Copyright © 1978 Ohio Acad. Sci. 0030-0950/78/0006-0323$1.5()/0 INTER AND INTRASPECIFIC INTERACTIONS BETWEEN RED-TAILED HAWKS AND GREAT HORNED OWLS IN CENTRAL OHIO1 MARK ANDREW SPRINGER, Department of Anatomy, Wright State University, Dayton, OH 45435 JOHN STEPHEN KIRKLEY,2 Department of Zoology, Ohio Weselyan University, Delaware, OH 43015 Abstract. Interspecific relationships between Red-tailed hawks and Great horned owls reveal distinct chronological activities that facilitate their inter-related nesting patterns. The hawk to owl ratio in the study area in 1976 was approximately 1.3 : 1. This proximal nesting may have had some effect upon Red-tailed Hawk success, in that Great Horned Owl predation on Red-tailed Hawks was related to their inter- specific nesting distances. Although Great Horned Owls partially depend upon Red- tailed Hawks for their nest sites (59%) and the diet of these two raptors do overlap in some prey selected, direct competition for these resources is minimized by the temporal segregation of their breeding chronologies and their daily activity patterns. OHIO J. SCI. 78(6): 323, 1978 Interspecific relationships between area located in Delaware County, Ohio. To- pography in Delaware County is flat to gently Great Horned Owls (Bubo virginianus) rolling and land is devoted largely to agricul- and Red-tailed Hawks (Buteo jamaicen- ture, with small percentages of pasture and sis) have raised many questions concern- woodlots. Most of the continuous woodlots ing the status of their coexistence. Pre- were located along the 3 major drainages traversing the study area from north to south. ceived as ecological counterparts, these To determine nesting populations of RTHs two large, cosmopolitan raptors reveal and GHOs, foot and aerial surveys were con- distinct chronological activities that al- ducted. The reproductive chronology was de- ternate their dominant-subordinate rela- veloped by interpolating hatch dates from known body weights of each nestling. Age of tionships, facilitate their inter-related the nestlings was determined by comparing nesting patterns, and stabilize their inter- body weights of each nestling to known growth specific competition and predation (Craig- rates of 5 nestling RTHs and 2 GHOs taken in head and Craighead 1965). Do Red- the study area in 1975 and 1976. This informa- tion together with the natural growth rates re- tailed Hawks and Great Horned Owls ported by Hoffmeister and Setzer, (1947) en- operate independently of one another, or abled reliable approximations of nestlings' age have their survival strategies mutually for the first 3 weeks of growth. Beyond this coevolved? Although little quantitative time, age was determined by the linear measure- ment of the 7th primary and/or overall size and information addresses the behavior of plumage development. Length of incubation, such mixed populations, studies which in- 34 days, was used for both GHOs (Baumgartner vestigate both species simultaneously 1939) and RTHs (Hardy 1939 and present may help to shed some light on such study). Length of nestlife for GHO's and RTHs was 45 days (Hoffmesister and Setzer questions. 1947) and 42 days (this study), respectively. Inter and intraspecific internest distances of STUDY AREA AND METHODS RTHs and GHOs were calculated by plotting Nesting populations of Red-tailed Hawks the location on topographical maps and cal- (RTH) and Great Horned Owls (GHO) were culating the linear measurements between nest studied from 1974 to 1976 on a 197 square mile sites. Manuscript received 16 August 1977 and in RESULTS AND DISCUSSION revised form 5 April 1978 (#77-63) The foot surveys and aerial surveys 2Affiliated with the Raptor Survey Project of the Ohio Biological Survey. Current address: revealed that density and productivity Utah State Dept. Botany, Logan, UT. of both raptors in 1976 were similar to 323 324 M. A. SPRINGER AND J. S. KIRKLEY Vol. 78 the averages of those values reported 21 February with a mean at 5 February. elsewhere in North America, but our Hatch dates ranged from 15 February to density figures for both species were 26 March with the mean at 10 March. considerably higher than those reported Fledging dates ranged from the last week previously in Ohio. In 1976 we located in March to the first week in May. 59 active GHO nests (1 pair/3.16 sq. mi.) Laying dates at 55 RTH nests in 1976 and 78 RTH nests (1 pair/2.39 sq. mi.). ranged from 6 March to 6 April with the Each season, the GHO is notably the mean at 16 March. Hatching dates first raptor to begin its nesting, and this ranged from 9 April to 10 May with the behavior, together with its noctural