Accipiter Striatus Research Articles

Differential Autumn Migration of Sharp-Shinned and Cooper's Hawks in Western North America

Sharp-shinned Hawks (Accipiter stria tus) and Cooper's Hawks (A. cooperii) showed differ ential migration timing during autumn migration in New Mexico and Nevada. Age-sex classes passed through both sites in this order: juvenite females, juvenile males, adult females, and adult males. We com pared the magnitude of differential migration timing in these two species. The number of days separating mean passage dates of age classes (juvenile and adult) was greater for Sharp-shinned Hawks than for Cooper's Hawks. The number of days separating the mean passage dates of sex classes was mostly similar be tween the two species. In the Manzano Mountains of New Mexico, however, adult male and female Sharp shinned Hawks were more separated than adult male and female Cooper's Hawks. In Sharp-shinned and Cooper's Hawks, it seems likely that differences in rate of travel between males and females may best explain sex-specific differential migration timing and that for aging efficiency (i.e., hunting skill level) may best ex plain age-specific differential migration timing.

Management of fat reserves in tufted titmiceBaelophus bicolorin relation to risk of predation

Fat reserves are an important energy source for animals wintering in temperate zones. Long nights, low ambient temperature and often unpredictable food all increase the probability of death from starvation, and to survive, animals carry energy reserves as fat. Based on the assumption that extra weight makes birds more vulnerable to diurnal predators, it has been hypothesized that predation risk places an upper limit on avian fat reserves. The hypothesis leads to the prediction that birds should decrease their mass in response to increased risk of predation. We tested this prediction with a resident species, the tufted titmouse,Baelophus bicolor. During a 3-day period, we presented a taxidermic mount of a sharp-shinned hawk,Accipiter striatus(predator treatment) or a mourning dove,Zenaida macroura(control treatment) to seven titmice housed individually with food ad libitum. Even though vigilance and delay in time to resume foraging were significantly higher after exposure to the hawk model, the body mass of titmice after exposure to the hawk increased rather than decreased, a result opposite to that predicted. In particular, the birds significantly increased their evening body mass and mean daily mass gain during the treatment period. Our results suggest that risk-dependent foraging appears to regulate avian body fat reserves by limiting foraging time. When food is unlimited, as in our experiment, birds may be able to obtain high energy reserves even while maintaining intense vigilance for predators, and under such conditions, the energetic costs of flying may control upper level energy reserves.

Long-Term Counts of Migrating Raptors: A Role for Volunteers in Wildlife Research

The science of conservation biology and the practice of wildlife management depend upon long-term databases, but collecting such data can be difficult, expensive, and labor intensive. Conservation biologists and wildlife managers have long used nonprofessional volunteers to collect much of the information needed to make informed decisions concerning the resources they are attempting to understand and protect. The advent of the modern field guide in the 1930s, together with the growing availability of prismatic binoculars, heralded the modern age of recreational bird watching. Since then, bird watchers have made significant and substantial contributions to our understanding of bird populations in North America. Hawk Mountain Sanctuary in eastern Pennsylvania has used volunteer hawk watchers, in conjunction with its own paid staff, to help create the longest and most complete record of raptor migration in the world. The Sanctuary's annual counts of migrating raptors have proved a critical resource in assessing long-term trends of raptor populations in northeastem North America. The extensive database played a key role in exposing the threat of organochlorine pesticides to bald eagles (Haliaeetus leucocephalus) and other predatory birds earlier this century, as well as in tracking more recent recoveries in many of the same populations. Recent analyses of the database are yielding insights into (1) how cold fronts affect counts of raptors at migration watchsites, (2) the extent to which climate change affects the timing of raptor migration, and (3) changes in the migratory habits of sharp-shinned hawks (Accipiter striatus) in eastern North America. I submit that volunteers will play increasingly important roles in wildlife conservation wherever their efforts can be coupled with those of professional practitioners in the field.

