Responses of Nesting Ravens to People in Areas of Different Human Densities

Adult Common Ravens (Corvus corax) respond in at least two ways to human beings at their nests (Hooper, Wilson Bull. 89:233-242, 1977; Stiehl, Aspects of the ecology of the Common Raven in Harney Basin, Oregon, Ph.D. diss., Portland State Univ., Portland, OR, 1978). Some birds leave the nest immediately and remain at a distance, soaring and calling (a timid response), whereas others closely approach the intruder, diving frequently and calling rapidly (an aggressive response). To determine whether different responses were related to human density and accompanying persecution,.I measured responses of nesting ravens toward me in a moderately populated agricultural area and in a sparsely populated rangeland area. I conducted my study in Franklin, Adams, and Grant counties in eastern Washington, during spring and summer 1978 to 198 1. Lands in the study area were either farmland with nonirrigated crops, or extensive shrub and grass rangeland where domestic cattle and sheep grazed. The farmland area was mostly level with few natural nesting sites. Rolling hills in the rangeland were interspersed with extensive basalt cliffs providing suitable nesting sites (Knight and Smith, Northwest Sci. 56:303-309, 1982). I located active nests each year by systematically searching all portions of each area. Of the 43 active nests used in this study, all of the nests in farmland (26 nests, 10 territories) were on man-made strucures (e.g., grain-storage elevators, highway bridges, abandoned barns) while in rangeland, all of the nests (17 nests, 8 territories) were situated on basalt cliffs. From current U.S. Geological Survey maps, I randomly picked 20 sections from each area (1 section = 2.5 sq. km) and counted the number of occupied human dwellings and roads. The farmland had more dwellings (t = 4.84; P < 0.0005) and roads (t = 7.39; P < 0.0005) than the rangeland. Raven nests in rangeland were significantly farther from the nearest highway (t = 4.62; P < 0.0025) and the nearest dwelling (t = 2.87; P < 0.025) than nests in farmland (Table 1). I noticed that there were unused cliffs near highways and buildings in rangeland, which suggested that ravens were attempting to avoid human contact by nesting on cliffs farther away. Farmland-nesting ravens usually lacked this choice, as nest sites were man-made structures and therefore situated close to roads, but not necessarily close to occupied homes. Apart from highway traffic, there was much more human activity in farmland than in rangeland. During 164 days spent in both areas, I saw people daily in farmland (usually associated with farming activities), whereas in rangeland I saw people only twice. Responses of adults were measured each year when the young were between two and three weeks old and when both adults were present. Accompanied by an assistant, I approached each nest at distances that exceeded adult flushing distances and from a direction with a clear line of sight to the nest. At each nest we recorded (1) the distance between us and the nest when each raven initially flew, (2) how closely each raven approached us while we stood at the base of the nest structure (i.e., cliff or manmade structure) during a 3-min period, (3) how closely each raven approached me while I climbed to the nest, (4) the number of calls given during my timed ascent to the nest, and (5) the number of times the birds dived at me during the ascent to the nest. Distances were measured using a rangefinder except when ravens were very close (less than approx. 20 m), in which case distances were estimated. Responses 1 and 2 were measured at nests between 1978 and 1981 while responses 3, 4, and 5 were measured at nests in 1981. In all cases, one adult stayed farthe from us than the other; therefore, in these analyses, I used the responses of the closer raven in each visit. R vens nesting in farmland flew sooner (i.e., at significantly greater distances; t = 18.95; P < 0.0005) as we approache the nests than those nesting in rangeland (Table 2). Likewise, farmland ravens stayed significantly (t = 9.64; P < 0.0005) farther away from intruders at the base of the st structure than did rangeland ravens. Ravens in rangeland approached significantly closer (t = 5.51; P < 0.0005) to a per on climbing to the nest than farmland ravens. Lastly, ravens in rangeland called significantly more often (t = 5.11; P < 0.0005) and dived significantly more often (t = 5.47; P < 0.0005) at a person climbing to a nest than ravens in farmland.