Foraging Behavior of Northern Harriers Wintering in Southeastern Salt and Freshwater Marshes

-We conducted a comparative study of the foraging behavior, hunting success, and diet of wintering Northern Harriers (Circus cyaneus) at North Inlet Marsh, a salt marsh in South Carolina, and Paynes Prairie, a freshwater marsh in Florida. The relative use of five hunting methods in the salt and freshwater marshes differed significantly (P < 0.0005). Harrier pouncing rates at Paynes Prairie were three times greater than at North Inlet Marsh, but their hunting success was significantly less (5.8% vs. 15.1%, P < 0.0005). Greater capture success at North Inlet Marsh was caused by differences in diet and vegetation structure rather than by differential use of hunting techniques. Harriers hunting at North Inlet Marsh captured exclusively birds, whereas at Paynes Prairie harriers captured primarily cotton rats (Sigmodon hispidus). Although differences in pouncing rate, capture success, and diet occurred between the two study areas, prey capture rates at North Inlet Marsh (27 g/h) and Paynes Prairie (23 g/h) were similar. Our finding that harriers had higher capture success hunting smalland medium-size birds on a salt marsh than cotton rats on a freshwater marsh contradicts the generalization that more mobile prey are more difficult to capture and illustrates the importance of comparative foraging studies. Received 3 January 1986, accepted 4 May 1986. accepted 4 May 1986. NORTHERN Harriers (Circus cyaneus) winter in a variety of habitats in North America, including pasturelands, croplands, dry plains, unforested uplands, estuaries, and freshwater marshes (Brown and Amadon 1968, Bildstein in press). As a consequence of this widespread distribution, their diet varies. Small rodents are the typical winter food in many areas (see Clark and Ward 1974, Sherrod 1978 for reviews). Craighead and Craighead (1956) found meadow voles (Microtus pennsylvanicus) comprised over 90% of the winter diet of harriers in Michigan. In Ohio, however, birds comprised 40% of the prey taken by males and 4% of the prey taken by females; meadow voles comprised most of the remainder (Bildstein 1978). In the southeastern United States, cotton rats (Sigmodon hispidus) are an important winter food in freshwater marshes (Jackson et al. 1972), while in coastal salt marshes, marsh rabbits (Sylvilagus palustris; Tomkins in Bent 1938) and Clapper Rails (Rallus longirostris; Sprunt and Chamberlain 1970, Bildstein pers. obs.) often are important. Schipper et al. (1975) reported that diet and hunting behavior of Hen Harriers (C. cyaneus cyaneus) wintering in Europe varied with habitat and that sexual differences existed in foraging behavior. In North America there have been no comparative studies that examine the influence of winter habitat and prey taxa on the hunting behavior of Northern Harriers. We compared the foraging behavior, hunting success, and diet of harriers wintering in salt and freshwater marshes, two important wintering habitats in the southeastern United States. STUDY AREA AND METHODS We observed Northern Harriers hunting over a salt marsh in South Carolina and a freshwater marsh in Florida. North Inlet Marsh (NIM) is a 3,000-ha, highsalinity salt marsh 6 km east of Georgetown, South Carolina. The site is approximately 60% Spartina alterniflora salt marsh; 10% mud flats, sandbars, and oyster reefs; and 30% open water (Forth 1978). Paynes Prairie (PP), 3 km south of Gainesville, Florida, is a 5,600-ha basin that contains plant communities characteristic of freshwater marshes, wet meadows, and pastures (Easterday 1982). Harriers on Paynes Prairie typically were found in wet meadows dominated by 11 The Auk 104: 11-16. January 1987 This content downloaded from 157.55.39.59 on Sun, 16 Oct 2016 04:25:45 UTC All use subject to http://about.jstor.org/terms 12 COLLOPY AND BILDSTEIN [Auk, Vol. 104