Factors Affecting Interannual Movements of Snowy Plovers

Abstract

-We studied the interannual movements of 361 individually color-banded adult Snowy Plovers (Charadrius alexandrinus nivosus) at Great Salt Lake, Utah from 1990 to 1993. In northern Utah, Snowy Plovers nested in a dynamic environment; suitable breeding habitat declined by 50% at two study areas in four years. Male Snowy Plovers were more site faithful than females; 40% of males exhibited fidelity compared with 26% of females (P < 0.01). However, as the amount of available suitable nesting habitat declined, male site fidelity diminished, whereas female fidelity remained relatively constant. We found strong evidence that female site fidelity was affected by nesting success in the previous year. Females that nested unsuccessfully were less likely than successful females to exhibit site fidelity the following year; males did not exhibit this nest-success bias. In addition, unsuccessful females breeding at sites with high densities of nests tended to disperse the following year, whereas male site fidelity did not appear to be affected by either a study site's overall nesting success the previous year or a study site's nest density the previous year. Female avoidance of areas with high densities of nests may be an antipredator strategy. Snowy Plovers in northern Utah have biparental incubation duties, but only males care for broods. Familiarity with brood-rearing areas was one plausible explanation for male-biased fidelity. However, we could not eliminate an alternative hypothesis that both focal study sites represented scarce breeding areas due to the presence of freshwater, and male Snowy Plovers preferred to use the same areas rather than disperse. We propose that more landscape-level studies are needed to address questions concerning local and regional movement patterns. Received 22 August 1994, accepted 27 January 1995. MIGRATORY BIRDS face a dilemma when returning to their breeding grounds: Should an individual breed at the same site as the previous year, or disperse to a new area? Individuals faithful to their previous breeding grounds presumably have some knowledge concerning the distribution of essential resources, predation risks, and potential mates (Shields 1982, Oring and Lank 1984, Gratto et al. 1985, Wiens and Cuthbert 1988). However, dispersing individuals have the opportunity to find a new territory or another mate, either of which could change reproductive output (Ollason and Dunnet 1978, Cuthbert 1988). Birds typically exhibit female-biased natal and breeding dispersal (i.e. females disperse farther and more often; Greenwood 1980, Greenwood and Harvey 1982; but see Oring and Lank 1984). The proximate and ultimate causes for sex-biased dispersal are still 3 Present address: Department of Natural Resources Science, University of Rhode Island, Kingston, Rhode Island 02881, USA. E-mail: ppaton@uriacc.uri.edu under debate (Bollinger and Gavin 1989, Anderson et al. 1992). Previous studies of avian populations have found a strong tendency for individuals to exhibit a higher degree of site fidelity after a successful nesting attempt than after an unsuccessful one (Greig-Smith 1982, Oring and Lank 1982, Gratto et al. 1985, Cuthbert 1988, Part and Gustafsson 1989, Hepp and Kennamer 1993; but see Haig and Oring 1988a, Schamel and Tracy 1991). In addition, older birds are often more likely to return to the same area than younger birds (Oring et al. 1983, Oring and Lank 1984, Thompson and Hale 1989, Montalvo and Potti 1992). Site quality, measured in terms of the number of young fledged from an area, also can affect return rates, with greater return rates to higher-quality sites regardless of an individual's nesting success the previous year (Bollinger and Gavin 1989, Montalvo and Potti 1992). Charadrius plovers provide an excellent opportunity to investigate proximate causes of dispersal. Birds in this genus often forage on unvegetated substrates (making it easy to see