Migration Ecology, Stopover and Winter Habitats, and Mercury Concentrations of Red-shouldered Hawks (Buteo lineatus) Breeding in Central and Northeastern Wisconsin
Hanneman, Matthew James
College of Natural Resources, University of Wisconsin-Stevens Point
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Red-shouldered Hawks (Buteo lineatus) are forest-dwelling raptors that are designated as a threatened species in Wisconsin and a species of conservation concern throughout much of the Great Lakes region. While many studies have focused on their breeding ecology, little is known regarding their migration ecology and winter habitats. Furthermore, as top predators that forage on semi-aquatic prey, Red-shouldered Hawks may be at an elevated risk of mercury (Hg) contamination, however, little is known about mercury concentrations in the species. I used Global Positioning System (GPS) transmitters to describe the migration ecology, identify stopovers and winter habitats, and assess transmitter performance of adult Red-shouldered Hawks that bred in central and northeastern Wisconsin, USA. I also analyzed blood and feather samples to report total mercury (THg) concentrations in adults and nestlings, tested for differences in adult feather THg concentrations between study sites and between sexes, compared inter- and intra-nest blood THg concentrations of nestlings, and compared adult inter- and intra-tract feather THg concentrations of Red-shouldered Hawks in central and northeastern Wisconsin, USA. From 2018-2021, I observed migration routes and wintering territories for five adult Red-shouldered Hawks that bred in Wisconsin with GPS transmitters. On average, Red-shouldered Hawks migrated 919.7 km over 29.8 days at a rate of 30.9 km/day to their wintering territories. Red-shouldered Hawks wintered in Wisconsin, Illinois, Kentucky, Alabama, and Louisiana and spent an average 91.4 days on their wintering territories. In spring, Red-shouldered Hawks migrated on average 920.2 km over 11.4 days at a rate of 77.4 km/day to return to their breeding territories. During migration stopover events, Red-shouldered Hawks were primarily observed in deciduous (55%) and mixed forests (13%), and woody wetlands (12%). On average, during winter, Red-shouldered Hawks were primarily observed in deciduous (29%) and mixed forests (20%), and woody wetlands (26%). These results show that some Red-shouldered Hawks that breed in central and northeastern Wisconsin will undergo migration and winter in states south of Wisconsin, while others will over-winter in Wisconsin. These results also show that, in general, Red-shouldered Hawks in central and northeastern Wisconsin primarily use deciduous and mixed forest and woody wetlands during the breeding season, migration, and in wintering areas. In June 2020, we also collected blood and feather samples to investigate Hg concentrations in nestling and adult Red-shouldered Hawks. The average (± SE) total mercury (THg) concentration (blood) in adult Red-shouldered Hawks (n = 4) was 0.43 ± 0.05 µg/g and was 0.02 ± 0.003 µg/g in nestlings (n = 17). The average THg concentrations (feathers) for adult Red-shouldered Hawks were 1.88 ± 0.21 µg/g (breast, n = 10), 2.21 ± 0.22 µg/g (back, n = 8), and 3.81 ± 0.70 µg/g (s1, n = 8). THg concentrations (breast) did not differ significantly between sexes (t0.05(2), 8= 1.03, p = 0.34), or between study sites (t0.05(2), 8= -0.32, p = 0.76). Average blood THg concentrations across nests ranged from 0.003-0.042 µg/g. Inter- and intra-tract feather THg concentrations were highly variable with the highest concentrations and variability found in s1 (3.81 µg/g, range: 1.57-7.48 µg/g). Percentage of inter-tract THg variability ranged from 16 – 86% across individuals. The concentrations I reported may reflect average concentrations in Red-shouldered Hawks within central and northeastern Wisconsin, USA. Furthermore, concentrations may be directly related to diet and Hg intake from prey items. Nestlings exhibited very low blood THg concentrations likely due to the dilution effect and excretion of Hg into feathers during feather growth. Inter-nest variability suggests that Hg concentrations may be related to specific nest-site habitat characteristics or simply the byproduct of a limited sample size. The inter- and intra-tract variability may reflect THg concentrations at the time of feather growth. Due to the high inter-tract variability and the wide acceptance for using breast feathers, I suggest continuing to sample breast feathers, but collect multiple feathers in order to provide a more representative sample of the bird. This is the first study to use GPS transmitters to track movements of Red-shouldered Hawks. Understanding the migration ecology and winter habitats is critical for management due to the species’ secretive nature and specific habitat preferences. Additionally, to the best of my knowledge, this is the first study to report Hg concentrations in Red-shouldered Hawks in Wisconsin. Reporting these concentrations is important for evaluating the toxicological risks for a threatened species associated with floodplain forests and riparian areas.