Ferruginous and Swainson's Hawk Abundance and Distribution in Relation to Land Use in Southeastern Alberta

Abstract

Eighty 41-km2 (4.4% of total area) sample plots were selected at random in a 74,686-km2 area in southeastern Alberta and searched for nesting ferruginous (Buteo regalis) and Swainson's hawks (B. swainsoni). The estimated number of nesting ferruginous hawks was 1,082 pairs (?429) and the number of Swainson's hawks 3,879 pairs (?802). Ferruginous, but not Swainson's, hawks declined in abundance as the proportion of land under cultivation increased. Swainson's hawks nested in close proximity to human habitation whereas ferruginous hawks did not. The abundance of ferruginous hawks may be limited by the availability of grassland habitat, or by the survival of the hawks with only preferred habitat types occupied. The size and number of plots sampled was more than adequate to minimize sample variance but confidence intervals were wide because only 4.4% of the total area was sampled. J. WILDL. MANAGE. 48(4):1180-1187 Ferruginous hawks, the largest North American buteos, are endemic to prairie habitat in western North America (Brown and Amadon 1968). The breeding range of ferruginous hawks overlaps with the prairie portion of the Swainson's hawk's range. The two species have similar ecological requirements during the nesting period except that ferruginous hawks are more common in arid, treeless grasslands. Their degree of habitat segregation was probably greater historically than it is today (Olendorff 1973, Schmutz et al. 1980). Habitat quality for nesting hawks is probably determined by accessibility of food and nesting sites. However, availability of food is probably the most important factor determining the abundance of hawks (Newton 1980). Ferruginous hawks have declined in abundance in recent years (Tate and Tate 1982). The size of the North American population was estimated as 3,000-4,000 nesting pairs, including 500-1,000 pairs in Canada (J. K. Schmutz and S. M. Schmutz, unpubl. rep. 1980). Because ferruginous h wks nest only in or near grassland habitat, cultivation is considered a major factor in the decline of this species (Olendorff 1973, Houston 1979). This study was designed to determine the present abundance and distribution of ferruginous hawks in the prairie region of southeastern Alberta. Past and present distribution was compared and the relationship between nesting density and land use (cattle grazing vs. cultivation) examined. There is no evidence that the population size of Swainson's hawks has changed. The distribution of this species was also studied and serves to evaluate whether a change in distribution of ferruginous hawks reflects an area's unsuitability for nesting by raptors or the possible absence of requirements specific to ferruginous hawks. I gratefully acknowledge the financial support of the Alberta Environ. Res. Trust and the Fish and Wildl. Div. of Alberta Energy and Nat. Resour. I thank H. P. Samoil for his enduring and conscientious 'Present address: Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan S7N OWO, Canada. 1180 J. Wildl. Manage. 48(4):1984 This content downloaded from 207.46.13.11 on Mon, 17 Oct 2016 05:36:14 UTC All use subject to http://about.jstor.org/terms FERRUGINOUS AND SWAINSON's HAWK ABUNDANCE * Schmutz 1181 assistance in the field and S. M. Schmutz for her help throughout the project. The comments of J. W. Grier and H. R. Postovit were very helpful in revising earlier versions of this manuscript. STUDY AREA AND METHODS The study area encompassed the prairie region of southeastern Alberta (Fig. 1). The 80 quadrats sampled were 41 km2 each. Plot boundaries corresponded to existing section lines, and sections (2.56 km2) are used as units of area measurement because their boundaries were discernible on maps and in the field. The 80 study plots were searched for nests occupied by buteos from 18 May to 13 July 1982. This period corresponded to the latter half of the incubation period and the nestling period of ferruginous hawks and to incubation and nestling periods of Swainson's hawks. All clumps of trees within study plots were searched from a truck, trailbike, or on foot. A windowmounted spotting scope was used when foliage was sparse or absent. To search large coulees and eroded banks, two observers drove simultaneously along the top and bottom of embankments stopping intermittently to use binoculars to search for nests. Hawks usually flushed from their nests at these times. Gently undulating grasslands were searched by driving along ridges or across hilltops using trailbikes. The accuracy of this counting method was not statistically determined but 19 unoccupied ground nests found compared to 11 occupied by ferruginous hawks suggests that this searching method was effective. Nests were considered occupied if an adult, eggs, or young were seen in the nest, or if a completed nest cup, feathers, and droppings were found (Postupalsky 1974, Schmutz et al. 1980). The nests of two pairs C :: ?'';?'':::::il; ?: .;?:.;.::;:::-:: :~::: :ji: ::j:: '(~~'~:'~:::j. iji:~~i