Susan L. Earnst

DOUBLE SAMPLING TO ESTIMATE DENSITY AND POPULATION TRENDS IN BIRDS

Abstract We present a method for estimating density of nesting birds based on double sampling. The approach involves surveying a large sample of plots using a rapid method such as uncorrected point counts, variable circular plot counts, or the recently suggested double-observer method. A subsample of those plots is also surveyed using intensive methods to determine actual density. The ratio of the mean count on those plots (using the rapid method) to the mean actual density (as determined by the intensive searches) is used to adjust results from the rapid method. The approach works well when results from the rapid method are highly correlated with actual density. We illustrate the method with three years of shorebird surveys from the tundra in northern Alaska. In the rapid method, surveyors covered ∼10 ha h–1 and surveyed each plot a single time. The intensive surveys involved three thorough searches, required ∼3 h ha–1, and took 20% of the study effort. Surveyors using the rapid method detected an average of 79% of birds present. That detection ratio was used to convert the index obtained in the rapid method into an essentially unbiased estimate of density. Trends estimated from several years of data would also be essentially unbiased. Other advantages of double sampling are that (1) the rapid method can be changed as new methods become available, (2) domains can be compared even if detection rates differ, (3) total population size can be estimated, and (4) valuable ancillary information (e.g. nest success) can be obtained on intensive plots with little additional effort. We suggest that double sampling be used to test the assumption that rapid methods, such as variable circular plot and double-observer methods, yield density estimates that are essentially unbiased. The feasibility of implementing double sampling in a range of habitats needs to be evaluated.

FACTORS INFLUENCING NEST SUCCESS OF SONGBIRDS IN ASPEN AND WILLOW RIPARIAN AREAS IN THE GREAT BASIN

Abstract ABSTRACT Recent studies have examined the effects oflivestock grazing, agriculture, and humanhabitation on nest predation and brood parasitismin riparian areas in the western United States.However, we know little about factors influencingnest success in riparian areas lacking suchanthropogenic influences, in part because theinfluences are so pervasive. We studied riparianbird communities in a 115 000 hawildlife refuge where livestock grazing wasdiscontinued >10 years ago, and which haslittle nearby agriculture or human habitation. Wemonitored nests on 24 aspen (Populustremuloides) and 10 willow(Salixspp.) plots. Brood parasitism rates weresubstantially lower than at other western sites anddid not differ between aspen and willow habitats.Nest success in aspen was relatively high comparedto that reported for other western sites and higherthan in willow. Predators may have been able tofind nests more efficiently in willow than in aspenbecause territory densities were higher in willow(40 versus 30 pairs per ha,respectively), because willow had lessstructural heterogeneity, or both. We did not findstrong evidence that nest success was influenced byaspen patch size or distance to riparian edge,indicating that even small aspen patches providevaluable nesting habitat. Weather was an importantcause of nest failure, particularly at higherelevations during late-spring snowstorms. Ourresults indicate that riparian areas without majoranthropogenic impacts, especially aspen stands,constitute high-quality breeding habitat andwarrant conservation focus.

The timing of wing molt in tundra swans: energetic and non-energetic constraints

Date of wing molt initiation, based on the regression of tenth primary length on capture date, was calculated for breeding and nonbreeding Tundra Swans (Cygnus columbianus columbianus) on the Colville River Delta, Alaska. Breeding females initiated wing molt significantly later than breeding males and nonbreeding males and females; the molt of breeding females was correlated with the date on which their eggs hatched. Breeding males did not differ significantly from nonbreeding males and females in the date of molt initiation. Timing of molt in breeding males and females was consistent with the views that females delay molt while replenishing energy spent on reproduction, but was also consistent with the breeding pairs need for primaries to defend territories and to defend and brood young. Other results, including an increase in an index of female body condition throughout most of the molt period, and a positive correlation between clutch size and female hatchto-molt interval, were not predicted by the hypothesis that past energy expenditures constrain the timing of molt. Patterns of wing molt within and among other Northern Hemisphere geese and swans are also difficult to explain on the basis of energetics alone. For example, breeding females initiate molt before breeding males in many species. Also, there is extreme asynchrony between mates in two swan species; one of those species also exhibits variation in which sex initiates wing molt first. Both patterns suggest that asynchrony, per se, is important, probably to facilitate brood protection or territory defense. In Tundra Swans and other northern breeding geese and swans, the non-energetic demands of territory defense, brood defense, and brooding are probably important constraints on the timing of wing molt.

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.

