We tested whether captive breeding and release is a feasible restoration strategy for the Harpy Eagle (Harpia harpyja) where suitable unoccupied habitat remains within its former range. From 1987 through 2006, 18 Harpy Eagles participated in a captive breeding program started in Boise, Idaho, and continued in Panama from 2001. From 131 eggs laid, 44 eagles were fledged. Most young were produced by just three females in the program, and at a higher annual rate after the birds were moved from Boise to Panama. Re-laying induced by collecting eggs for artificial incubation increased the number of viable eggs laid per female each breeding season up to six, but may have reduced female reproductive lifetime. Including rehabilitated eagles hatched in the wild, we released 49 eagles from 1998 through 2009. When the last released eagle with a functioning radio transmitter died in 2011, 63% were known or presumed to be dead, 31% were missing and possibly alive, and 6% were back in captivity. Shooting (44%) was the primary cause of death. Behavior interpreted as aggression toward humans was sufficiently frequent (23% of released eagles) in captive-raised and wild-rehabilitated eagles after release to be a concern for public safety and a potential cause of shooting deaths. This study demonstrated that it is feasible to breed Harpy Eagles in captivity at high rates needed for species restoration. It is possible to release captive-reared and rehabilitated Harpy Eagles to the wild, and is most cost effective (i.e., resulting in the highest survival to hunting-independence) when eagles are released close to the age of independence. Preventing shooting and other kinds of human persecution, and protecting remaining forest habitat, are the most urgent conservation needs for the Harpy Eagle.

Causes of admission to rehabilitation centers can provide valuable information about factors that cause mortality in the wild. We studied causes of admission to a rehabilitation center for 108 Andean Condors (Vultur gryphus) in Chile. Seventy-nine, 28, and one condor came from central, south, and northern Chile, respectively. From central Chile, an area with high human population, the majority of condors received were adults. The most frequent causes of admission to the rehabilitation center were poisoning (52%) and collisions with power lines (13%). Seventy-two percent of the radiographed birds showed ammunition in their bodies. Almost all the condors (85%) were received during the wintering period, when condors use the lowlands, thus increasing the probability of interaction with humans. The condors admitted from southern Chile, an area with low human pressure, were mainly juveniles. The most frequently admitted birds in the south were young birds that were trapped just after fledging (68%), which made up only 4% of the cases in central Chile. There were no poisonings or collisions with power lines. Only 25% of the radiographed birds were positive for ammunition. No seasonal variation in admissions was observed, indicating that risk factors in the southern zone did not operate on a seasonal basis. The sample of birds admitted from central Chile had similar sex and age structure as the wild population, with some bias toward juveniles, in contrast with the sample from southern Chile, in which young birds dominated. In conclusion, we observed an important anthropogenic effect on causal and temporal patterns of admissions to a rehabilitation center for Andean Condors; for the segment of the population in central Chile, the mortality pressure is apparently higher than expected under natural conditions, which could promote a demographic sink in this region.

Eradication is often the preferred method of invasive species management on islands; however, its consequences may affect native communities. Feral goats (Capra hircus), donkeys (Equus asinus), and pigs (Sus scrofa) were eradicated from Santiago Island in the Galapagos Archipelago by 2005. Because feral goats were the dominant herbivores on Santiago Island until their eradication, we examined the consequences of goat eradication on the diet of territorial Galapagos Hawks (Buteo galapagoensis) through a comparative study of observations of prey deliveries to nests before (1999–2000) and after (2010–2011) eradication. We predicted that vegetation recovery after eradication would limit the hawks’ hunting success of terrestrial prey and they would therefore switch to predominantly arboreal prey. We did not observe the predicted switch from terrestrial to arboreal prey in the diet; on the contrary, after goat eradication, hawks delivered significantly fewer arboreal prey items. However, introduced black rats (Rattus rattus) represented a significantly greater proportion of the hawks’ diet after eradication, particularly in moderate to dense vegetation (arid and transition habitats), replacing other prey items. Overall, 73% of total prey biomass delivered after eradication consisted of introduced rats, compared to only 20% before eradication. This study documents the complex interaction of predators and introduced prey, even in relatively simple ecosystems.

