T. Luke George

On the Determinants of Extinction

We have extended the analyses of Pimm et al. on the risks of extinction to include attributes of the environment, as well as species characteristics, as potential determinants of the risk of extinction. We find their earlier conclusion that large-bodied species are at greater risk of extinction at high population sizes is not supported by the data, nor is the contention that more variable populations are at greater risk of extinction. Our analyses suggest that small populations of small-bodied birds on oceanic islands are more vulnerable to extinction than are large populations of large-bodied birds on more protected islands. Moreover, a significant interaction of body size with type of island (channel vs. oceanic island) indicates that body size influences time to extinction differently depending on the type of island. The potentially complex interactions among determinants of extinction suggested by these data and analyses indicate that conservation policies will need to consider the intricate interactions among characteristics of both the species and the environment. Furthermore, our analysis illustrates the inability of current models to account for any more than about 45% of the variance in extinctions of populations from an extensive data set on bird populations from small islands surrounding Britain. Thus, the ability of models to predict the risk of extinction of particular species on particular islands is still very limited. Policy concerning the management of reintroduced species, therefore, should include data from the species and environments in question as well as model predictions of the risk of extinction.

NEST PREDATORS, NEST-SITE SELECTION, AND NESTING SUCCESS OF THE DUSKY FLYCATCHER IN A MANAGED PONDEROSA PINE FOREST

Abstract Among hypotheses explaining nest-site selection, the potential-prey-site hypothesis predicts that birds place nests in areas with many potential nest sites, while the nest-concealment hypothesis predicts nest placement in sites with greater surrounding vegetation. We examined these hypotheses by comparing habitat attributes between Dusky Flycatcher (Empidonax oberholseri) nest sites and random sites, successful and unsuccessful nests, and nests depredated by birds versus mammals. Nesting success was 28% (n = 167), and predation was the most important cause of nest failure (96%) during the study (1998–2000) at our site in northeastern California. We identified 28 nest predators using surveillance cameras. Sciurid mammals were the most common predators (17 of 28, 61%), with Coopers Hawks (Accipiter cooperii; 25%), small owls (11%), and Stellers Jays (Cyanocitta stelleri; 3%) accounting for the rest. Raptors preyed on nestlings more frequently than on eggs, while small mammals depredated nestlings and eggs in proportion to their availability. Dusky Flycatchers nested in larger shrub patches with greater shrub cover than at random sites, supporting the potential-prey-site hypothesis. Successful nests were in larger shrub patches surrounded by fewer seedlings and saplings compared to depredated nests, providing equivocal support for the potential-prey-site hypothesis. Nests depredated by birds were less concealed, located in smaller shrub patches, surrounded by fewer seedlings and saplings, and were closer to the shrub edge than nests depredated by mammals. Our results, strengthened by identification of nest predators, suggest that differences in search strategies among predators may constrain the ability of Dusky Flycatchers to optimize nest-site selection. Depredadores de Nidos, Selección de Sitios de Anidación y Éxito de Anidación de Empidonax oberholseri en un Bosque Manejado de Pino Ponderosa Resumen. Entre las hipótesis que explican la selección de sitios de anidación, la de “presa potencial” predice que las aves ubican sus nidos en lugares con muchos sitios de anidación, mientras que la de “ocultación del nido” predice la ubicación de nidos en sitios con mayor vegetación circundante. Nosotros examinamos estas hipótesis comparando características del hábitat entre sitios de anidación del mosquerito Empidonax oberholseri y sitios aleatorios, entre nidos exitosos y no exitosos, y entre nidos depredados por aves y por mamíferos. Entre 1998 y 2000 en un área del noreste de California, el éxito de anidación fue del 28% (n = 167) y la depredación fue la causa más importante del fracaso de los nidos (96%). Identificamos 28 depredadores de nidos utilizando cámaras. Los mamíferos sciúridos fueron los depredadores más comunes (17 de 28, 61%), mientras que los demás nidos fueron depredados por Accipter cooperii (25%), búhos pequeños (11%) y Cyanocitta stelleri (3%). Las aves rapaces se alimentaron con mayor frecuencia de pichones que de huevos, mientras que los mamíferos pequeños depredaron pichones y huevos en proporción a su disponibilidad. Los mosqueritos anidaron en parches con mayor cobertura de arbustos que sitios aleatorios, lo cual apoya la hipótesis de presa potencial. Los nidos exitosos se ubicaron en parches de arbustos más grandes rodeados por menos plántulas y renovales en comparación con los nidos depredados, lo que apoya equívocamente la hipótesis de presa potencial. Los nidos depredados por aves estuvieron menos ocultos, ubicados en parches de arbustos más pequeños, rodeados por menos plántulas y renovales, y estuvieron más cerca del borde del matorral que los nidos depredados por mamíferos. Nuestros resultados, fortalecidos por la identificación de los depredadores, sugieren que las diferencias en las estrategias de búsqueda entre depredadores pueden limitar la habilidad de los mosqueritos para optimizar la selección de sitios de anidación.