Changes in Avian and Plant Communities of Aspen Woodlands over 12 Years after Livestock Removal in the Northwestern Great Basin

Riparian and quaking aspen (Populus tremuloides) woodlands are centers of avian abundance and diversity in the western United States, but they have been affected adversely by land use practices, particularly livestock grazing. In 1990, cattle were removed from a 112,500-ha national wildlife refuge in southeastern Oregon. Thereafter, we monitored changes in vegetation and bird abundance in years 1-3 (phase 1) and 10-12 (phase 2) in 17 riparian and 9 snow-pocket aspen plots. On each 1.5-ha plot, we sampled vegetation in 6 transects. Three times during each breeding season, observers recorded all birds 50 m to each side of the plots 150-m centerline for 25 minutes. We analyzed data with multivariate analysis of variance and paired t tests with p values adjusted for multiple comparisons. In both periods, riparian and snow-pocket aspen produced extensive regeneration of new shoots (stems/ha and 7079 stems/ha, respectively). By phase 2, a 64% increase in medium-diameter trees in riparian stands indicated successful recruitment into the overstory, but this pattern was not seen in snow-pocket stands, where the density of trees was over 2 times greater. By phase 2 in riparian and snow-pocket stands, native forb cover had increased by 68% and 57%, respectively, mesic shrub cover had increased by 29% and 58%, and sagebrush cover had decreased by 24% and 31%. Total avian abundance increased by 33% and 39% in riparian and snow-pocket aspen, respectively, ground or understory nesters increased by 133% and 67% and overstory nesters increased by 34% and 33%. Similarly, ground or understory foragers increased by 25% and 32%, aerial foragers by 55% and 57%, and overstory foragers by 66% and 43%. We interpreted the substantial regeneration of aspen shoots, increased densities of riparian forbs and shrubs, and increased avian abundances as a multitrophic-level response to the total removal of livestock and as substantial movement toward recovery of biological integrity.

Avian response to wildfire in interior Columbia Basin shrubsteppe.

Abstract. Wildfire and conversion of sagebrush (Artemisia spp.) shrublands to cheatgrass (Bromus tectorum) grasslands is a serious threat to the shrubsteppe ecosystem, but few studies have documented wildfires effects on birds with multiple years of pre-and post-fire data. Using data from avian point counts recorded 4 years before and 7 years after a large-scale, severe wildfire in the Columbia Basin of south-central Washington, we found significant effects of fire on population trends or mean abundance of nearly all species investigated. The Sage Sparrow (Amphispiza belli), a sagebrush obligate, was decreasing at a high rate both pre- and post-fire. Among species inhabiting more open shrubsteppe or grasslands, the mean abundance of three (Grasshopper Sparrow, Ammodramus savannarum; Western Meadowlark, Sturnella neglecta; Vesper Sparrow, Pooecetes gramineus) was lower post-fire and one (Lark Sparrow, Chondestes grammacus) showed an initial, but short-lived, increase post-fire before dropping below pre-fire levels. Only one (Horned Lark, Eremophila alpestris) increased steadily post-fire and had higher post-fire mean abundance.

Status of the White-faced Ibis: Breeding colony dynamics of the Great Basin population, 1985-1997

-The status of the White-faced Ibis (Plegadis chihi) in the Great Basin is of concern because of its small population size and the limited and dynamic nature of its breeding habitat. We analyzed existing annual survey data for the White-faced Ibis breeding in the Great Basin and surrounding area for 1985-1997. Methods varied among colonies and included flight-line counts and fixed-wing aircraft and helicopter surveys. The number of White-faced Ibis breeding pairs in the Great Basin area has nearly tripled since 1985, despite years of severe flooding and drought at major breeding areas. This growth is reflected in both peripheral (i.e., Oregon, California, Idaho) and core (i.e., Nevada and Utah) components of the population. Our data on colony dynamics in Oregon and Nevada illustrate the ability of the highly nomadic White-faced Ibis to compensate for poor conditions at traditional colony sites by moving among colonies and rapidly colonizing newly available wetlands. We suggest that the White-faced Ibis would benefit from a landscape mosaic of well-distributed peripheral wetlands and persistent colony sites. The nomadic nature of the White-faced Ibis and the dynamic nature of their breeding habitat necessitates that wetland management decisions and population monitoring be conducted in a regional context. Received 30 March 1998; accepted 22July 1993.

Habitat Selection by Tundra Swans on Northern Alaska Breeding Grounds

Abstract Habitat selection by the Tundra Swan (Cygnus columbianus columbianus) was evaluated on the Colville River Delta prior to oil field development (1982-1989). Tundra Swan territories comprised a lake, used for refuge and foraging, and terrestrial habitats and ponds near the lake’s perimeter used for foraging and nesting. Tundra swan sightings from early and late summer aerial surveys were used to investigate habitat selection at the territory and within-territory scale. At the territory or lake scale, swan sightings/lake increased with lake size, and increased from discrete to tapped (i.e., connected to a river channel) to drained lakes within size categories. Overall, 49% of the variation in swan sightings/lake was explained by lake size and type, a size-x-type interaction term, and the proportion of lake perimeter comprised of Halophytic Ponds and Halophytic Wet Meadows. At the within-territory or within-lake scale, foraging swans significantly selected Halophytic Ponds, Halophytic Wet Meadows, and Fresh Ponds relative to Uplands; nesting swans significantly selected Halophytic Ponds and significantly avoided Fresh Wet Meadows relative to Uplands. Vegetation sampling indicated that sites used by Tundra Swans on river channels and tapped lakes were significantly more likely to have Sheathed Pondweed (Potamogeton vaginatus) than control sites. The three major components of Tundra Swan diet were Carex sedges, Sheathed Pondweed, and algae, together comprising 85% of identifiable plant fragments in feces.