Differential migration by age and sex has been observed within several raptor species. Here, we report differential autumn migration timing between adult and juvenile Red-tailed Hawks (Buteo jamaicensis), and between adult males and females at the Golden Gate Raptor Observatory in the Marin Headlands of northern California. Because Red-tailed Hawks cannot be easily sexed in hand, we used discriminant function analysis (DFA) to identify morphometric measures useful for sexing adult Red-tailed Hawks captured during migration in the Marin Headlands and created a flowchart for in-hand field sexing of adult Red-tailed Hawks. The DFA correctly assigned sex 95% of the time, and provides an improved method for sexing adult Red-tailed Hawks in the Marin Headlands when compared to existing DFAs developed for other populations of this species in the western United States. Our ability to sex adult Red-tailed Hawks permitted examination of fall migration phenology, which differed markedly for adult and juvenile Red-tailed Hawks in the Marin Headlands. Juveniles displayed two distinct peaks of migration, one in mid-September and a second in mid-November. In contrast, the number of migrating adults increased steadily through mid-November, and declined thereafter, with adult females migrating earlier than adult males and the mean passage date for both sexes much later than documented at other North American hawk watch sites.

Nest boxes for American Kestrels (Falco sparverius) may alleviate local nest site limitation, but there is concern that periodically opening nest boxes or handling adults may negatively affect nesting success. I monitored 536 kestrel breeding attempts (≥1 egg laid) in about 100 nest boxes in northwestern New Jersey, 1995–2012. To study return rates, I opportunistically captured adults in nest boxes and marked them with U.S.G.S. leg bands and patagial tags. To examine possible effects of this disturbance, I compared nesting success (≥1 nestling surviving to banding age) of marked and unmarked adults. Nesting success was 67% for 270 unmarked pairs, 76% for 25 pairs with only the male marked, 82% for 206 pairs with only the female marked, and 91% for 35 pairs with both adults marked. This significant difference likely reflects differences in the probability of capture: successful attempts last longer and successful parents may be more attentive. To control for these correlations, I examined attempts for which the first disturbance was encountering an adult in the nest box; that bird either flushed from the nest box or was captured and marked. Abandonment was not significantly related to this initial disturbance: breeding attempts continued for 94.3% of attempts in which the male flushed, 93.1% for males handled, 93.7% for females flushed, and 93.2% for females handled. Nesting success also did not differ significantly among these four treatment groups. The timing of the first disturbance did not significantly affect abandonment; breeding attempts continued for 90% of attempts in which males were first disturbed (flushed or handled) during the laying period, 94% for males during incubation, 97% for females during laying, and 93% for females during incubation. Nesting success also was not significantly related to timing of the initial disturbance. Thus, it appears that both the intensity (handling or not) and timing of disturbance had no substantial effect on abandonment or nesting success for this population.

Most species of owls lack distinctive sexual color dimorphism, and plumage is not considered reliable for distinguishing sex. In North America, Long-eared Owls (Asio otus) are generally considered monomorphic in color, although there are subtle color differences between the sexes. From 1987 to 2015, we investigated differences in plumage coloration of male and female Long-eared Owls in western Montana. We initially used an observational method (1987–1993), followed by a quantitative method (1994–1999), and then a simplified method (2000–2015). When we used the observational method, we correctly sexed all 22 Long-eared Owls. For the quantitative method, we used a Munsell Soil Color Chart to score underwing coverts, tarsometatarsus, and facial disc of breeding males and females and museum specimens purportedly sexed correctly. We found significant sex-specific color differences: underwing coverts (G = 136.77, df = 5, P < 0.01), tarsometatarsus (G = 44.50, df = 4, P < 0.01), and facial disc (G = 50.62, df = 7, P < 0.01). Underwing coverts differed the most between sexes. Based on these plumage color differences, we then correctly sexed all 19 owls captured during fall and winter and later recaptured as breeding birds. Using the simplified method, we correctly predicted the sex of 55 of 58 (93%) owls captured during fall and winter and later recaptured as breeders. Overall, we correctly predicted sex of 96 of 99 (96.9%) Long-eared Owls in Montana. We suggest that plumage coloration differences should be investigated in other study areas outside of Montana.

