivity is an important parameter for the management of waterfowl populations. Fall age ratio (juveniles:total birds) is an index of productivity of the preceding breeding seas...

Sabine's Gull (Xema sabini) Nesting Aggregations in Western Alaska

Abstract We analyzed data on the size of nesting aggregations of Sabines Gulls (Xema sabini) observed in the Yukon–Kuskokwim Delta (YKD), Alaska, between 1986 and 2013, to determine whether the species is a “solitary low-density nester” there, as in other parts of its range. Data for 1087 nests show that 56% of Sabines Gull nests were solitary and that <3% of aggregations exceeded 10 nests, although factors positively correlated with the evolution of colonial nesting, including exposure of nests to predators, aquatic habitat, and absence of feeding territories, apply to Sabines Gulls nesting in the YKD. Although Sabines Gulls in the YKD were most often solitary nesters, in 2010 we found a 102-nest aggregation on a small island in the study area, larger than any reported for the species. Among all aggregated nests, 75.4% occurred on islands, and only 13.9% along shorelines, while 39.8% of all solitary nests occurred on islands, and 39.3% along shorelines. The proportion of Sabines Gull nests in the YKD located on islands ranged from 0.15 (in 1991) to 0.78 (in 2010) and showed no significant trend between 1986 and 2013, despite an increase in population size during the period. Although islands may provide greater protection from mammalian predators, and the majority (54.6%) of Sabines Gull nests in our study area occurred on islands, thousands of similar, apparently suitable islands remained unoccupied by Sabines Gulls and other nesting species every year. Given the potential benefits of nesting on islands, in colonies, or in colonies on islands, the YKD Sabines Gull population maintains considerable behavioral variability, in terms of nest-site selection and size of nesting aggregations, perhaps to take advantage of locally abundant food resources.