Buteo Lineatus Research Articles

Propensity of Predator Mimicry in Steller’s Jays

Avian vocal mimicry has been described in a variety of contexts, suggesting its function is multifaceted within and across species; however, basic empirical data describing mimetic signal prevalence and context are lacking for numerous species. We examined the occurrence and context of mimicked Red-shouldered Hawk (Buteo lineatus) calls over a 12-month period in a population of 49 individually color-marked Steller’s Jays (Cyanocitta stelleri). We documented mimicry of Red-shouldered Hawk calls in 14 of 49 (28.6%) jays during this 12-month period. We also reviewed the occurrence of the behavior in historic observation data. Hawk mimicry occurred more often during the early breeding season when jays were within home territories, their mates were present, and aggression was absent. Younger, larger, and bolder jays were most likely to perform imitations. These results suggest jays individually vary in mimetic propensity, and individuals’ proclivity for mimicry may be influenced by social and ecological contexts, physical characteristics, and personality traits.

Effects of urbanization on the efficiency and composition of vertebrate scavengers

As human populations continue to expand, urbanization will increase and impact ecosystem processes, contribute to habitat fragmentation, and alter community composition of species. While some species can adapt to the rapidly changing environment, urbanization can favor generalist species and reshape food webs, which may result in ecological instability. Determining how wildlife respond to urbanization is necessary for management and city planning purposes in order to create suburban areas where humans and wildlife can coexist. One area that needs more focus is the effects of suburbanization on scavenging species, and how land development can alter scavenging dynamics and the redistribution of carrion-derived nutrients within food webs. We used motion activated cameras to monitor experimentally placed fish carcasses in riparian zones of suburban and rural areas in northeast Georgia, USA. We conducted 300 trials across both landscapes that were scavenged by 7 mammalian, 2 avian, and 2 reptilian species. Our results revealed fish carrion within riparian areas was readily consumed by terrestrial scavengers. However, carcass fate (whether the carcass was scavenged or not) and carcass persistence differed between suburban and rural landscapes. Carcasses were scavenged by vertebrates less often and persisted longer in rural landscapes. Species richness was similar and mesocarnivores were the predominant scavengers in both landscapes. However, scavenging by the Virginia opossum (Didelphis virginiana) was more prevalent in the suburban landscape relative to the rural landscape (61% and 36% of scavenging events, respectively). While raccoons (Procyon lotor) scavenged carcasses fairly equally across both landscapes, the American black bear (Ursus americanus) and red-shouldered hawk (Buteo lineatus) scavenged more frequently in rural areas. Our data suggest that suburban species, especially opossums, may be highly adapted to foraging in an anthropogenic environment. Additionally, nutrients from aquatic carcasses near water sources can readily move into terrestrial environments through scavenging. These results contribute to the growing body of scavenging ecology and the effects of urbanization on wildlife.

Documentation of microplastics in the gastrointestinal tracts of terrestrial raptors in central California, USA

Microplastics are persistent environmental contaminants that to date have been studied primarily in aquatic systems, but few studies have examined their prevalence or impact in terrestrial trophic networks. A recent study documented microplastics in the gastrointestinal (GI) tracts of terrestrial raptors in central Florida. We used protocols based on that study to develop a baseline dataset of microplastics in the GI tracts of terrestrial raptors on the central coast of California. In 2021, Pacific Wildlife Care, a wildlife rehabilitation center in Morro Bay, CA, provided 16 raptor carcasses: three red-tailed hawks (Buteo jamaicensis), four red-shouldered hawks (Buteo lineatus), two great horned owls (Bubo virginianus), and seven barn owls (Tyto alba). We found microplastics in the GI tracts of all 16 birds, with a mean (± SE) of 12.25 (± 1.89) microplastic particles per bird. Of the 196 microplastic particles we observed, microfibers were the most abundant (58%), followed by microbeads (34%), and microfragments (8%). However, microbeads were the most prevalent, found in all 16 birds, followed by microfibers (15 birds), and microfragments (6 birds). The chemical composition and source of these contaminants remains unclear, along with the physiological and ecological implications to raptor populations and their ecosystems, and the extent to which microplastics occur in other terrestrial species and regions in California.