Effects of the Exxon Valdez Oil Spill on Habitat Use by Birds along the Kenai Peninsula, Alaska

We examined habitat use by 34 species of marine-oriented birds along the Kenai Peninsula, Alaska, in 1989-1991 to measure effects of the Exxon Valdez oil spill of March 1989 outside of the immediate spill area in Prince William Sound. Overall, 22 species (65%) did not exhibit statistically significant initial negative impacts on their use of oil-affected habitats. Of the 12 species that did exhibit significant negative impacts on habitat use, 6 showed evidence of recovery by the end of the study in late 1991. Of the 6 species that failed to show clear evidence of recovery, two (Common Merganser and Glaucous-winged Gull) showed no evidence of recovery by 1991, three (Common Loon, Double-crested Cormorant, and Sharp-shinned Hawk) may have begun recovery by 1991, and evaluation of recovery of the sixth species (Ancient Murrelet) was precluded by a lack of data. On individual surveys, the proportion of species recorded that exhibited negative impacts declined through time, from 36% on the first survey after the spill in 1989 to 19% in late 1991. These results indicate that the Exxon Valdez oil spill had significant initial effects on habitat use by some marine-oriented birds along the Kenai, although the majority of species analyzed showed no obvious spill-related effects on habitat use and impacts on several other species were not prolonged.

Radio sociology [noise investigation

The article discusses the use of radio telemetry in wildlife research. An aircraft was used to locate a migrating, radio-tagged, sharp-shinned hawk. A 222 MHz four-element Yagi receiving antenna was clamped to the entrance step on the right side of the aircraft, so that it pointed in the direction normal to the direction of flight. The idea was to climb to an appropriate altitude, and fly in large circles to scan a substantial area for the missing bird. While the antenna beam swept across a small town a rush of noise-like sound was heard. This resulted in an investigation of the relationship between a town's radiation (radio noise) signature and its other attributes, such as economic type, population, geographic concentration, etc.

Forest Structural Characteristics of Accipiter Nesting Habitat: Is There an Allometric Relationship?

In montane forests of the western United States, a general correlation of Accipiter body size and scaling of the vegetation component of nesting sites and nest trees used by sympatric Accipiter species has been reported. We evaluated this pattern with vegetation data collected at Northern Goshawk (Accipiter gentilis), Cooper's Hawk (A. cooperii), and Sharp-shinned Hawk (A. striatus) nest sites in the Jemez Mountains and Pajarito Plateau of north-central New Mexico. We selected habitat variables at the nest tree and nest site scale that would allow us to evaluate the prediction that accipiters use nesting habitat in which their body size is positively correlated with tree size and tree spacing, and inversely correlated with tree density, basal area, and percent canopy closure. At the nest-site level, density of larger diameter trees should be positively correlated with body size, and density of smaller diameter trees should be inversely correlated with body size. Our results suggest that nest tree height and diameter support body size predictions about nesting habitat for accipiter hawks. None of the nest-site parameters measured in this study supported the body size predictions due to a large amount of intra-specific variation. As a result ofthis variation, it was difficult to differentiate between Cooper's Hawk and Northern Goshawk nest sites for most site variables. These results suggest there is a correlation between accipiter size and nest tree size, but that a correlation between nest site structural size and accipiter body size may not be a widespread phenomenon for all vegetation variables for all three species. Many commonly measured forest stand structural characteristics such as basal area and total tree densities may not be adequate for predicting suitable accipiter nesting habitat in all areas, particularly in the absence of comparisons with available habitat.

Wintering Sharp-Shinned Hawks (Accipiter striatus) in an Urban Area of Southwestern Idaho

