Red-Shouldered Hawk Study in Southern Ohio

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RESEARCH ON SUBURBAN RED-SHOULDERED HAWKS

1997 to 2014

Since 1997, a team of ornithologists has been studying suburban Red-shouldered Hawks in the Cincinnati area, in an effort to understand the ecology of these interesting birds and the ways that they have managed to survive despite increasing urbanization.

After quantifying habitat surrounding suburban nests and comparing it to that surrounding nests in a rural forested control area (the Hocking Hills region of south-central Ohio), the researchers concluded that suburban hawks chose nests sites very similar to those in the rural area in most ways (See Journal Article). Nests were in large trees, situated among other larger-than-average trees, and within 27-33 m of seasonal or permanent streams, ponds, or other water. As expected, nests were closer houses and roads in the Cincinnati area than they were in the rural Hocking Hills region.

Red-shouldered Hawks in the suburban area had reproductive rates that were similar to those from rural Hocking Hills, averaging 1.7 young per active nest in suburban southwest Ohio and 1.6 young per active nest in Hocking Hills (See Journal Article). In the suburban southwestern Ohio study area, some nest areas consistently fledged more young than others. Some nest areas never fledged young, despite being active for 5–6 years.

Researchers have banded over 2000 nestling Red-shouldered Hawks with an aluminum USGS bird band on one leg and a plastic band (blue, black, green or yellow) on the other leg. The plastic bands have letters and numbers that can be seen using binoculars or a small scope. The purposes of banding are to individually identify hawks for documentation of movements and mortality. Of 899 nestlings banded by 3 teams of banders in suburban southwestern from 1955-2002, 43 (4.8%) were encountered (dead or alive) some time after fledging (See Journal Article). Mean distance from natal nest at time of encounter was 38.5 + 13.6 km and was related to with hawk age. Analysis of distance from natal nest at time of encounter indicated that 50% of suburban hawks were found <15 km from where they were hatched, 75% were <29 km away, and 95% were <62 km away. Average age of hawks found dead was 1.9 yr (N = 31). Analysis of age at recovery indicated that 50% of Red-shouldered Hawks were dead by age 1.2 yr, 75% by 2.4 yr, and 95% by 5.2 yr, although one hawk banded as an adult in 1998 was still alive in spring 2014. Most hawks died of unknown causes, but of those for whom the cause of death was known, 38% died by collision with motor vehicles and another 31% died of electrocution on utility wires or electric fences. This information suggests that there may be an increased risk to living in suburban area. In rare cases, hawks nested on human-made structures. One pair nested successfully on the rooftop of a 3-story apartment building for 4 years; another pair nested on a grill located on the deck of a suburban home for 3 years, but were not successful, and a third pair nested once on a rooftop of a house.

The abundance of Red-shouldered Hawks in study sites along streams in southern Ohio was negatively correlated to the abundance of Red-tailed Hawks there (See Journal Article), which was expected since breeding hawks of both species are highly territorial and will defend their territories from intruders of any species. The abundance of hawks along the streams was positively related to the number of small ponds within the stream corridor; this finding was consistent with other studies in the eastern U.S., many of which have shown an affinity of Red-shouldered Hawks for ponds, wet woods, wetlands, and riparian areas.

The researchers used radio-telemetry to measure the home ranges used by 11 Red-shouldered Hawks in suburban southwestern Ohio (See Journal Article). On average, the hawks used 90 ha (222 acres) during the breeding season and 165 ha (408 acres) annually. The average breeding home range contained 169 houses. The typical annual hawk home range was made up of 40.9% forested habitat, 50% suburban areas, 8.8% fields and pastures, and 0.3% wet areas. The suburban hawks spent more time in wet areas (along rivers, streams, and ponds) than was expected, based on the availability of these habitats within their home ranges, and they used suburban areas (houses, roads, and lawns) less than expected. The hawks did use suburban areas at times, hunting at bird feeders and ornamental ponds, and perching on playground equipment and utility lines.

