This review examines the main developments that have occurred over the past 50 years in our understanding of three aspects of raptor biology: (1) natural factors that limit breeding densities; (2) influences of toxic chemicals; and (3) movements and migrations. Early evidence indicated that raptor breeding densities were limited naturally by the availability of either prey or nest sites, whichever was in shortest supply in the area concerned. More recent evidence has shown that predation can have additional influence, with larger raptors and owls limiting the numbers of smaller ones to below what food or nest sites would permit. In addition, it has become apparent that some migratory raptors, like other migratory birds, can be limited in their migration and wintering areas to levels below those that conditions in breeding areas would permit. As many raptor populations have recovered from the effects of organochlorine pesticides, attention has switched to other limiting agents, including lead (from ammunition), which is currently preventing California Condors (Gymnogyps californianus) from establishing self-sustaining populations in the wild, and anti-inflammatory veterinary drugs, which have caused massive declines in Asia vultures (Gyps spp.). The development of radio-tracking enabled studies of the local movements of individual raptors, providing new information on territories and ranging behavior, while satellite-based tracking has revealed the migration routes, wintering areas, and behavior of hundreds of individual birds.

The study of individual movement patterns using stable hydrogen isotopes (δ ²H) as a natural marker has grown; however, recent studies have suggested that measurement of δ ²H in feathers (δ ²H_f) may prove unreliable as a means for identifying region of origin of migrating or wintering birds, particularly raptors. In this study, we examine whether differences in body condition could explain some of the variability in δ ²H in feathers. We analyzed growing feathers of 21 Swainson's Hawks breeding in northern CA for δ ²H, nitrogen (δ ¹⁵N), and carbon (δ ¹³C) stable isotopes in relation to body condition. We found that δ ²H was variable (range = 40‰), and that variability was significantly associated with body condition. Raptors derive most or all of their moisture from prey. Therefore, we suggest that individuals in poor condition have an enriched pool of body water relative to individuals in good condition, due to fractionation of body water stores during respiratory water loss and metabolic processes. Body condition was also negatively correlated with δ ¹⁵N_f. However, δ ²H_f, δ ¹⁵N_f, and δ ¹³C_f were not correlated, suggesting that the relationship between δ ²H_f and body condition is a result of physiological processes rather than differences in dietary δ ²H. We used an isotopic basemap of δ ²H_f values to assess individual origin as if they were encountered naively on the migration or wintering grounds, and all individuals fell within the 95% confidence interval of our study area. Conversely, the 95% confidence interval of δ ²H_f values obtained encompassed almost the entire breeding range of this species, indicating little ability to differentiate origins of this species.

The Short-eared Owl (Asio flammeus) is a widespread raptor whose abundance and distribution fluctuates in response to the varying amplitudes of its prey, which are predominately microtines. Previous efforts to describe the seasonal movements of Short-eared Owls have been hindered by few band recoveries and the species' cryptic and irruptive behavior. We attached satellite transmitters to adult Short-eared Owls at breeding areas in western and interior Alaska in June 2009 and July 2010, and tracked their movements for up to 19 mo. Owls initiated long-distance southward movements from Alaska and most followed a corridor east of the Rocky Mountains into the Prairie provinces and Great Plains states. Four owls followed a coastal route west of the Rocky Mountains, including one owl that crossed the Gulf of Alaska. Completed autumn migration distances ranged from 3205–6886 km (mean = 4722 ± 1156 km [SD]). Wintering areas spanned 21° of latitude from central Montana to southern Texas, and 24° of longitude from central California to western Kansas. Subsequent seasonal migrations were generally northward in spring and southward in autumn; these movements were comparatively short-distance (mean = 767.5 ± 517.4 km [SD]) and the owls exhibited low site fidelity. The Short-eared Owls we tracked from two relatively local breeding areas in Alaska used a patchwork of diverse open habitats across a large area of North America, which highlights that effective conservation of this species requires a collaborative, continental-scale focus.