During the three winters (1980 to 1984) in Nampa, Canyon County, of southwestern Idaho, 17 sharp-shinned hawks (Accipiter striatus) were trapped, banded and measured at a backyard bird feeder trapping station. Mean mass of 5 females was 172.4 g (range 163 to 185) and mean wirlg chord for 6 females was 201.8 mm (range 197 to 209). Mean mass for 12 males was 103.1 g (range 94 to 115) and mean wing for 18 males was 174.8 mm (range 168 to 180). Five, 6 and 10 sharp-shinned hawks were trapped during the 3 winters with 6 individuals being recaptured during either the same winter (2) or a 2nd winter (4) resulting in 23 captures with additional sightings of marked birds for a total of 29 encounters. Repeat encounters during the same, 2nd, and 3rd winters strongly suggest some winter fidelity for this accipiter in urban areas of southwestern Idaho. Urban bird feeders concentrate prey of appropriate body size in suitable habitat which enhances opportunities for sharp-shinned hawk foraging. The sharp-shinned hawk (Accipiter striatus), smallest and most migratory of the three North American accipiters, is generally regarded as somewhat common throughout the western United States. Although its population trends are not well known outside of Oregon where it is listed as stable (Reynolds 1989), Bednarz and others (1990) reported migration counts indicating the northeastern United States sharpshinned hawk population was stable. Recent unpublished reports suggest declines in the East, but no rigorous analysis has yet to be published (J. C. Bednarz, pers. comm.). In Idaho this species is considered an uncommon breeder (Larrison and others 1967), common transient, and an uncommon and rather local winter resident (Burleigh 1972). Tripp and Powers (1990) recorded a few sharp-shinned hawks on winter car surveys of diurnal raptors in vicinity of Deer Flat National Wildlife Refuge in southwestern Idaho, 1983 to 1986. Otherwise, I know of no published wintering data on this

Decreases in Migrating Sharp-Shinned Hawks (Accipiter striatus) at Traditional Raptor-Migration Watch Sites in Eastern North America

ABSTRACr.-Each fall, migrating Sharp-shinned Hawks (Accipiter striatus) are counted at traditional raptor-migration watch sites in eastern North America. During the 1980s and early 1990s, declines in the numbers of Sharp-shinned Hawks sighted at many of these sites raised concerns about the population status of the species. Using count data from Hawk Mountain Sanctuary, Pennsylvania, and Cape May Point, New Jersey, we offer additional evidence that the reported decline is greater at raptor-migration watch sites along the Atlantic coast than at sites farther inland. Band-recovery data for fall migrants indicate that Hawk Mountain Sanctuary and Cape May Point count birds that essentially are from the same eastern population. An analysis of Christmas-bird-count data for easternmost North America during the 1980s indicates significant increases in the numbers of Sharp-shinned Hawks observed north of the two sites. We suggest that at least part of the decline in numbers of Sharp-shinned Hawks sighted at traditional watch sites in eastern North America may result from migratory short stopping. Received 29 September 1994, accepted 18 March 1995. EARLIER THIS CENTURY, declines in the numbers of birds of prey at traditional raptor-migration watch sites were correctly interpreted

HEMOGRAMS AND HEMATOZOA OF SHARP-SHINNED (ACCIPITER STRIATUS) AND COOPER'S HAWKS (ACCIPITER COOPERII) CAPTURED DURING SPRING MIGRATION IN NORTHERN NEW YORK

Hemograms were determined for 26 Cooper's (Accipiter cooperii) and 55 sharp-shinned hawks (Accipiter striatus) captured during spring migration (27 March to 12 May 1987) on the south shore of Lake Ontario, New York (USA). No significant differences were noted in packed cell volume and estimated total solids between the species. However, Cooper's hawks had significantly higher total white blood cells counts and higher concentrations of heterophils, monocytes, and eosinophils. Proportionally, lymphocytes made up a smaller percentage of the differential count in the Cooper's hawk. Eosinophil concentrations and percentages of the differential count were significantly higher in the females of both species. Both species had a high prevalence of Leucocytozoon toddi and Haemoproteus spp. infection. Haemoproteus nisi and H. elani were identified in both hawks. Trypanosoma avium was identified in a single Cooper's hawk and Plasmodium circumflexum was identified in a sharp-shinned hawk. Prevalence of Leucocytozoon toddi and Haemoproteus spp. infections were significantly higher in the birds caught late in the spring as compared to those caught earlier in the spring; this was evidence for a spring recrudescence of patent parasite infections.

