The use of artificial nest boxes to bolster populations of endangered cavity-nesting birds has increased significantly, but spatial variation of nest-box occupancy rates and breeding success within a nest-box population has been little studied. In a case study with 798 Little Owl (Athene noctua) nest boxes established in central Germany, we analyzed the dependence of occupancy rate and breeding success on the characteristics of the surrounding habitat. The analysis focused on two aspects of general concern for nest-box management: (1) whether nest boxes were occupied for breeding or were left unoccupied, and (2) whether Little Owls had different reproductive rates, depending on the location of nest boxes. A high resolution (1 × 1 m) land-use map was used to analyze species-habitat relationships, and Generalized Linear Mixed Models were used to predict suitable nest-box locations. During the period from 2004 to 2006, 544 (68%) of the nest boxes were never occupied, 144 (18%) housed birds with low breeding success and only 108 (14%) housed pairs that produced more than 2.35 nestlings annually, a reproductive rate thought necessary for population stability. Nest boxes were more likely to be occupied if they were located near orchards, at lower altitude, and in areas of higher densities of fields and less forest. Higher breeding success was associated with fallow fields and field margins, and with greater distance to roads and forests. Our results suggested that the efficiency of this nest-box program could be substantially increased if unoccupied nest boxes were relocated to sites where occupancy is more likely, and if unproductive nest boxes were relocated to locations that would enhance breeding success.

Nest predation is the primary cause of nest failure in birds. Individuals should therefore adjust parental investment to minimize the costs associated with this constraint; evidence suggests that nest predation influences nest-site selection, and drives variation in both clutch size and parental behavior. Here, we test how the perception of the risk of nest predation from red squirrels (Tamiasciurus hudsonicus) influenced nest-site selection and reproductive investment of American Kestrels (Falco sparverius) breeding in the boreal forest. For this purpose, we conducted audio playbacks of squirrel vocalizations and altered nest boxes to experimentally increase cues of the presence of Red Squirrels in the vicinity of potential nests. Experimental manipulations of the risk of nest predation did not influence nest-site selection; however, experimentally increasing the perceived risk of nest predation induced kestrels to initiate breeding later, and to lay larger clutches. Parents did not appreciably alter incubation behavior in response to our manipulation, although the duration of incubation was longer where natural squirrel threat was higher. Our results showed that kestrels are capable of making facultative adjustments to current reproductive investment in response to their perception of the risk of nest predation.

Some raptors are considered to be generalist predators and switch among different species of prey according to their availability in the environment. In contrast, other raptor species are specialists with narrow dietary niche widths, selecting only one or a few types of prey; reproductive success in these species is often closely related to variability in abundance of preferred prey species. Determining whether raptors are generalist or specialist predators is important for understanding the degree to which fluctuations in their population sizes are influenced by individual responses to changes in prey availability. We assessed diet components based on prey remains collected at nest ledges to determine whether Peregrine Falcons breeding along the Yukon River in northern Canada were specialist feeders or, conversely, whether they took prey according to its relative abundance. We also examined whether the use of certain types of prey had consequences for reproductive success. Compositional analyses showed that peregrines exhibited nonrandom prey selection, with shorebirds, jays and thrushes, and waterbirds being used more than expected based on their abundance, whereas sparrows and other small passerines were used significantly less than expected. Similarly, medium-sized prey was selected by peregrines, but small (<50 g) and large (>1000 g) prey were used significantly less than expected. Pairs occupying territories in which the diet included large numbers of waterbirds, and few sparrows and small passerines, fledged more young. Although prey >1000 g were among the least selected by pairs in our study population, pairs that used these larger prey species fledged more offspring. These patterns may relate directly to the contribution of female parents to provisioning. Females that are able to capture large prey items may deliver larger amounts of biomass to their nests and, as a consequence, may have high reproductive output. We conclude that Peregrine Falcons in the Yukon are selective predators and that some of the variation in reproductive rates in this population may be attributed to variation in the types of prey available to and utilized by individual pairs.

We estimated annual survival rates (S) of 23 radio-marked Northern Spotted Owls (Strix occidentalis caurina) at the Quartz and Timbered Rock burns and adjacent areas in southwest Oregon. We used known-fate models in program MARK to test for differences in survival among three groups (owls dwelling inside of burned areas, owls displaced by wildfire, and owls dwelling outside of burned areas) and to investigate the potential effects of fire severity and cover type on survival. We found that annual survival rates of Spotted Owls that maintained a territory within the burn perimeter (Ŝ = 0.69 ± 0.12) or that had been displaced by the burn (Ŝ = 0.66 ± 0.14) were lower than those of owls that had territories outside but adjacent to the burn perimeter (Ŝ = 0.85 ± 0.06). This result indicated that wildfire had a negative impact on Spotted Owl survival, at least in the short-term (i.e., 3–5 yr postburn). We were unable to identify a significant effect of fire severity or quantity of habitat on Spotted Owl survival, but this inability was likely related to our small sample size and high variability in habitat composition. We recommend long-term monitoring of survival on larger samples of owls to identify the effects of fire severity and habitat on Spotted Owl survival and to determine if wildfire poses a significant threat to the long-term persistence of Spotted Owls.