Sibling Aggression, Feeding Rates, and Hatch Rank of Nestling Red-Shouldered Hawks (Buteo lineatus)

ABSTRACT In many raptor species, hatching asynchrony results in a size hierarchy among nestlings, which may facilitate brood reduction by means of sibling aggression-induced mortality, especially in times of low food resources. However, few researchers have investigated the relationships among hatching asynchrony, sibling aggression, and nestling feeding rates in raptors. We studied the influence of hatch rank, brood size, and brood age on aggression and feeding rates among Red-shouldered Hawk (Buteo lineatus) nestlings in suburban southwestern Ohio. Despite relatively modest hatching asynchrony, nestling hatch rank (along with brood size and brood age) was an important predictor of the amount of aggression exhibited by individual nestlings. The oldest nestlings exhibited more aggressive behavior than younger nestlings, although they did not consume significantly more food. Aggression rates were higher in broods of three than broods of two nestlings, and aggression decreased from week 2 to week 3, post-hatching. However, hatch rank was not an important determinant of nestling food consumption, and food consumption rates were unrelated to sibling aggression rates. Identifiable prey delivered to nestling Red-shouldered Hawks was diverse and consisted of 28.3% birds, 25.7% mammals, 23.4% amphibians, 16.4% reptiles, and 6.2% invertebrates (by biomass). The relatively high percentage of birds in the diet may reflect food availability in the suburban habitat surrounding the nests. Sibling aggression did not result in direct siblicide in our study, but four nestlings apparently starved. Overall, the oldest Red-shouldered Hawk nestlings in our study established their dominance early but this was not associated with a detectable advantage in terms of food consumption.

Breeding season diet of the Red-shouldered Hawk (Buteo lineatus extimus) in southern Florida

La dieta del gavilán Buteo lineatus ha sido descrita en al menos 12 estados. La dieta de esta especie no ha sido aún descrita en Florida, aunque en el estado se ha registrado una subespecie endémica (Buteo lineatus extimus) descrita por primera vez hace más de 100 años. Este estudio caracteriza las presas que los gavilanes Buteo lineatus usaban para alimentar a sus polluelos en el sudeste de Florida usando cámaras en 2 nidos en la primavera del 2018 y en 4 nidos en la primavera del 2019. Setenta y dos por ciento de las 1024 presas llevadas registradas fueron identificadas a clase. Reptiles y anfibios eran las clases de presas más frecuentemente consumidas, como era esperado, ya que los gavilanes Buteo lineatus consumen más mamíferos en la parte norteña de su distribución y más anfibios y reptiles en la parte sureña. La amplitud de dieta era menor en la mayoría de nidos urbanos en este estudio. Cada pareja tuvo éxito en emancipar a los polluelos que fueron grabados, aunque les dieron presas que variaron ampliamente en tipo y cantidad. Palabras clave: amplitud de dieta análisis de hábitat cámaras en el nido gradiente urbano, presas no nativas

Estimating population dynamics trajectories of raptors from a multi-species hierarchical distance sampling model

Community dynamics are essential for ecosystems as they support the functioning of food webs and maintain biodiversity. However, the fluctuations of species abundances are rarely analyzed simultaneously despite their interconnectedness across space and time. We developed a Bayesian multi-species model that estimates community- and taxon-level parameters related to dynamics, abundance, detection probability, and environmental stochasticity. From this model, we analyze the variability of populations of a community of raptors composed of accipiters (sharp-shinned hawk [Accipiter striatus], and Cooper's hawk [A. cooperii]), buteos (red-tailed hawk [Buteo jamaicensis], and red-shouldered hawk [B. lineatus]) and owls (great horned owl [Bubo virginianus], and barred owl [Strix varia]). We show that migration pulses of accipiters, buteos, and dispersal of fledging owls drive variation in raptor abundances, indicating a defined production and dispersal period. Also, large-scale climatic processes such as the North Atlantic Oscillation (NAO) influenced the variation in abundances of raptors. When the NAO was in a positive phase, the abundance of raptors decreased, and inversely. Local weather did not affect raptor population abundances. Our approach facilitated the modelling of species-specific effects of environmental variation and guild-level dynamics that could be used for ecosystem-based conservation measures.