Feeding Ecology of Sharp-Shinned Hawks Nesting in Deciduous and Coniferous Forests in Colorado

Feeding ecology of 11 Sharp-skinned Hawk (Accipiter striates) pairs nesting in aspen (Populus tremuloides), conifer (Abies, Picea spp.), and mixed aspen-conifer habitats in southwest Colorado was investigated during 1988-1989. Small birds (x= 20.9 g, SE = 0.8 g) and mammals (X = 41.1 g, SE = 3.3 g) comprised 91 and 9% of 5 13 prey identified at nests that fledged at least one young, respectively. Sixty percent of the birds eaten during the hawks' nestling and fledgling stages were nestlings or fledglings. Accordingly, median mass of birds eaten decreased from 17.4 g during incubation to 12.1 g during the nestling stage. Although more birds were consumed than mammals during all nesting stages (birds = 91 .I%; mammals = 8.9%), the proportion of birds relative to mammals in diets pro- gressively decreased from incubation through fledging. Taxa of birds in the diet were con- sumed in proportion to their occurrence in the most abundant of three different habitats surrounding nests, whereas some mammalian taxa were consumed in greater proportion than their relative availability in these habitats. This suggested that Sharp-shinned Hawks foraged opportunistically for birds, but may have selectively foraged for mammals. Differ- ences in the habits of mammals (e.g., fossorial vs. terrestrial behavior), and hence their relative availability, may explain the apparent selection for certain species of mammals by Sharp-shinned Hawks.

Chlorinated hydrocarbons and shell thinning in eggs of (Accipiter) hawks in Ontario, 1986-1989.

Eggs of sharp-shinned hawks (Accipiter striatus), Cooper's hawks (A. cooperii), and northern goshawks (A. gentilis) were collected from nests in south-central Ontario from 1986 to 1989. Detectable levels of the chlorinated hydrocarbons DDE, DDT, DDD, mirex, photomirex, cis-nonachlor, trans-nonachlor, oxychlordane, dieldrin, heptachlor expoxide, HCB and PCB were present in all the eggs analyzed. Mean contaminant levels were highest in sharp-shinned hawks, slightly lower in Cooper's hawks, and significantly lower in goshawks; for example, mean DDE values were 7.23, 4.48 and 0.90 mg kg(-1) (wet weight), respectively. Total PCB levels were generally less than 1.5 mg kg(-1), lower than those associated with reproductive effects in field and laboratory studies with other species. Mean shell thicknesses of sharp-shinned and Cooper's hawk eggs were significantly less than mean pre-DDT era thicknesses (-10 and -8%, respectively), reductions that were probably not relevant to reproductive success. Goshawk eggs exhibited no significant reduction in shell thickness. Contaminant levels among the three species probably reflect differing food habits and migratory patterns.

Prevalence of Encysted Toxoplasma gondii in Raptors from Alabama

Little is known about the prevalence of encysted Toxoplasma gondii in wild birds. We examined the hearts and breast muscles from 101 raptors for encysted T. gondii. All of the raptors had been submitted for necropsy to the State Veterinary Diagnostic Laboratory, Auburn, Alabama. Tissues were digested in acid-pepsin solution and inoculated into groups of 3-5 laboratory mice. Toxoplasma gondii was isolated from 27 of 101 (26.7%) raptors: 8 of 12 (66.7%) red-shouldered hawks (Buteo lineatus), 13 of 27 (41.1%) red-tailed hawks (Buteo jamaicensis), 1 of 4 (25%) Cooper's hawks (Accipiter cooperi), 1 of 5 (20%) great horned owls (Bubo virginianus), 4 of 15 (26.7%) barred owls (Strix varia), and 1 of 3 (33.3%) kestrels (Falco sparverius). Toxoplasma gondii was not isolated from 3 broad-winged hawks (Buteo platypterus), 3 sharp-shinned hawks (Accipiter striatus), 6 barn owls (Tyto alba), 9 screech owls (Asio otus), a Mississippi kite (Ictinia misisippiensis), 2 golden eagles (Aquila chrysaetos), a bald eagle (Haliaeetus leucocephalus), 4 ospreys (Pandion haliaetus), 4 turkey vultures (Cathartes aura), or 2 black vultures (Coragyps atratus). No significant difference (P > 0.05) in prevalence was detected based on sex using chi-square analysis. Chi-square analysis of the data demonstrated that adult raptors had encysted stages of T. gondii significantly (P < 0.05) more often than did immature raptors.