The sex determination of Eurasian Sparrowhawks (Accipiter nisus) is not apparently a problem, as sexual dimorphism seems to be evident, at least in adults. However, there is a clinal geographic variation expressed as increasing size and decreasing color saturation from west to east. Hence, in wintering regions such as the Iberian peninsula, where migrant birds headed toward Africa also occur, sedentary birds are mixed with migrant and wintering birds from September to April. Here, we would expect to find birds from different breeding areas and therefore, possessing notable differences in biometrics, body mass, or color. We trapped eighteen adult sparrowhawks during the breeding seasons between 2001 and 2007 in Bizkaia, northern Spain, and we measured 91 dead and 38 rehabilitated sparrowhawks from Spanish Wildlife Rehabilitation Centers (WRC) between 1999 and 2007. A total of 147 full-grown sparrowhawks were sexed, aged, and measured. Some overlap between sexes was found for all but four measurements (eighth primary length [P8], wingspan, forearm, and mass). Only bill length differed significantly between age classes (juvenile/adults) and wingspan differed between sedentary and wintering birds for both sexes. Three variables: bill length, forearm, and P8 were retained in the discriminant function, allowing us to determine the sex with 100% accuracy. It is possible to correctly sex sparrowhawks, both adults and juveniles, using the three measures, even when the origin of the bird is unknown.

Following the reintroduction of the Bearded Vulture (Gypaetus barbatus) in the western Alps in 1987, the species reappeared in the Gran Paradiso National Park, Italy, where it had not been recorded since 1930. We analysed 1157 sighting records collected inside the park borders from 1989 to 2007. The number of sightings per year was not correlated with the number of captive-bred individuals released during the same year in the western Alps but was positively correlated with the number released one year prior and two years prior. Bearded Vultures were recorded mainly at higher altitudes during warmer months, and at lower altitudes when the terrain was mostly covered by snow, as were the two most abundant ungulates of the park, the alpine ibex (Capra ibex) and the alpine chamois (Rupicapra rupicapra), whose carcasses were primary food sources for vultures. Three habitats were used with a frequency significantly higher than expected based on availability: vegetated cliffs and screes, forest-scrub mosaic, and agriculture. Bare rocks and deciduous forests were used less frequently than expected, and other habitats were used in the same proportion as expected, including alpine grassland, coniferous forests, and alpine heaths and scrubs.

Winter diet of the Short-eared Owl (Asio flammeus) in Texas is little known. We investigated the diet of Short-eared Owls wintering in McMullen County, in subtropical Texas, by analyzing the contents of 129 pellets collected over two winters (28 November 2007 to 22 February 2008 and 11 December 2008 to 11 February 2009) and conducted a latitudinal-based comparison of published diet studies of Short-eared Owls. In southern Texas, we recovered the remains of 162 prey items, 98% of which were vertebrates. Hispid cotton rats (Sigmodon hispidus) were the most important prey species in terms of percent of total number (67%) and percent of total biomass (87%). Most (86%) Short-eared Owl diet studies (based on ≥100 pellets) have been conducted north of 35°N, with only six studies, including the present study, conducted at or south of 35°N latitude. Voles (primarily Microtus spp.) were the dominant prey in North American studies (71%), but microtines were not the dominant prey in any of the six studies conducted south of 35°N latitude. We suggest that Short-eared Owls do not specialize on microtines, as is often implied, but rather depend on rodents with cyclic populations, such as the hispid cotton rat in southern areas.

Barn Owls (Tyto alba javanica) have been widely introduced in Malaysian oil palm plantations to control rodent pests. However, their effectiveness in regulating rodent populations is unknown. We investigated whether Barn Owls selected prey with respect to size and sex classes based on data from 128 pellets of Barn Owls compared to 1292 live-trapped rats in an oil palm plantation in Malaysia. The birds mostly fed on Rattus rattus diardii, the most commonly trapped species. Body mass of prey consumed was predicted based on models derived from measurements from trapped rats. Sex of prey was determined by pelvic measurements with reference to those taken from specimens of known gender. There was no clear selection of prey by Barn Owls in relation to size or sex of prey, and no difference in the body mass of prey between the owls' breeding and nonbreeding seasons. The absence of differential predation in Barn Owls may partly explain the lack of clear evidence that they regulate rodent populations and thus act as successful biological control agents.

Two similar-sized scops-owl species, the Elegant Scops-Owl (Otus elegans) and the Japanese Scops-Owl (O. semitorques) occur syntopically in a forest on Okinawa Island, Japan. We investigated diets of these owls to evaluate the breadth of their food niches and examine potential mechanisms that might differentiate their food niches. Elegant Scops-Owls fed predominantly on orthopterans, whereas Japanese Scops-Owls took prey from many taxonomic groups. Although we observed dietary overlap, food niches were differentiated by prey size and habitat. Elegant Scops-Owls mostly foraged on arboreal prey, whereas Japanese Scops-Owls took both arboreal and terrestrial prey. Elegant Scops-Owls specialized on small prey, whereas Japanese Scops-Owls preyed equally on small- to large-sized prey. Foraging habitat of these owls may reflect differences in prey locomotion, and differences in prey sizes may correspond to body-size differences between the two species. Food-niche differences may relax competition between these species.