Composite population trends reveal status of wintering diurnal raptors in the Northwestern USA

Integrated models can combine data from disparate monitoring programs to improve inference into population dynamics because trends in disparate datasets can be interpreted using a single index. We present a composite index of raptor population trends that combines data from two monitoring programs into a single estimate. The National Audubon Society's Christmas Bird Count and the East Cascade Audubon Society's Winter Raptor Survey use different methods to monitor the same populations of wintering raptors. Observers record all birds detected within 452 km2 ‘circles’ during the Christmas Bird Count, and during the Winter Raptor Survey observers count raptors along roadways. We present the results of a joint likelihood model that combines trends of raptors counted during both programs into a single population trend from 2005 to 2020. Population indices of the Bald Eagle (Haliaeetus leucocephalus), Ferruginous Hawk (Buteo regalis), Cooper's Hawk (Accipiter cooperii), Red-shouldered Hawk (Buteo lineatus), and Red-tailed Hawk (Buteo jamaicensis) increased over the course of our study, whereas Prairie Falcon (Falco mexicanus) and White-tailed Kite (Elanus leucurus) population indices declined. Further, the composite index for White-tailed Kite declined at a rate of 22 % (80 % CRI = 17 %–28 %) per year—supporting evidence of declines from breeding season surveys. We recommend further research into population trends of this species within its core California range. Our study advances efforts to combine trends in datasets collected using differing methodologies to estimate large-scale population trends from disparate data sets.

Blood Lead Concentrations of Free-Ranging North Florida Raptors: 2008-17.

Whole blood samples for lead analysis were collected from 441 raptors admitted to the Zoological Medicine Service at the University of Florida (US) between 1 January 2008 and 31 December 2017. The species included Bald Eagles (Haliaeetus leucocephalus), Ospreys (Pandion haliaetus), Black (Coragyps atratus) and Turkey (Cathartes aura) Vultures, Barred (Strix varia) and Great Horned (Bubo virginianus) Owls, and Red-tailed (Buteo jamaicensis) and Red-shouldered (Buteo lineatus) Hawks. Our hypothesis was that geography, seasonality, and hunting strategies of these species would all affect the blood lead concentrations. Blood lead concentrations were found to vary significantly between species, with known scavengers having higher values. Additionally, seasonal differences were seen between winter and summer, but the county in which these individuals were found did not alter the blood lead concentrations. We found lead contamination to be a common and considerable problem in Bald Eagles and vultures but not as evident in nocturnal and other diurnal raptors in North Florida. Furthermore, concentrations were highest during winter in this temperate location, suggesting a possible relationship with hunting seasons despite a lack of big game.

Project Review: SwampScapes

Project Review: <i>SwampScapes</i>

Falcons From the United Arab Emirates Infected With Chlamydia psittaci/C abortus Intermediates Specified as Chlamydia buteonis by Polymerase Chain Reaction.