Impacts of a Severe Drought on Grassland Birds in Western North Dakota

We studied the effect of a severe drought on the population dynamics and community structure of grassland birds in western North Dakota. During the spring and summer of 1988 the northern Great Plains suffered one of the warmest, driest periods in its recorded history. We compared the changes in bird populations and nesting productivity over a 3-yr period before, during, and after the drought. Total grassland bird density declined 61% (P < .05) between June 1987 and June 1988. Densities of six of eight common species declined significantly during the drought. Populations of all but two species recovered in 1989 and total bird density in June 1989 did not differ significantly from June 1987. Species richness and species diversity both declined significantly during the drought and recovered to predrought levels in 1989. Species richness declined more on fair condition than on good condition range during the drought. Vesper Sparrow (Pooecetes gramineus) hatching success, number of young fledged per successful nest, and nesting success were significantly lower in 1988 than either 1987 or 1989. Clutch size did not differ among the three years. The decline in nesting success in 1988 was primarily due to nest abandonment during incubation. Nesting of Vesper Sparrows, Horned Larks (Eremophila alpestris), and Western Meadowlarks (Sturnella neglecta) ended abruptly in mid-June 1988 during a period of extremely hot weather. In 1987 and 1989, nesting continued into July. Despite substantial reductions in bird density and productivity during the drought, many species recovered to predrought levels 1 yr following the drought. This suggests that year-to-year fluctuations in densities of some of these species may not be tightly linked to short-term changes in local productivity. However, sequential years of low productivity may have more substantial effects on these short-lived species. Thus, if drought conditions in North American grasslands become more frequent, as some climate models predict, there could be related changes in the avifauna of the region.

Greater Land Bird Densities on Island vs. Mainland: Relation to Nest Predation Level

Densities of Black—throated Sparrows (Amphispiza bilineata) are greater on Coronados Island in the Gulf of California than on the adjacent Baja peninsula, while those of Verdins (Auriparus flaviceps) and Costas Hummingbirds (Calypte costae) are not. I tested the hypothesis that the higher sparrow densities on the island were influenced by greater nesting success there. Nesting success for 1984 and 1985 combined was significantly higher on the island than the adjacent Baja California peninsula for the sparrows, but not for Verdins or hummingbirds. Predation on artificial nests placed in sites similar to those used by Black—throated Sparrows was significantly lower on the island. Damage to eggs in artificial nests indicated that birds were responsible were responsible for at least some of the nest failures; correspondingly, densities of avian nest predators were significantly higher on the mainland. These data are consistent with the hypothesis that higher densities of Black—throated Sparrows are due to re...

Priority research needs for the conservation of Neotropical migrant landbirds

Abstract Partners in Flight (PIF) is a consortium of professional and volunteer scientists and educators that promotes the conservation of landbird species. Central to the PIF conservation effort is the development of Bird Conservation Plans specific to each physiographic region of the United States. Without a coordinated prioritization of research needs, land managers, researchers, and funding agencies seeking to conserve landbirds lack direction. To address this issue, we (the Research Working Group of Partners in Flight) identified research priorities that have emerged recently as a result of Bird Conservation Plan development. Research priorities for the coming decade focus on habitat, specifically the identification of high-quality habitats and landscapes for breeding, migration, and wintering. Identification of the scale of breeding and natal dispersal and describing linkages between wintering and breeding populations are also research priorities for the coming decade. A summary of research priorities for each of the PIF regions (Northeast, Midwest, West, and South) is also provided. Specific research needs associated with priority species and habitats in each physiographic area can be accessed in a searchable database: http://www.partnersinflight.org/pifneeds/searchform.cfm.

RESPONSE OF PASSERINE BIRDS TO FOREST EDGE IN COAST REDWOOD FOREST FRAGMENTS

Abstract The response of passerine birds to forest edge was examined in old-growth and mature second-growth coast redwood (Sequoia sempivirens) forest in northern California. The study objectives were to determine which common passerine species are sensitive to edges during the breeding season and to estimate edge width for forest interior species. Response to edge was measured along twelve 100 × 400 m plots extending from the edge into the forest interior to obtain relative density of birds. Plots were surveyed 4 to 5 times in 1996 and 8 to10 times in 1997. We found that 14 common passerines showed a gradient of edge sensitivity. Stellers Jay (Cyanocitta stelleri) and Swainsons Thrush (Catharus ustulatus) had higher relative densities near edges than in the forest interior (P < 0.05) and were categorized as edge species. Brown Creeper (Certhia americana), Winter Wren (Troglodytes troglodytes), Pacific-slope Flycatcher (Empidonax difficilis), and Varied Thrush (Ixoreus naevius) had lower relative densities near edges (P < 0.05) and were categorized as interior birds. Based on exponential regression models, estimated edge widths were 140 m for Varied Thrushes, 85 m for Brown Creepers, 120 m for Winter Wrens, and 125 m for Pacific-slope Flycatchers. Creation of edges would probably benefit Stellers Jays (which may be a nest predator), may not benefit Swainsons Thrushes, and may be detrimental to species that avoid edges. We recommend that edge effects be taken into consideration when planning for the conservation of bird species in the region.