We present wing-loading measurements for 33 Golden Eagles (Aquila chrysaetos) trapped during autumn migration in Montana and Wyoming, and we compare wing loading and other related variables between age classes (hatch-year and adult) and sexes. Adult females had significantly greater wing loading than hatch-year females and both adult and hatch-year males. Adult and hatch-year males had similar wing loading. Hatch-year females weighed less than adult females, whereas the mass of hatch-year and adult males did not differ. Although our sample of wing-loading estimates for 33 Golden Eagles is small, it is the largest currently available for this species and this manuscript is the first to present age- and sex-specific comparisons of this important flight parameter. Our study distinguishes interesting and previously unidentified differences in mass and wing loading between sex and age categories, which may have important implications for energetics during foraging and migration, and merit further investigation.

The Golden Eagle (Aquila chrysaetos) and Bald Eagle (Haliaeetus leucocephalus) are the largest avian predators in North America, and are thus species of great ecological importance and cultural significance. There is a long history of human use of eagle body parts, and this use continues today: Bald and Golden eagles are among the North American birds most affected by the illegal wildlife trade. Detached eagle talons are often recovered in both law enforcement and archaeological contexts, but data to allow morphological identification of these talons have been lacking. This study documents measureable differences in the morphology of Bald Eagle and North American Golden Eagle talons, which can be used to identify the detached talons of these two species. We measured talon samples of both species from the U.S. Fish and Wildlife Service's National Fish and Wildlife Forensics Laboratory and other collections and categorized them according to species, sex, age, and digit number (Digits I–IV). We then conducted ANOVA and principal components analysis to test for statistical differences in the talon measurements of these two species. Although species identification was not always possible, due to overlap in the morphology of the talons of the two eagles, our results demonstrated that measurements allow identification of many talons, especially the large talons of Digits I and II, which are most commonly recovered in law enforcement cases. These results will be valuable for researchers studying North American eagle remains in the contexts of law enforcement, archaeology, and anthropology.

Bird species, including raptors, can often be categorized into two groups depending upon their response to alterations in clutch size while laying. For some, clutch size is predetermined prior to the start of laying (i.e., determinate species). In contrast, the clutch size of indeterminate layers can be influenced by external factors present at the time of laying. Using field experiments, our objective was to examine the egg-laying responses of female Western Burrowing Owls (Athene cunicularia hypugaea) to manipulations of clutch size. To assess whether Burrowing Owls are determinate or indeterminate egg-layers, we altered female clutch size by removing or adding one egg during the laying period. At the time of egg removal or addition, the resident female had 1–5 of her own eggs present in her clutch. We compared the size of completed clutches of both removal and addition nests to the size of completed clutches at control nests. Mean clutch size at removal nests (x̅ = 8.3 eggs, SE = 0.44, n = 9) did not differ from controls (x̅ = 8.8 eggs, SE = 0.18, n = 17), whereas mean clutch size at addition nests (x̅ = 10.7 eggs, SE = 0.66, n = 9) was significantly larger than that of control nests. These findings demonstrate that female Burrowing Owls responded to the removal of an egg by laying a replacement, yet they did not curtail laying in response to the addition of an egg to their nest. Thus, female Burrowing Owls may be described as removal indeterminate and addition determinate. These results have implications for understanding aspects of Burrowing Owl nesting biology, such as selective advantage following the partial or total loss of a clutch of eggs, and they also may provide insight into how this behavior could facilitate conspecific brood parasitism or mitigate its costs in this species.