Chlamydiaceae are obligate intracellular bacteria with a broad host range. Several studies have found chlamydial species that are genetically intermediate between Chlamydia psittaci and Chlamydia abortus in various avian species. One of these intermediate Chlamydia species, found in a red-shouldered hawk (Buteo lineatus), was recently classified as a new species Chlamydia buteonis. This newly described Chlamydia species has, so far, only been reported in hawks exhibiting clinical signs of conjunctivitis, dyspnea, and diarrhea. In the present study, fecal samples of 5 gyrfalcons (Falco rusticolus), 3 gyr/peregrine falcon hybrids (Falco rusticolus × Falco peregrinus), and 15 falcons of unknown species presented to falcon clinics on the Arabian Peninsula were shipped to the Vetsuisse Faculty, University of Zurich (Zurich, Switzerland), for examination for the presence of Chlamydiaceae. A step-wise diagnostic approach was performed to identify the chlamydial species involved. Chlamydiaceae were detected in 21/23 falcons by a family-specific real-time quantitative PCR (qPCR). Further identification with a 23S ribosomal RNA-based microarray assay and 16S conventional PCR and sequencing yielded inconclusive results, indicating the presence of an intermediate Chlamydia species. Because none of the falcons tested positive for Chlamydia psittaci by specific qPCR, all 23 samples were subjected to a Chlamydia buteonis-specific qPCR, which was positive in 16/23 samples. Detailed information regarding clinical history was available for 8 falcons admitted to a falcon clinic in Dubai, United Arab Emirates. Six of those birds that were presented to the clinic because of loss of performance and poor general condition, including vomiting and diarrhea, were positive for C buteonis. In 2 birds without clinical disease signs admitted for a routine health examination, 1 was positive for C buteonis, and 1 was negative. It is yet unknown whether Chlamydia buteonis causes disease in birds, but the findings in this study indicate that Chlamydia buteonis may be an infectious pathogen in falcon species.

The mantling behavior of Red-shouldered Hawk (Buteo lineatus) as a defensive strategy against pedestrians

The mantling behavior of Red-shouldered Hawk (Buteo lineatus) as a defensive strategy against pedestrians

Breeding Phenology of Red-Shouldered Hawks (Buteo lineatus) Is Related to Snow Cover and Air Temperature During the Pre-Laying Period

Global climate change has advanced the breeding phenology of many avian species. However, raptors’ breeding phenologies may not respond in the same way to the factors that influence passerine breeding dates. We studied reproduction of suburban and rural Red-shouldered Hawks (Buteo lineatus) in southern Ohio, United States, from 1997 to 2020. Mean hatching dates for 786 broods were 24 April [Julian day: 114.1 ± 0.3 d (SE)] for suburban birds and 25 April (Julian day: 114.5 ± 0.4) for rural birds. Egg-laying date averages approximately 33 days before hatching date, or about the third week of March. We used mixed models to test which factors influenced nestling hatching dates from 1997 to 2020. The best model included year, days of snow cover during the pre-laying period (February–March), and mean March temperature, with days of snow cover having the largest effect. Hatching date (in Julian days) was positively related to snow cover and negatively related to air temperature, i.e., young hatched earlier in years with fewer days of snow cover and in warmer years). Young also hatched slightly later as the study progressed. Overall, neither mean hatching date nor any of the weather variables showed a significant trend over the course of the study. Previously published reports indicate that many raptor species do not exhibit advancing hatching dates, and breeding phenologies often reflect local weather conditions. The complexity and diversity of raptor responses to climate change underscore the importance of long-term studies of raptors at multiple locations.

Toxoplasma gondii prevalence in carnivorous wild birds in the eastern United States.

Toxoplasma gondii is an important zoonotic protozoan parasite that can infect all warm-blooded animals including mammals and birds. Raptors can be intermediate hosts for T. gondii and the infection may be dependent on their feeding habits. In this study, we investigated the seroprevalence of T. gondii in ten raptor species from Florida, Pennsylvania, and Tennessee followed by a parasite bioassay on select seropositive samples. From a total of 155 raptors, we detected T. gondii antibodies using a modified agglutination test (cutoff 1:25) in 32 (20.6%) birds. The T. gondii seroprevalence was 44.8% in Falconiformes (13/29), 75% in Strigiformes (15/20), and 3.8% in Ciconiiformes (4/106). All Ciconiiformes samples (hearts and sera) were collected from Pennsylvania during nuisance wildlife removal projects and all birds were apparently healthy. Falconiform and Strigiform samples were collected from an exotics clinic in Tennessee and a rehabilitation center in Florida. All sampled birds were dead or euthanatized due to failure of rehabilitation or treatment. There was no statistically significant difference in T. gondii seroprevalence between Tennessee and Florida in the tested raptors. There was also no statistically significant difference in T. gondii exposure between males and females or adults and subadults. Mice bioassay attempts using fresh brain and/or heart tissue were performed on four seropositive birds. We isolated viable T. gondii tachyzoites from one red-shouldered hawk (Buteo lineatus) and genotyped the isolate using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of ten genetic markers. The isolated strain was designated as TgHawkFL1, which is ToxoDB PCR-RFLP genotype #28. Further research is needed to investigate the prevalence of T. gondii in raptors in the United States to obtain a better understanding of the life cycle, wildlife population impacts, and transmission dynamics of the parasite.