Monitoring organochlorines in blood of sharp‐shinned hawks (Accipiter striatus) migrating through the great lakes

AbstractBlood samples from sharp‐shinned hawks (Accipiter striatus) were collected from 1985 to 1989 during the fall migration at Hawk Cliff, Ontario, and during the spring migration at Whitefish Point, Michigan, and analyzed for organochlorine contaminants. The study was designed to investigate the potential of using blood samples from birds of prey trapped by banders as a means of estimating the organochlorine exposure of migrant birds on their breeding and wintering grounds. Mean plasma levels averaged over the entire sample were highest forp,p′‐DDE (0.28 mg/kg, wet weight), followed by total polychlorinated biphenyls (PCBs) (0.08 mg/kg, wet weight). Mean levels of DDE, mirex, oxychlordane, heptachlor expoxide, dieldrin, hexachlorobenzene, and PCBs increased significantly in hatch‐year birds from their first southward migration to their return flight the following spring as second‐year birds. Thereafter, mean residue levels of the more persistent chemicals, including DDE, oxychlordane, and PCBs, did not change significantly between seasons in adult age classes. In contrast, dieldrin and heptachlor epoxide, which have a shorter half‐time in the body, increased significantly in the blood of all age classes after each winter and decreased significantly after each breeding season. The results were interpreted in terms of the clearance rates of the compounds as well as the diet of the birds on the breeding grounds. The sharp‐shinned hawk is a useful indicator species for contaminant‐monitoring programs involving birds of prey.

The humeroscapular bone of the great horned owl (Bubo virginianus) and other raptors.

A small, separate, bony density dorsal to the shoulder joint is radiographically visible in several species of large hawks and owls. Gross dissection and histological examination show the bone to lie on the deep surface of the major deltoid muscle in intimate association with the dorsal coracohumeral ligament of the shoulder joint. The tendon of the supracoracoideus muscle passes immediately cranial to the humeroscapular bone. Two ligaments distinct from the shoulder joint capsule attach the humeroscapular bone to the proximal humerus: one passes to the proximal edge of the pectoral crest of the humerus, and the other passes to the ventral tubercle of the humerus. The bone was described as the humeroscapular bone in reference to a similar fibrocartilaginous structure possessed by some birds. The humeroscapular bone is present in the great horned owl (Bubo virginianus), the screech owl (Otus asio), the barred owl (Strix varia), the red-tailed hawk (Buteo jamaicencis), the Cooper's hawk (Accipiter cooperii), and the sharp-shinned hawk (Accipiter striatus). The bone is absent in the barn owl (Tyto alba), the osprey (Pandion haliaetus), the golden eagle (Aquila chysaetos), and the turkey vulture (Cathartes aura), though some of these species possessed a similar fibrocartilaginous structure. Whether the humeroscapular structure develops as bone or cartilage in a given species may be related to other morphological features of the wing, and/or to characteristics of the predatory behavior of the species. Clinicians and anatomists dealing with birds of prey must be aware of the presence of the humeroscapular bone to avoid misinterpreting it as a fracture fragment.