hunt- mean at 19 April. Fledging dates ranged ing abilities, has enabled it to dominate from the last week in May to the last over other nesting raptors (Craighead and week in June with the mean at 3 June. Craighead 1956). In the spring of 1976, A number of reasons for the staggering we noted that nesting of GHOs preceeded of nesting sequences have been proposed that of the RTHs by approximately 40 by Craighead (1956) and Orains and days (fig. 1). Laying dates at 26 GHO Kuhlman (1967). Perhaps the early nests in 1976 ranged from 12 January to nesting of the GHO reduces interspecific 130- Eggs •• • • • Hatchlings - Fledglings -• Potential Nest Lives Time FIGURE 1. Reproductive chronology of 63 Red-Tailed Hawk nests and 39 Great Horned Owl nests in Central Ohio, 1976. Here fledglings are denned as young ready to leave the nest; thus, maximum number of fledglings decreases as they leave the nest. Potential nest lives represent the total number of nestlings produced before inherent egg loss (7.3% and 8.8%) and nestling mortality (19.7% and 13%) for Red-Tailed Hawks and Great Horned Owls, respectively. Ohio J. Sci. INTERACTIONS BETWEEN RTH AND GHO 325 competition for prey species which both average internest distance of 2.9 km it and the RTH utilize, or perhaps prey (Hagar 1975) and 3.1 km (Mclnvaille species are more vulnerable during the and Keith 1974). The shortest distance early spring months (before new vegeta- between active GHO nests observed in tion provides sufficient cover). Others 1976 was approximately 0.9 km. Ad- postulate that early nesting is a mechan- jacent RTH nests were spaced an average ism by which GHOs obtain their limiting an average distance of 1.5 km apart, with resource, nest sites; therefore early nest- a minimum distance between active ing enables GHO to gain first access to nests of 0.6 km. The average internest old RTH nests before RTHs have a distance for RTH reported by other re- chance to reoccupy them (Craighead and searchers averaged 2.07 km (table 1). Craighead 1956, Orains and Kuhlman The relative densities of RTH and 1956). Whatever the reasons for its GHO populations can be expressed in existence, the staggering of the nest terms of a simplified ratio of active hawk activities is a widely reported phenome- to owl nests. Those previous studies non which may have evolved in response highest disparity of RTHs to GHOs oc- to the interspecific relationships of the curred in sparsely wooded study areas, GHOs and other nesting raptors. whereas those with the most equal num- Intraspecific nesting distances of GHOs ber of RTHs to GHOs occurred in heavily averaged 2.0 km, which was below the wooded areas (table 2). In contrast, our TABLE 1 Intra and Interspecific nesting distances (km) of Red-tailed Hawks (RTH) and Great Horned Owls {GHO). Avg. RTH Avg. GHO Avg. Internest Reference Internest Internest Distance Distance Distance RTH and GHO Springer & Kirkley (1978) 1.5 2.0 1 Hagar (1957) 1.8 2.9 — Mclnvaille & Keith (1974) 2.1 3.1 — Seidensticker & Reynolds (1971) 2.1 — — Smith & Murphy (1973) 3.3 — — Wiley (1975) 0.84 TABLE 2 Nesting densities of Red-tailed Hawks and Great Horned Owls with respect to percent wooded study area. GH Owl RT Hawk Density Density Researcher and Ratio (km2) (km2) (Study Site) % Wooded Hawk to Per Per Owl Nesting Nesting Nesting Pair Pair Kirkley & Springer (1978) 9.0 1.3:1 8.2 6.2 (OH) Mclnvaille & Keith (1974) 34.0 1.3:1 10.1 7.5 (Canada) Hagar (1957) 50.0 1.5:1 12.2 8.0 (NY) Orians & Kuhlman (1956) — 2.3:1 14.5 7.3 Cornman (1973) 3.0 5:1 82.9 24.9 Gates (1972) 0.3 10:1 108.8 10.6 (WI) 326 M. A. SPRINGER AND J. S. KIRKLEY Vol. 78 study area was not extensively wooded evidence indicates predation by GHOs. (9%) but supported the highest nesting GHO feathers were found in 3 nests, and density of RTHs and GHOs, as well as in 1 case, time lapse photography docu- having at 1.3:1 RTH to GHO ratio, mented the predation by the GHO. equal to that of the heavily wood study As a rough test to determine whether areas. We believe this was due, in part, the success of RTH nests was related to to our extensive coverage of the study their internest distance from GHOs, 69 area and not to any difference in habitat RTH nests were categorized according to per se. their distance from the nearest GHO Internest distances between nearest activity center using the following four RTH and GHO in 1976 averaged 1.0 km, categories: less than 0.5 km, between 0.5 which fell considerably below the aver- and 1.0 km, between 1.0 and 2.0 km, and ages of either hawk to hawk (1.5 km) or greater than 2.0 km. Scattergram plots owl to owl (2.0 km) nest distances.