Burrowing Owl (Athene cunicularia) population declines have led to the owl's designation as a species of conservation concern in South Dakota. Burrowing Owls nest primarily in black-tailed prairie dog (Cynomys ludovicianus) colonies, but a significant proportion of colonies in South Dakota are not occupied by owls. We studied the influence of landscape-level habitat variables on colony selection by Burrowing Owls. We used call-playback surveys to document presence or absence of Burrowing Owls at 613 prairie dog colonies throughout western and central South Dakota. We used a geographic information system to calculate the percent cover of prairie dog colonies, grassland, cropland, and tree canopy in the surrounding landscape at four buffer sizes. We modeled Burrowing Owl occupancy of prairie dog colonies using logistic regression, and ranked models using Akaike's Information Criterion. All competitive models contained a tree-canopy-cover variable. Increasing tree canopy cover within 800 m and 1200 m of colony centers was associated with decreasing likelihood of occupancy by Burrowing Owls. Grassland, cropland, and prairie dog colony cover variables did not influence occupancy by Burrowing Owls, and these variables did not improve model fit or discrimination. In landscapes where the presence of nesting burrows is not a limiting factor, as in central and western South Dakota, Burrowing Owls occupied colonies based on the absence of trees. Trees provide habitat for avian and mammalian predators and reduce the available foraging area for Burrowing Owls around prairie dog colonies. Management for Burrowing Owls should include conserving prairie dog colonies in landscapes with few trees and preventing the establishment of trees near occupied colonies.

Urbanization has increased with human population growth and the responses from raptor species are gaining more attention from both researchers and the public. Northern Goshawks (Accipiter gentilis) now breed in urban areas in Japan and Europe; however, there are few studies examining the factors that influence their reproductive rate in urban areas. We investigated the reproductive rate (number of fledglings per nest) of the Northern Goshawk population in an urbanized area of Japan from 2014 to 2016, and used a binomial mixture model to examine the relationship between the number of fledglings per nest and environmental factors such as nesting and foraging environments, anthropogenic disturbance, predation risk, and intraspecific competition. The goshawk nesting success rate from 2014 to 2016 was 71.6%, with an average reproductive rate of 1.7 fledglings per occupied nest. The percentage of canopy cover of nesting stands had a significant positive effect on fledgling numbers, and the number of adjacent occupied nests had a significant negative effect on fledgling numbers. The positive effects of canopy coverage may be explained by the protection offered by canopy against direct sunlight, wind, and rain. The negative effect of the adjacent occupied nests may result from an increase in the amount of time and energy goshawks spent in territory defense, and a decrease in available foraging habitat due to intraspecific competition.

We recorded molt patterns of migrant Northern Saw-whet Owls (Aegolius acadicus) at three locations in the Great Lakes region: Stevens Point, Wisconsin; Whitefish Point, Michigan; and Duluth, Minnesota. We then analyzed the molt patterns of recaptured birds that were banded in a previous season and originally classified as hatch-year birds (i.e., they had no remigial molt). Of these 286 known-age birds included in our analysis, 155 (54%) were second-year (SY), 72 (25%) were third-year (TY), 39 (14%) were fourth-year (4Y), 16 (6%) were fifth-year (5Y), three (1%) were sixth-year (6Y), and one (0.30%) was eighth-year (8Y). Using these known-age birds, we were able to identify definitive molt patterns for the SY and TY age classes. All 155 (100%) SY birds and 52 (72%) of the TY birds matched our ageing criteria. The remaining 28% of TY birds did not match our ageing criteria and were classified as after-second-year (ASY). Of the entire sample (n = 286), only three birds (1%) matched an incorrect age class, including one 4Y that matched the SY age class and two 5Y that matched the TY age class.

We investigated Golden Eagle (Aquila chrysaetos) diet on the Seward Peninsula, Alaska, at 35 and 30 occupied eagle nest sites in July 2014 and 2015, respectively. We used 52 prey remains and 456 regurgitated pellets collected from nest and perch sites to identify 727 individual prey items representing 18 prey categories. Golden Eagles in our study area primarily consumed Arctic ground squirrels (Urocitellus parryii; 52% of total biomass), ptarmigan (Lagopus spp.; 20%), and waterfowl (15%). Dietary breadth differed between years. We detected 13% more ptarmigan and 12% fewer ground squirrels at perch sites compared to nest sites. Our results suggest that Golden Eagles have a relatively broad dietary niche in western Alaska during the first half of the breeding season.