Recent Trends in Counts of Migrant Hawks from Northeastern North America

Using simple regression, pooled-sites route-regression, and nonparametric rank-trend analyses, we evaluated trends in counts of hawks migrating past 6 eastern hawk lookouts from 1972 to 1987. The indexing variable was the total count for a season. Bald eagle (Haliaeetus leucocephalus), peregrine falcon (Falco peregrinus), merlin (F. columbarius), osprey (Pandion haliaetus), and Cooper's hawk (Accipiter cooperii) counts increased using route-regression and nonparametric methods (P 0.10). We found no consistent trends (P > 0.10) in counts of sharp-shinned hawks (A. striatus), northern goshawks (A. gentilis), red-shouldered hawks (Buteo lineatus), red-tailed hawks (B. jamaicensis), rough-legged hawks (B. lagopus), and American kestrels (F. sparverius). Broad-winged hawk (B. platypterus) counts declined (P < 0.05) based on the route-regression method. Empirical comparisons of our results with those for well-studied species such as the peregrine falcon, bald eagle, and osprey indicated agreement with nesting surveys. We suggest that counts of migrant hawks are a useful and economical method for detecting long-term trends in species across regions, particularly for species that otherwise cannot be easily surveyed. J. WILDL. MANAGE. 54(3):463-470 Wildlife managers have responsibilities to monitor a wider variety of wildlife species than in the past, and cost-effective and efficient methods are needed to detect trends in populations that might warrant management action. Monitoring populations of nesting raptors over large areas is difficult because most species occur in low densities, and some species are secretive or nest in remote areas (Fuller and Mosher 1987). Although nesting season surveys have been used to monitor populations of endangered raptors such as the peregrine falcon (Cade et al. 1988), such long-term surveys are expensive and labor intensive. Most other raptors have received less study on their breeding grounds. However, migratory species have been counted annually during their migrations at certain concentration points in the eastern United States (e.g., Broun 1935, Allen and Peterson 1936). Ward and Berry (1972) suggested that counts of migrant hawks might provide a useful method for indexing population trends. Assuming that hawk count data can be used to detect trends in numbers of migrants and that these counts are a reliable index to populations, then analyses of hawk counts could be used for raptor management. Qualitative analyses of hawk count data have been used to infer population declines, increases, or stability (e.g., Nagy 1977, Dunne and Sutton 1986). The usefulness of these eyeball analyses and the quality of data on which they are based have been questioned (e.g., Kerlinger 1985). Hussell (1985) used quantitative methods to allay some previous criticisms of the use of migration count data, and Titus et al. (1989a, 1990) and Bednarz and Kerlinger (1989) discussed methods for analyzing count data. Svensson (1978) and others suggested that counts of migrants might be a useful method for detecting population trends. Indeed, counts of migrating hawks have been used for policy decisions about raptor management (e.g., U.S. Dep. Inter. 1983) and the status of raptors. Our objective was to estimate trends in 14 species of migrant hawks using regression and nonparametric techniques with single-site and pooled-sites analyses and to compare our results with information available from other population studies. We thank the observers who counted the migrant raptors. Data collection was supported, in part, by the Office of Migratory Bird Management (OMBM), U.S. Fish and Wildlife Service, through contracts with Cape May Bird Observatory, Derby Hill Bird Observatory-Onondaga Audubon Society, Whitefish Point Bird Observatory, and the Duluth Audubon Society. The Hawk Mountain Sanctuary Association (L. 'Present address: Alaska Department of Fish and Game, Division of Wildlife Conservation, P.O. Box 20, Douglas, AK 99824.

Breeding Biology of the Puerto Rican Sharp-Shinned Hawk (Accipiter Striatus Venator)

Abstract We studied the breeding biology of the Sharp-shinned Hawk (Accipiter striatus venator ) in Maricao forest of western Puerto Rico from 1978 to 1985. Sharp-shinned Hawks are year-round residents and establish nesting territories only during the breeding season, which coincides with the dry and beginning of the wet seasons. Nesting sites were occupied in December and January every year and reoccupancy rates were 50% or higher. Prolonged and intense territorial conflicts between mated and unmated males were common. The onset of egg-laying was in late March or early April approximately 3-4 months after occupancy of nesting sites. Laying of first clutches peaked in early April and spanned 38 days (n = 19 clutches). Laying of second clutches occurred irregularly and spanned 55 days (n = 8 clutches). Females renested only after the initial clutch or brood was lost. The incubation period was 32 days (n = 13 clutches), similar to the duration reported in temperate North America. Nestling females attained larger asymptotic mass than males, but the latter grew faster; although the slopes of the regression lines were statistically homogeneous. Males fledged at an average age of 28.2 days and females at 32.1 days. Young were slightly heavier than adults at fledging, but the wing chord and tail lengths were approximately 50% shorter than those of adults. Fledging occurred at the peak of prey abundance. The breeding cycle in Puerto Rico was approximately 2 months longer than that recorded in Oregon and Utah. The time that elapsed from occupancy of nesting sites to egg laying accounted for the differences. Juveniles departed from nesting sites when prey was still abundant but delivery rates had declined considerably. A total of 105 eggs was laid in 40 nests (average clutch size 2.6), of which 63% hatched and 47% of the nestlings fledged. A total of 0.8 young fledged per breeding attempt. Overall nest success was 29%, 36% (n = 33 nests) in first nesting attempts and 0% in second nesting attempts (n = 9 nests). Most reproductive losses in 28 nests resulted from nestling mortality from Philornis sp. (Diptera, Muscidae) (n = 9) and desertion of clutches (n = 11). Fecundity and reproductive success was lower in Puerto Rico than in Oregon, Utah, and Wyoming. Despite low reproductive success in Maricao forest, the breeding population did not decline during our study.