Yearly temperature fluctuations and survey speed influence road counts of wintering raptors

Globally, evaluation of population trends is the most pressing research need for many species of conservation concern. Road counts for birds of prey are useful for monitoring long‐term population trends and examining year‐to‐year variations in abundance. We examined data from 2155 road surveys conducted from 2001 to 2018 by community scientists who recorded &gt; 85 000 individuals of 14 species of raptors while participating in the Pennsylvania Winter Raptor Survey, in Pennsylvania, USA. We estimated abundance and population growth rates while accounting for observation error by using dynamic Bayesian state‐space models. Model estimates indicated that counts of wintering Bald Eagles Haliaeetus leucocephalus, Red‐shouldered Hawks Buteo lineatus and Black Vulture Coragyps atratus increased over the course of the study. Counts of Rough‐legged Buzzard Buteo lagopus, Red‐tailed Hawk Buteo jamaicensis, Northern Harrier Circus hudsonius, Turkey Vulture Cathartes aura and American Kestrel Falco sparverius varied more (CV &gt; 5.0) over the duration of the study than other species did. Higher winter temperatures were associated with increases in counts of species whose local populations are partially migratory (American Kestrel and Red‐tailed Hawk), and with lower counts of a long‐distance arctic migrant – Rough‐legged Buzzard. Counts of these species were therefore correlated such that more American Kestrels and Red‐tailed Hawks were counted during years when Rough‐legged Buzzards were less frequently seen. Generally, the number of individuals counted declined as survey speed increased. A general rule for road counts therefore seems to be ‘slower is better’, consistent with past recommendations that observers travel at a speed &lt; 40 km/h during road counts. Our study highlights the utility of road surveys and advances analytical approaches to monitor raptors.

Partial Incubation and Hatching Asynchrony in the Red-Shouldered Hawk (Buteo lineatus)1

Like many raptors, Red-shouldered Hawks (Buteo lineatus) exhibit asynchronous hatching, which is thought to be a parental strategy for rearing the maximum number of offspring under conditions of unpredictable prey availability. Current knowledge of incubation patterns and the associated behavioral mechanisms by which asynchronous hatching occurs in raptors is limited, and few detailed quantitative studies are available for any species of Accipitriformes. We investigated the effects of egg-laying intervals and parental behavioral incubation patterns during egg-laying on hatching asynchrony in the Red-shouldered Hawk. We used 24-hr/d digital color/infrared video cameras at 15 Red-shouldered Hawk nests to record egg-laying and hatching, and to quantify the proportion of time parents spent incubating, standing, or absent from the nest. The mean egg-laying interval was 2.9 ± 0.1 d or 69.8 ± 2.2 hr (range = 65.8–74.2 hr), and the mean duration of incubation (from the onset of full incubation to hatch) for last-laid eggs was 33 d (32–34 d, n = 15). Red-shouldered Hawks exhibited partial incubation behavior, with the delay in the onset of full incubation varying by clutch size and among individuals; full incubation generally began with or just after the laying of the penultimate egg of the clutch. The mean interval between the hatching of the first and second egg was 0.56 ± 0.49 d; between the second and third eggs, 1.24 ± 0.71 d (P= 0.0215); and between the third and fourth eggs, 2.42 ± 0.38 d (P < 0.0004). The total amount of time parents at each nest spent incubating during the egg-laying period explained most of the variation in the amount of time between the hatching of their first and last egg (the hatching span; r2 = 0.83, P < 0.0001). Hatching patterns were the result of both the egg-laying intervals and the pattern of partial incubation, which may represent an adaptive mechanism to reduce the hatching interval while maintaining the hatching asynchrony.