Anticoagulant rodenticides (ARs) are a source of mortality to nontarget wildlife, including raptors. Raptors that consume a large quantity of rodents, such as Red-tailed Hawks (Buteo jamaicensis), are especially at risk of secondary exposure due to bioaccumulation and biomagnification of ARs. However, few studies have focused on the sublethal effects of ARs and how they can lead to potentially lethal outcomes. We investigated the effects of ARs on both parasitemia and body condition in juvenile Red-tailed Hawks from the Marin Headlands—the site of a migratory bottleneck for raptors on the west coast of the United States—north of San Francisco, California. We investigated the presence of several ARs in the plasma of 116 Red-tailed Hawks. We used the ratio of heterophils to lymphocytes (H/L ratio), levels of haemosporidian parasitemia, and polychromatic erythrocytes in blood smears as indices of body condition. Based on studies of other carnivores, we predicted higher levels of parasitemia and increased H/L ratios in raptors with ARs in their blood serum, but we found no significant difference in parasitemia or H/L ratio between raptors with and raptors without ARs in their blood. In contrast, individuals exposed to ARs had increased levels of polychromasia, or immature red blood cells, possibly as a result of AR exposure, though levels of polychromasia were within the typical range. Because ARs in blood have a short half-life, we do not know each individual's exposure history, which may play a role in subsequent parasitemia. Future studies should focus on individuals with known exposure history or obtain a longer temporal resolution using liver samples.

Scavenging is an important ecological function that increases individual fitness and transfers energy between trophic levels. Scavenging by owls has been documented opportunistically through direct observations, camera trapping, and pellet analyses, but it is unknown how frequent or widespread the behavior is. We documented three new scavenging events from North America and Europe, and also performed a systematic literature review of the reports documenting scavenging by owls. The number of such reports was similar in each decade from the 1970s to the 2000s, but the decade of the 2010s had more reports than all previous decades combined. Owls scavenged primarily on mammals (81%), followed by birds (16%) and reptiles (3%); almost half (47%) of carrion scavenged were Artiodactyla (hoofed mammals) and most of the species scavenged were larger than the feeding owl. Most reports documenting scavenging by owls were from either Europe (n=14) or North America (n=11), with few reports from Asia (n=2), South America (n =2), or Australia (n=1), and none from Africa. The most frequent type of report was direct observations (n= 14), followed by camera trapping (n=9) and pellet analyses (n=6). Our review indicates that scavenging is a widespread behavior among owl species, but most observations were opportunistic, suggesting that additional incidents of scavenging by owls are likely unobserved. Further research is needed to establish the frequency of scavenging by owls and the effects that scavenging may have on owl populations and scavenging communities.

Species considered raptors are subjects of monitoring programs, textbooks, scientific societies, legislation, and multinational agreements. Yet no standard definition for the synonymous terms “raptor” or “bird of prey” exists. Groups, including owls, vultures, corvids, and shrikes are variably considered raptors based on morphological, ecological, and taxonomic criteria, depending on the authors. We review various criteria previously used to define raptors and we present an updated definition that incorporates current understanding of bird phylogeny. For example, hunting live vertebrates has been largely accepted as an ecological trait of raptorial birds, yet not all species considered raptors are raptorial (e.g., Palm-nut Vulture [Gypohierax angolensis]), and not all raptorial birds are considered raptors (e.g., skuas [Stercorariidae]). Acute vision, a hooked bill, and sharp talons are the most commonly used morphological characters for delineating raptors; however, using those characters as criteria may cause confusion because they can be vague and exceptions are sometimes made. Old World vultures, for example, are in the family Accipitridae along with hawks and eagles, and thus are usually considered raptors despite their lack of sharp talons. We define raptors as species within orders that evolved from raptorial landbirds (Telluraves) in which most species maintained raptorial lifestyles. Raptors are therefore all species within Accipitriformes, Cathartiformes, Falconiformes, and Strigiformes. Importantly, we believe that seriemas (Cariamiformes) should also be considered raptors. Our definition combines phylogeny with morphology and ecology, and avoids ambiguity associated with owls, vultures, and shrikes. Establishing a common definition of raptors should improve interpretability across studies and lessen ambiguity of research and management recommendations.