EFFECTS OF AVIAN PREDATION ON GRASSHOPPER POPULATIONS IN NORTH DAKOTA GRASSLANDS

We experimentally studied the effects of avian predation on grasshopper abundance in western North Dakota during the summers of 1988 and 1989. Grasshopper densities in 15 x 15 m plots from which birds were excluded (NO BIRDS) were compared with similar-sized plots where birds were allowed to forage (BIRDS). Plots were established in early June at 16 sites (8 per year), and grasshopper densities were estimated from hoop counts in NO BIRDS and BIRDS plots at 2-wk intervals until the end of July. There were significantly more grasshoppers in NO BIRDS plots than in BIRDS plots in 1989 (P .388). After the final hoop count each year, sweep-net sampling was also used to estimate grasshopper densities. By this method, average grasshopper density in late July was 26% and 37% lower in BIRDS plots than in NO BIRDS plots in 1988 and 1989, respectively. Average length, total biomass of grasshoppers, species richness, and species diversity, however, did not differ between the treatments. In 1988, 2 of 15 grasshopper species were significantly more abundant in the NO BIRDS plots. There was no difference between the treatments among 16 species identified in the 1989 samples, but the power of our tests to detect differences for individual species was low. Our results support the hypothesis that avian predation reduces insect populations at low and moderate densities.

Comparison of mechanically egg-triggered cameras and time-lapse video cameras in identifying predators at Dusky Flycatcher nests

Abstract We compared the effectiveness and reliability of mechanically egg-triggered set-cameras and time-lapse video cameras in identifying nest predators at active Dusky Flycatcher (Empidonax oberholseri) nests in Siskiyou County, California. We monitored 72 active flycatcher nests using camera systems from 1998 to 2000. Nest abandonment, hatching success, and daily survival rate did not differ between camera systems. Set-cameras were less reliable than video systems in successfully recording predation events (45% versus 95%). The nest predator assemblage between camera types differed significantly with more avian predators (primarily raptors) recorded at video-monitored nests. This may have been due to the failure of hawks and owls to take the trigger egg at set-camera nests during the nestling stage. We did not find a strong association between the condition of the nest (disturbed or undisturbed) after the predation event and the identity of the predator (mammalian or avian). We also documented disruption of the nest by visitors other than the original predator. These observations suggest that identification of nest predators based on nest remains can be highly inaccurate. We recommend the use of videography for accurate identification of nest predators. Results from studies identifying predators with egg-triggered cameras should limit conclusions to the incubation stage.

SONG POST AND FORAGING SITE CHARACTERISTICS OF BREEDING VARIED THRUSHES IN NORTHWESTERN CALIFORNIA

Abstract We used a hierarchical approach to describe habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius) in coastal redwood (Sequoia sempervirens) forests of northwestern California. We measured mesohabitat (0.04-ha circular plots) and microhabitat (0.5-m radius) scale attributes centered on occupied and random song posts and foraging locations at four study sites during March–August of 1994 and 1995. Ninety-five percent of song posts were in trees or snags. Male thrushes used song posts with low foliage density near the top of large conifers (microhabitat scale), located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water (mesohabitat scale) as compared to random locations. Varied Thrushes foraged predominantly on the ground early in the breeding season, then subsequently included fruit in their diet after the young had fledged. Although many variables were correlated with ground foraging locations, microhabitat foliage density had the greatest explanatory power, indicating thrushes selected foraging locations primarily at the microhabitat scale, and emphasizing the importance of measuring habitat characteristics at the appropriate spatial scale. Abrupt forest edges, such as those produced by clearcuts, may reduce habitat suitability for Varied Thrushes possibly explaining their absence from small forest fragments during the breeding season.

Survival and Growth of Nestling Vesper Sparrows Exposed to Experimental Food Reductions

We examined the effects of experimental food reductions on the reproductive biology of nestling Vesper Sparrows (Pooecetes gramineus) during 1991 in the Little Missouri National Grasslands, North Dakota. Grasshopper densities on territories around 24 individual nests were experimentally reduced while 31 nests served as controls. There were no significant differences between treatment and control nests in number of young fledged per nest, 2-8 day nestling growth rate, or final 8 day nestling mass. Nest failure was due to predation (83.3% of nests that failed), and abandonment or parental death (16.7% of nests that failed). Grasshoppers were the principal food, comprising 67.7% of all identified food items brought to all nests when nestlings were 7-8 days old. Breeding adults on treated territories foraged significantly further from the nest than control birds suggesting that birds on treated sites compensated for the reduction in food by altering foraging behavior.