The research team observed 21 Red-shouldered Hawk nests for a total of 256 hours, in order to measure food delivery rates to the nestlings, to identify prey types, and to quantify adult and nestling behaviors (See Journal Article). Small mammals made up the largest percentage of the identified prey (31.5%), followed by reptiles (22.7%), invertebrates (18.8%), amphibians (17.7%), birds (6.9%), and fish (2.5%). Season-long prey delivery rate averaged 3.4 prey items delivered per 4-hr observation period, or 116 g biomass delivered per 4-hr observation period. As the nestling aged, the parent birds spent less time brooding them, feeding them, and attending the nest. During the first two weeks of the nestlings’ lives, at least one adult was present in the nest about 70% of the day; nestlings <2 weeks old cannot maintain their own body temperature or feed themselves.

Red-shouldered Hawks typically line their nests with fresh branches of coniferous and deciduous trees. Researchers recorded all species of green vegetation present in 63 nests in southwestern Ohio and 35 nests in rural Hocking Hills. For comparison, they identified all trees in areas near nests in southwestern Ohio and Hocking Hills. Red-shouldered Hawks at both sites used branches from black cherry trees very often; more than 80% of nests at both sites contained black cherry branches as nest lining (See Journal Article). This was unexpected since black cherry trees made up only 4- 5% of the total trees in the areas near the nests. Other tree species selected by the Red-shouldered Hawks included white pine, red pine, eastern hemlock, and red cedar. It is not known why hawks selected these species, but black cherry is cyanogenic (the leaves produce hydrogen cyanide when crushed), and it is possible that it functions as a natural pesticide or pest repellent.

Like most raptors, Red-shouldered Hawks have reversed sexual size dimorphism, that is, the females are larger than the males. Differences in size can be used to distinguish the sexes in some species. Multiple body measurements were made on adult and nestling Red-shouldered Hawks. Adult males could be distinguished from the females by body mass alone (birds weighing less than 623.5 g were males). For nestlings older than 3 weeks, the length of the footpad and the body mass could be used to differentiate males from females (See Journal Article). Being able to sex adults and nestlings allows researchers to better understand sex-based differences in dispersal, survival, and behavior.

Body measurements can also be used to estimate age of nestling raptors. Researchers measured multiple body parameters of known-age Red-shouldered Hawks in Cincinnati and compared them to those of nestlings in Quebec. The length of the secondary feathers on the nestlings' wings was the most valuable parameter for estimating nestling age. Ohio nestlings differed only slightly from Quebec nestlings, with the Ohio nestlings have smaller secondaries relative to age.

While banding nestlings, researchers observed that some were infested with larvae of bird blow flies in their ears in some locations. Systematic study showed that 15 of 25 Hocking Hills nests had Protocalliphora avium larvae on one or more nestlings and/or pupae in the nest material. In contrast, no nestlings were parasitized in the southwest Ohio (suburban Cincinnati) area. Reproductive rates at parasitized and non-parasitized areas were similar, so it is likely that Protocalliphora avium infestations did not have a negative effect on fledging rates of young Red-shouldered Hawks (See Journal Article).

Suburban Red-shouldered Hawk nests were occasionally usurped by Barred Owls, which nest approximately 2 weeks earlier than Red-shouldered Hawks. Barred Owls have been considered as the nocturnal equivalent to Red-shouldered Hawks, but little was known about the habitat and ecology of suburban Barred Owls. Researchers located 21 nests of Barred Owls and compared nesting habitat to that of Red-shouldered Hawks. The primary habitat type surrounding nests of both species was forest (41% for Barred Owl nests and 46% for Red-shouldered Hawk nests), and the second-most common was low-intensity residential land (30% for Barred Owls and 29% for Red-shouldered Hawks). Pasture made up <15% for both species. No variables discriminated habitat surrounding nests of owls from those of hawks, suggesting that the two species use similar suburban habitat in southwest Ohio (See Journal Article).