Land managers regularly use temporal nest protections to reduce the likelihood of raptor nest disturbance or abandonment, but guidelines are not consistent across management boundaries. We assessed alternative nest use (i.e., egg-laying) and nest spacing at 28 Golden Eagle (Aquila chrysaetos) territories that were monitored ≥25 yr between 1976 and 2013 (all except seven territories were monitored annually without interruption). Territories contained 1–8 nests (x¯ = 2.9), and average spacing between alternative nests was 0.5 km. Inspection of 21 territories monitored for 26–38 yr without interruption suggested eagles used individual nests an average of every 3.3 yr, laid eggs in any nest within territories an average of every 1.8 yr, and switched nests between 43.3% of consecutive nesting attempts (i.e., egg-laying in discrete breeding seasons). Protecting individual nests for 7 yr, or protecting all nests within a territory for 4 yr after the last documented use of any nest when alternative nests were considered would have protected >90% of all consecutive nesting attempts. These temporal protections are longer than individual nest protections commonly applied by land management agencies (e.g., 3 yr since last use), but shorter than those suggested by Golden Eagle data collected in southwestern Idaho in an area with more alternative nests per territory. We recommend that land managers take a territory approach to Golden Eagle nesting protection, including consideration of local alternative nest-use patterns when possible. Management decisions should be based on the last use of any nest within a territory, including all potential eagle nests within a biologically meaningful distance of one another (e.g., based on local alternative nest spacing) when nest-monitoring data are limited; longer protections should be applied when knowledge of alternative nests is likely incomplete.

Gamla Nature Reserve once held the largest colony of nesting and roosting Griffon Vultures (Gyps fulvus) in Israel, with 45 to 57 pairs nesting at the colony during our study years (1998–2002), and up to 140 individuals roosting on the canyon's cliffs. Nevertheless, the fledging success there was very low: only 34% of breeding attempts (nest with eggs laid) resulted in fledged young during our study. Poisoning and hunting were the main causes of mortality, but in addition, a shortage of appropriate nesting places may also have been an important limiting factor. Fledging percentage was correlated with nest-site use and “attractiveness”: nest sites with greater fledging percentage also had more breeding attempts and were inhabited earlier in the nesting season. The main physical characteristic that enhanced breeding success was the type of nest site; nests in caves were more successful and were used for more breeding attempts than nests that were exposed from above. The influence of microclimatic conditions on nesting success was emphasized by the differences in the intensity of parental care, particularly activities associated with thermoregulation, between parents at the different types of nest sites. Parents at exposed nests invested substantially more time in thermoregulation (i.e., brooding or shading the young), an investment that was negatively correlated with breeding success.

We conducted a hacking project in 1986–1994 to restore a population of breeding Bald Eagles (Haliaeetus leucocephalus) in central California, where the species had not nested in more than a half-century. We first documented breeding among release cohorts in 1993, and the population increased to 26 known occupied breeding territories by 2012, exceeding the recovery plan goal for central California. Not all Bald Eagle nesting in the region can be attributed to the hacking project, but because the first seven nesting pairs each included at least one released eagle, we believe that the project expedited the recovery of a Bald Eagle breeding population in central California. The proportion of Bald Eagles returning to breed increased for the final three cohorts in 1991–1994, when we released eaglets younger than the standard fledging age. Eaglets released at or beyond the standard fledging age dispersed relatively quickly, whereas eaglets released at a younger age established more regular feeding patterns at the hack tower, and were more often seen in future seasons. Reintroduction in central California was supported by previous protective measures for the recovery of the global population, particularly the ban on DDT.

Using radiotelemetry we determined for a managed landscape, seasonal home-range sizes and habitat compositions of the threatened Haida Gwaii subspecies of the Northern Saw-whet Owl (Aegolius acadicus brooksi). We trapped 17 owls, and successfully monitored seven: four adult males from May−June 2011 (spring) and three owls (adult male, juvenile male, juvenile female) from January−February 2012 (winter). The adult males in spring moved up to 2.9 km in a 24-hr survey period and had nonoverlapping 95% fixed kernel home ranges (276−375 ha) that were composed of four defined habitat classes (open, early seral forest, mid-seral forest, older forest). Comparisons between internal core-use areas (50% and 30% fixed kernels) and the larger surrounding 95% fixed kernel home ranges suggested these males used foraging and roosting habitat proportional to its availability within the home ranges. The three tagged winter owls had larger 95% fixed kernel home ranges (690−1310 ha) than those in spring, and moved up to 4.4 km in a 24-hr survey period. Winter home ranges overlapped (14.7−78.4%) and included the four habitat classes. Roosts in spring and winter were confirmed in all but open habitat types. Although our findings suggest that spring and winter home ranges can be composed of a diverse mixture of habitat types, consistent with studies of the subspecies A. a. acadicus, differences in home-range size suggest possible differences in the two subspecies' ecology. Our findings will inform recovery planning for brooksi in managed landscapes.