We conducted acoustic lure surveys of Northern Pygmy-Owls (Glaucidium gnoma) on 224 transects that were systematically placed in forested areas in Oregon. Our objectives were to determine if pygmy-owls were associated with particular forest types, regions, or with large trees. We also examined temporal variation in numbers of owls located. We detected pygmy-owls at 161 of 224 transects. The mean number of owls detected per linear km on individual transects was 0.21 (SE  =  0.01, range of means  =  0–1.07, N  =  224). Mean detections per transect were highest in the Western Cascades, intermediate in the Coast Ranges, Klamath Mountains, Eastern Cascades, and Blue Mountains, and lowest in the Willamette Valley and High Desert Ecoregions. The mean number of owls detected per transect was highest in mesic Douglas-fir (Pseudotsuga menziesii) and ponderosa pine forests (Pinus ponderosa), intermediate in conifer/hardwood and mixed-conifer forests, and lowest in alpine conifers, deciduous hardwoods, and western juniper (Juniperus occidentalis) woodlands. Also, the mean number of owls located per transect increased with the average diameter ranking of the dominant overstory trees. The mean number of owls detected per transect declined in July, and there was evidence of a decrease in the number of owls detected per survey station with increasing time after sunrise. Our data suggested that removal of large trees may have reduced numbers of Northern Pygmy-Owls, but this finding needs to be viewed in the context that our surveys also show that this species is a widely-distributed and fairly common forest bird in Oregon.

Nesting habitat, predator type, and level of reproductive effort influence nest defense behaviors in many bird species, yet no study has examined these or other possible factors influencing nest defense in a cross-species comparison for raptors. Using data from the literature, we grouped the nest defense behaviors of 19 diurnal North American raptors into four categories based on a gradient of aggressiveness. For each species, we identified the cover types where nesting occurred, accessibility of nest location, assessed two indices of reproductive effort, and examined associations between these factors and nest-defense behavior. We also we examined responses by raptor species to different predator types including diurnal avian, nocturnal avian, mammalian (not human), and human. Most raptor species with high reproductive effort exhibited very aggressive nest defense. Most raptor species nesting in open cover types and species with accessible nests showed aggressive nest defense. While many raptors react aggressively toward diurnal and nocturnal avian predators, they exhibit less aggressive defense against potential human predators. Results from this study suggest that a variety of factors may influence nest-defense strategies used by diurnal raptors. However, more work is needed on the relative influence of these factors (including predation risk) and variation in raptor nest defense strategies before general patterns can be elucidated.

We examined the diet of Tawny Fish-Owls (Ketupa flavipes) living along Sakatang Stream, Taiwan, from January 1994–July 1995 and from February 1998–January 1999. In both number and biomass, aquatic prey constituted the majority of the owls' diet. The most frequently caught prey were small freshwater crabs (Candidiopotamon spp.). Nevertheless, the Taiwan mitten crab (Eriocheir formosa) and the Taiwan common toad (Bufo bufo gargarizans) made up the largest proportion of prey biomass in different years. The breeding activity of prey species appeared to influence their representation in the owls' diet. Large and sluggish prey species such as Taiwan mitten crab and Taiwan common toad were preyed upon more frequently than expected relative to their availability.

We compared the migration ecology of two owl species that exhibit different migration strategies: the Flammulated Owl (Otus flammeolus) and the Northern Saw-whet Owl (Aegolius acadicus). During fall 1999–2004, we captured 117 Flammulated Owls and 1433 Northern Saw-whet Owls in the southern Boise Mountains of southwestern Idaho. These owl species exhibited contrasting seasonal timing and body condition. Flammulated Owl captures peaked in mid-September and Northern Saw-whet Owl captures peaked in early to mid-October. Flammulated Owls displayed greater body condition than Northern Saw-whet Owls and increasing condition scores during the season, whereas Northern Saw-whet Owls had no apparent seasonal condition patterns. Based on seasonal timing of captures, both species showed unimodal movement patterns characteristic of fall migrants. However, in 1999 both species' capture rates were at least double those in other years of this study. Flammulated Owls' earlier arrival and departure, coupled with superior body condition, were consistent among years and typical of a long-distance migration strategy. In contrast, the Northern Saw-whet Owls' later arrival, more lengthy passage, and variable body condition were more characteristic of a short-distance migrant strategy. Furthermore, Northern Saw-whet Owls' body condition was significantly lower during the irruptive year than during nonirruptive years, supporting the notion that population density affects their migratory condition.