Hematocrits and Erythrocyte Numbers for Cooper's and Sharp-Shinned Hawks

Hematocrits and RBC counts have been used to evaluate the health of the oxygen transport system in raptors. Few values ofhematocrit and erythrocyte (RBC) numbers have been reported for North American accipiters. Values for Northern Goshawks (Accipiter gentilis) and Cooper's Hawks (A. cooperii) were reported by Elliott et al. (1974) and by Hunter and Powers (1980). Our objective was to provide a larger database of hematocrit values and RBC counts for healthy wild accipiters. The scant information on hematocrit and RBC counts of raptors is mostly from captive, relatively inactive birds at low altitudes. In contrast, the migrating accipiters in this study had been actively flying, probably for several hours each day at relatively high altitudes, prior to being captured. Indirect evidence suggest that healthy, migrating raptors may have significantly different hematocrits and RBC counts than healthy captives. For example, it is well known that living at high altitudes stimulates increased production of RBCs. Secondly, acute dehydration can increase the RBC count without a concomitant increase of red-cell mass; Hart and Berger (1972) speculated from experimental investigations of three avian species (non-raptors) that dehydration occurs in migrating birds.

Comparative flight behaviour of migrating hawks studied with tracking radar during autumn in central New York

Tracking radar with simultaneous visual observations was employed to study the flight behaviour of nine species of hawks during autumn migration, 1978–1979, in central New York. The predominant mode of flight for all species was thermal soaring and interthermal gliding. Although most species were seen in small flocks at some time, only Broad-winged Hawks (Buteo platypterus) could be considered flocking migrants, with most migrating in flocks &lt; 40 individuals. Altitude of flight increased through the day as convective depth developed, with approximately 85% of all individuals flying below 1000 m. Climb rates of individuals soaring in thermals averaged 3 ms−1 and were greater than previously reported for larger soaring species. Short- to medium-distance migrants (Accipiter striatus, Falco sparverius) tended to fly at lower altitudes than longer distance migrants. The direction realized during thermal soaring was positively related to wind direction and was oriented to the southeast, a function of the prevailing northwest winds. Orientation strategy was considered to be a compromise between drift and complete compensation, resulting in an elliptical migratory flight path, probably shaped by prevailing northwest winds. Such a compromise promotes a faster and more energetically efficient migration.

Hematozoa of raptors from southern New Jersey and adjacent areas.

Blood smears from 259 birds of 12 species, representing four families of raptors, from New Jersey, Pennsylvania, Delaware, and Virginia were examined for blood parasites. Infected birds constituted 59.1% of the total. Birds were infected with one or more of the following genera of protozoa: Leucocytozoon (43.2%); Haemoproteus (21.6%); Plasmodium (1.2%); and Trypanosoma (1.2%). Blood culture of 142 raptors of 11 species for Trypanosoma revealed a prevalence of 41.5%. Plasmodium circumflexum is reported for the first time in Accipiter striatus, and Trypanosoma sp. in Buteo jamaicensis.

Flight behaviour of sharp-shinned hawks during migration. I : Over land

We tested two hypotheses that have been proposed to explain why large numbers of sharpshinned hawks ( Accipiter striatus) are counted during fall migration at Cape May Point, New Jersey. The most popular hypothesis, which suggests that hawks are drifted to the coast by west to northwest winds, was rejected in favour of the alternative hypothesis, which suggests that the large numbers of hawks seen on west winds resulted from a sampling bias. Using modified marine radar, we found that sharp-shinned hawks flew significantly lower at Cape May on days with west winds than on days with other winds, making them easier to count. The altitude of flight on days with other winds was regularly greater than 400 m, and hawks were difficult to detect without the radar. Migration traffic rates at Cape May were consistently greater than the broad-front migration rates computed from counts taken 36 km north and inland from Cape May. Flight directions measured at the inland site, away from topographic leading lines, showed that sharp-shinned hawks compensate for different wind directions by adjusting their headings, and the direction realized on all winds brings them to the coast. Our results suggest that counts of migrating hawks at some topographic features are subject to systematic biases and the conclusions derived from these counts may be erroneous.