Competition between European Starlings and Native Woodpeckers for Nest Cavities in Saguaros

-European Starlings (Sturnus vulgaris) have recently invaded Arizona and breed in some areas but not in similar areas nearby. In Arizona, European Starlings commonly nest in cavities in saguaro cacti (Carnegiea gigantea) but do not excavate these cavities. To examine whether European Starlings compete with native woodpeckers for nest cavities in saguaros, we studied Gila Woodpeckers (Melanerpes uropygialis) and Northern Flickers (Colaptes auratus) in areas with European Starlings and in similar, nearby areas with no European Starlings. We determined which factors explained the variation in the number of nests of each species present on fifteen 10-ha plots. We also compared the location and dimensions of nest cavities used by each species to determine whether European Starlings use Gila Woodpecker nest cavities, Northern Flicker nest cavities, or both. We found that European Starlings compete with Gila Woodpeckers but not with Northern Flickers. This competition decreases the number of Gila Woodpeckers that nest in areas where European Starlings nest. European Starlings used Gila Woodpecker nest cavities, and there was a negative relationship between the number of European Starling nests and the number of Gila woodpecker nests that explained 46.7% of the variation in the number of Gila Woodpecker nests on the plots. European Starlings did not use Northern Flicker nest cavities, and we found no relationship between the number of European Starling nests and the number of Northern Flicker nests. In addition, the number of Gila Woodpecker nests was positively related to the number of large saguaros and negatively related to the slope of the plot. The number of Northern Flicker nests was positively related to the volume of ironwood (Olneya tesota). The number of European Starling nests was negatively related to the distance to agriculture and large lawns. Received 29 June 1989, accepted 4 December 1989. EUROPEAN Starlings (Sturnus vulgaris) were introduced into North America in 1890 and rapidly spread throughout most of the United States (Kessel 1953). They were not observed in Arizona until ca. 1946 (Monson 1948). In Arizona, European Starlings commonly nest in cavities in saguaro cacti (Carnegiea gigantea), but they do not excavate these cavities (Kessel 1957). Gila Woodpeckers (Melanerpes uropygialis) and Northern Flickers (Colaptes auratus) excavate these cavities in saguaros for nest sites (Gilman 1915, Bent 1939), and the nesting season of the European Starling overlaps that of the Gila Woodpecker and the Northern Flicker (Gilman 1915, Bent 1939, Royall 1966). Brenowitz (1978) observed European Starlings usurp nest cavities from three pairs of Gila I Present address: Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 USA. Woodpeckers. One pair of Gila Woodpeckers lost three successive cavities to European Starlings. European Starlings have been observed usurping nest cavities from Northern Flickers in New Hampshire (Shelly 1935), Maryland (Howell 1943), and Massachusetts (Bent 1950). European Starlings have also usurped nest cavities from many other species including Purple Martins (Progne subis) in Michigan (Allen and Nice 1952), Red-bellied Woodpeckers (Melanerpes carolinus; Kilham 1958) and Wood Ducks (Aix sponsa; McGilvrey and Uhler 1971) in Maryland, Acorn Woodpeckers (Melanerpes formicivorus) in California (Troetschler 1976), Eurasian Nuthatches (Sitta europaea) in Sweden (Nilsson 1984), and Buffleheads (Bucephala albeola) in British Columbia (Peterson and Gauthier 1985). Van Balen et al. (1982) concluded that by competing for nest cavities European Starlings decreased the number of Great Tits (Parus major) nesting in their study area in the Netherlands. Our objective was to determine whether Eu367 The Auk 107: 367-375. April 1990 This content downloaded from 157.55.39.78 on Sun, 19 Jun 2016 05:49:35 UTC All use subject to http://about.jstor.org/terms 368 KERPEz AND SMITH [Auk, Vol. 107 ropean Starlings competed with Gila Woodpeckers and Northern Flickers for next cavities in saguaro cacti, and if so, to evaluate the effects of this competition. European Starlings presently breed in some areas of Arizona but not in similar areas nearby. Therefore, we were able to study Gila Woodpeckers and Northern Flickers in areas with no European Starlings and in similar, nearby areas with various densities of European Starlings. To assess the effect of competition, we studied the relationship between the number of European Starlings nesting in an area and the number of Gila Woodpeckers and Northern Flickers nesting in that area. We examined the relationships between habitat variables and the numbers of nesting Gila Woodpeckers, Northern Flickers, and European Starlings to separate the effects of habitat and competition. We also compared the location and dimensions of nest cavities used by each species to determine whether European Starlings use Gila Woodpecker nest cavities, Northern Flicker nest cavities, or both.