Habitat and weather conditions influence reproductive rates of suburban and rural Red‐shouldered Hawks Buteo lineatus

Both habitat and weather can strongly influence reproductive rates of birds. We measured reproductive rates of suburban and rural Red‐shouldered Hawks Buteo lineatus in southern Ohio, USA, from 1997 to 2016, and then tested how weather conditions and habitat in the areas surrounding the nest‐sites were related to two measures of reproductive rate. Reproductive rates of Red‐shouldered Hawks did not differ between the suburban and rural study areas, and were relatively stable from year to year. Suburban Red‐shouldered Hawks produced 2736 young in 1773 nesting attempts (i.e. nest with eggs) at 302 territories (1.54 young per nesting attempt), and rural‐nesting Hawks produced 996 young in 640 nesting attempts at 108 territories (1.56 young per nesting attempt). Annual nesting success averaged 58.9 ± 1.1% for suburban birds and 58.9 ± 2.1% for rural birds. Several factors influenced Red‐shouldered Hawk reproductive rates, measured as either the number of young per nesting attempt or the percentage of nesting attempts that were successful. Higher air temperatures during May (the nestling period) and increasing amounts of coniferous forest were associated with higher reproductive rates, whereas increasing April air temperatures and increasing amount of grassland cover had the opposite effect. Land cover variables associated with suburban landscapes, such as the amount of residential development, did not influence the number of young per nesting attempt or nesting success, suggesting that Red‐shouldered Hawks are welladapted to human‐dominated landscapes in southern Ohio. Our study also illustrates the value of long‐term datasets for improving our understanding of factors that affect raptors’ demographic parameters.

Tolerance and avoidance of urban cover in a southern California suburban raptor community over five decades

We explored nest site placement and re-use relative to ornamental tree usage and urbanization level in a diurnal raptor community in southern California (USA) during three discrete time periods spanning five decades (1971–2018). Re-use of prior years’ nests varied among species, with Red-tailed Hawks (Buteo jamaicensis) and American Kestrels (Falco sparverius) showing moderate re-use rates (ca. 30%), and Red-shouldered Hawks (Buteo lineatus), and Cooper’s Hawks (Accipiter cooperii) showing almost none. Nearly all nests were in native and naturally-occurring trees during the 1970s, yet by 2018, most Cooper’s Hawk nests, and many Red-tailed Hawk nests, were located in ornamental vegetation such as pines (Pinus spp.) and eucalyptus (Eucalyptus spp.). The amount of urban cover around nest sites increased for Red-tailed, Red-shouldered, and Cooper’s hawks during the study period, but not for American Kestrels, which were confined to the least-urban areas. Cooper’s Hawks appear to now be selecting urban nest sites over wildland sites, based on the increase in surrounding urban cover, even as landscape urbanization has not substantially changed in the study area during the last two time periods. Our study illustrates the utility of long-term datasets in understanding how a species’ urban tolerance can change over time, and highlights species (including three extirpated taxa) that may be failing to adapt to local urbanization.

Microplastic accumulation in the gastrointestinal tracts in birds of prey in central Florida, USA

A study was conducted to quantify the abundance of plastic pollution in the gastrointestinal tracts in birds of prey. Data was collected from all birds retrieved from the Audubon Center for Birds of Prey in central Florida, USA from January to May 2018. Individuals were either dead prior to reaching the Center or died within 24 h of arrival with no food consumed during captivity. Sixty-three individuals representing eight species were dissected to extract the gastrointestinal (GI) tract from the esophagus to the large intestine. Microplastics were found in the GI tracts in all examined species and in all individual birds. The overall mean number (±S.E.) of microplastics for species of bird of prey in central Florida was 11.9 (±2.8), and the overall mean number of microplastics per gram of GI tract tissue was 0.3 (±0.1). A total of 1197 pieces of plastic were recorded. Microfibers accounted for 86% of total plastics followed by microfragments (13%), macroplastics (0.7%) and microbeads (0.3%). Most fibers were either clear or royal blue in color. Micro-Fourier-transform infrared spectroscopy (μ-FTIR) found that processed cellulose was the most common polymer identified in birds (37%), followed by polyethylene terephthalate (16%) and a polymer blend (4:1) of polyamide-6 and poly(ethylene-co-polypropylene) (11%). Two bird species, Buteo lineatus (red-shouldered hawk, n = 28) and Pandion haliaetus (osprey, n = 16), were sufficiently abundant to enable statistical analyses. Microplastics were significantly more abundant per gram in the gastrointestinal tract tissue of B. lineatus, that consumes small mammals, snakes, and amphibians, than in fish-feeding P. haliaetus (ANOVA: p = 0.013). If raptors in terrestrial food webs have higher densities of microplastics than aquatic top predators, then it potentially could be due to a combination of direct intake of plastics and indirect consumption via trophic transfer.