We studied home range and habitat use of radio-tagged Great Gray Owls (Strix nebulosa) in Yosemite, California. From 1986–90 we made 5338 relocations on nine adult and three juvenile owls. Home-range size was not correlated with number of locations and was significantly different between breeding and nonbreeding periods. Breeding female summer home range averaged 61.47 ha and during the winter 2457.27 ha, while males average 19.89 and 2112.87 ha, respectively. Juveniles and nonbreeding birds had home-range sizes intermediate between seasonal values of breeding owls. Home ranges for California Great Gray Owls were larger than has been recorded for all studies in North America, but smaller than in Europe. All owls were found to have intensive high-use activity centers (x ¯  =  17.56 ha) in summer, with use patterns influenced primarily by meadows. Over 60% of all relocations occurred within 100 m of a meadow. Great Gray Owls habitat usage during summer was concentrated in fir (Abies spp.) and lodgepole (Pinus contorta) habitat types, while during the winter, birds moved to lower elevations into Sierra mixed conifer habitats. This post-breeding movement was the cause of the large nonbreeding home ranges. During winter, paired birds did not remain together, even though all birds moved to lower elevation habitats below deep snow-pack levels. We suggest that Great Gray Owls in California have responded to the relatively hot and southern habitat with unique adaptations that have allowed several local populations to persist within the upper montane Sierra Nevada forest zone. The protection of meadow foraging habitat, as well as nesting locations, will be important for the continued preservation of this southernmost North American population of Great Gray Owls in Yosemite National Park.

We tested two methods of releasing endangered, captive-bred Burrowing Owls (Athene cunicularia) in Saskatchewan, Canada. The first technique involved releasing pairs of captive-reared, adult owls. Twelve of 26 pairs remained together using this technique, while another six individuals paired with wild owls. Pairing/nesting success was poor when enclosures were left in place for only 3 d prior to release; success improved when enclosures remained for 5 d or until clutch initiation. At least 19% of released adults died during the breeding season, compared to only 3.7% for wild owls. At least five released adults failed to migrate. None of the captive-release adults returned to the study area in subsequent years, whereas 19% of banded wild owls returned during the same period. One of 62 offspring from released pairs returned to breed in a subsequent year; this recruitment rate was not different than that of offspring produced by wild adults. The second release technique involved fostering captive-hatched owlets into wild nests. We fostered 54 owlets at three different ages. Fostered chicks were accepted by wild owls; their growth, survival, and behaviors did not differ from their wild siblings'. Our results suggest that adults raised in captivity can breed successfully in the wild, but there are questions about their ability to migrate successfully. Fostering captive chicks into wild nests showed some success, but also had some limitations.

Presentamos datos recolectados con posterioridad a la publicación de McClelland et al. (1994). En ese estudio, 303 águilas de la especie Haliaeetus leucocephalus fueron marcadas en las patas con anillos del U.S. Fish and Wildlife Service en el Parque Nacional Glacier, Montana, entre 1977 y 1993. Además, se colocaron marcadores en el patagio de las alas en 121 de las 303 águilas, y a 66 de éstas se les acoplaron transmisores de VHF. Las edades estimadas de ocho águilas encontradas muertas tras la publicación de McClelland et al. estuvieron entre 2 años y ocho meses, y 22 años y seis meses. Observamos un águila con una marca patagial aún intacta, con el código legible 22 años y dos meses después de haber sido marcada. Tres de las águilas para las que presentamos datos (dos de 22 años y siete meses y una de 22 años y seis meses de edad)parecen ser registros récords de longevidad para individuos de esta especie anillados al oeste del Río Mississippi en los Estados Unidos continentales.

Resumimos y discutimos observaciones recientes de migraciones nocturnas de Pandion haliaetus. En el otoño de 2004, desde la plataforma de observación (elevación 320 m) del edificio Empire State en la ciudad de Nueva York se realizaron dos avistamientos nocturnos de individuos migrando hacia el sur. De igual forma, el monitoreo satelital ha revelado que P. haliaetus migra durante la noche de manera regular tanto en la temporada migratoria de otoño como en la de primavera, particularmente en vuelos largos que implican el cruce de cuerpos de agua. Encontramos que 15 individuos migraron hacia el sur durante la noche en otoño (1) de Carolina del Norte a Florida, (2) de Florida a Cuba y (3) de Cuba a Centroamérica/México o América del Sur. En la migración hacia el norte, dos individuos migraron durante la noche desde Sudamérica hacia Cuba. Algunos de los vuelos sobre agua representaron entre 12–22 horas de vuelo continuo. También proveemos información que muestra que al menos otras 10 especies de rapaces migran durante la noche de manera regular.

Presentamos una revisión de casos descritos de incubación prolongada en diferentes especies de rapaces, tanto diurnas como nocturnas. Siendo un mecanismo para asegurar que los huevos eclosionen, la incubación prolongada también presenta costos potenciales en términos de la reproducción futura relacionados con la posible pérdida de condición durante la incubación, los riesgos de depredación o las posibilidades de volver a criar en la misma temporada reproductiva. Por ello, la incubación prolongada debería observarse más frecuentemente en especies con bajo riesgo de depredación, sin reemplazo de puestas, con poca asincronía en la eclosión y con cuidado parental compartido. Evaluamos estas hipótesis con datos existentes para diferentes especies. La información compilada que presentamos sugiere que la incubación prolongada de huevos que no eclosionan parece ser un hecho habitual, y no se conoce si se trata de un comportamiento adaptativo. Sería necesario disponer de datos cuantitativos sobre la frecuencia de este comportamiento a nivel de especies para poder probar si éste es atribuible a las características individuales de las especies o está relacionado con variables ecológicas.

La dieta de Parabuteo unicinctus fue analizada utilizando 23 egagrópilas y 41 restos de presa recolectados durante la temporada reproductiva 2003–04 en el Aeropuerto Carriel Sur de Concepción, en el sur de Chile. Los mamíferos formaron la mayor parte de la dieta (64.7% por frecuencia, 70.6% por biomasa). Aunque Oligoryzomys longicaudatus fue la presa consumida (36.7%) con mayor frecuencia, los lagomorfos representaron la mayor parte de la biomasa(62.5%). Las aves siguieron a los mamíferos en importancia (26.4% por frecuencia, 29.3% por biomasa). La contribución de los reptiles e insectos fue menor. El peso geométrico promedio de las presas vertebradas (85.4 g) fue marcadamente más bajo en nuestro sitio de estudio que en Chile central (202.4–257.1 g). En general, la composición de la dieta de P. unicinctus en nuestro sitio de estudio fue muy similar a la documentada para otras localidades en Chile y Norte América.

Las especies Tyto alba y Bubo magellanicus son simpátricas en el noroeste de la Patagonia argentina, donde comparten ambientes y hábitos similares. La principal diferencia entre estas especies radica en su tamaño (300 g en T. alba vs. 800 g en B. magellanicus respectivamente). Por estas razones, estas especies constituyen un buen modelo para estudiar la partición del recurso presa. Se analizó el tamaño de las presas consumidas a nivel individual por ambas especies de rapaces. El peso de las presas se estimó a partir de ecuaciones obtenidas de medidas mandibulares. Se encontró que T. alba consume los individuos más pequeños de todas las especies presa consideradas en comparación con B. magellanicus, excepto en aquellas especies de menor tamaño. Además, B. magellanicus consume preferentemente individuos en clases de tamaño más grandes dentro de cada especie. Estos resultados apoyan la hipótesis de que los depredadores más grandes consumen presas de mayor tamaño, y que esto puede observarse mejor en presas que presentan un amplio rango de variación en peso.

Reportamos el primer registro de un nido de Strix rufipes con la presencia de una cría. Adicionalmente, presentamos algunos datos de actividad nocturna y comportamiento reproductivo. Finalmente, discutimos las posibles razones de la escasez de registros de nidificación de esta especie.