Feeding Ecology Drives Lead Exposure of Facultative and Obligate Avian Scavengers in the Eastern United States

Lead poisoning of scavenging birds is a global issue. However, the drivers of lead exposure of avian scavengers have been understood from the perspective of individual species, not cross-taxa assemblages. We analyzed blood (n = 285) and liver (n = 226) lead concentrations of 5 facultative (American crows [Corvus brachyrhynchos], bald eagles [Haliaeetus leucocephalus], golden eagles [Aquila chrysaetos], red-shouldered hawks [Buteo lineatus], and red-tailed hawks [Buteo jamaicensis]) and 2 obligate (black vultures [Coragyps atratus] and turkey vultures [Cathartes aura] avian scavenger species to identify lead exposure patterns. Species and age were significant (α < 0.05) predictors of blood lead exposure of facultative scavengers; species, but not age, was a significant predictor of their liver lead exposure. We detected temporal variations in lead concentrations of facultative scavengers (blood: median = 4.41 µg/dL in spring and summer vs 13.08 µg/dL in autumn and winter; p = <0.001; liver: 0.32 ppm in spring and summer vs median = 4.25 ppm in autumn and winter; p = <0.001). At the species level, we detected between-period differences in blood lead concentrations of bald eagles (p = 0.01) and red-shouldered hawks during the winter (p = 0.001). During summer, obligate scavengers had higher liver lead concentrations than did facultative scavengers (median = 1.76 ppm vs 0.22 ppm; p = <0.001). These data suggest that the feeding ecology of avian scavengers is a determinant of the degree to which they are lead exposed, and they highlight the importance of dietary and behavioral variation in determining lead exposure. Environ Toxicol Chem 2020;39:882-892. © 2020 SETAC.

Intergeneric Hybridization of a Vagrant Common Black Hawk and a Red-Shouldered Hawk

Natural intergeneric hybridization occurs rarely in raptors. In 2012, L. Hug observed a juvenile hawk interacting with a banded adult Common Black Hawk (Buteogallus anthracinus) in Sonoma County, California, USA, and reported that the juvenile was a Common Black Hawk × Red-shouldered Hawk (Buteo lineatus) hybrid (Hug 2016; Western Birds 47:325–326). A disparity of opinions concerning the parentage of the juvenile prompted us to document the nesting efforts of this Common Black Hawk in 2013 and 2014. The 2013 nesting attempt failed during incubation before the identity of the mate could be confirmed. In 2014 the adult female Common Black Hawk bred with an adult male Red-shouldered Hawk (B. l. elegans), and this pair fledged one hybrid offspring. Evidence of this hybridization included observations of the pair engaged in courtship flights (n = 3), copulations (n = 2), and simultaneous nest attendance (n = 2), and the offspring's intermediate physical characters and vocalizations. Although the hybrid offspring's juvenile plumage more closely resembled a juvenile Common Black Hawk overall, the hybrid also exhibited some intermediate plumage, morphological, and vocal characteristics. This hybridization event appears to be an example of the desperation hypothesis, in which a rare vagrant, unable to secure a conspecific mate, settled on a species of raptor abundant in western California. Dietary overlap of the two parental species (e.g., crayfish, amphibians) and strong associations with aquatic habitats may also have facilitated this pairing and hybridization event.