Effects of prior nesting success on site fidelity and breeding dispersal : An experimental approach

-Based on more than 300 individually marked American Robins (Turdus migratorius) and Brown Thrashers (Toxostoma rufum), I tested three hypotheses to explain low return rates of birds whose nesting attempts are unsuccessful: (1) birds with low reproductive success are low-quality individuals that are more likely to suffer mortality between breeding seasons; (2) nesting failure increases reproductive effort by causing birds to renest, and this energetic stress increases the probability of mortality; and (3) birds use a "decision rule" based on prior experience to select nesting sites, such that individuals that experience low reproductive success are more likely to move to an alternate breeding site, whereas birds that nest successfully are more likely to breed in the same site again. Birds subjected to experimental nesting failure returned at a significantly lower rate (robins 18%, thrashers 12%) than birds that nested successfully (robins 44%, thrashers 29%). Birds that nested more than once in a season returned at rates (robins 43%, thrashers 21%) indistinguishable from birds that nested only once in a season (robins 36%, thrashers 23%). These results, as well as supplementary data, were inconsistent with hypotheses 1 and 2 and consistent with hypothesis 3. This study provides strong evidence that low return rates result from dispersal in response to nesting failure. Received 11 August 1997, accepted 12 March 1998. DESPITE THE OBVIOUS AND IMPORTANT DIFFERENCES between dispersal and mortality, we are unable to distinguish between them in many natural populations (e.g. Delius 1965, Harvey et al. 1985). Dispersal is difficult to study, and data on immigration and emigration largely are lacking for bird populations (Greenwood and Harvey 1982, Clobert and Lebreton 1991, Brawn and Robinson 1996). Birds that breed successfully in one year are more likely to return to the same site the following year than are those that fail to fledge young (e.g. Harvey et al. 1979, Gratto et al. 1985, Drilling and Thompson 1988, Gavin and Bollinger 1988, Part and Gustafsson 1989, Thompson and Hale 1989, Haas 1990, Payne and Payne 1993, Murphy 1996; but see Bollinger and Gavin 1989). When marked birds are not resighted on a study area, however, we generally do not know whether they have dispersed or died. Some authors have attributed low return rates of banded birds to mortality (e.g. Askenmo 1979, Harvey et al. 1985, Johnson and Marzluff 1990), whereas others have invoked dispersal (e.g. Hbgstedt 1981). Although evil Present address: Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. E-mail: cahaas@vt.edu dence suggests a link between reproductive success and survivorship (Pugesek and Diem 1990) and between reproductive success and breeding dispersal (Harvey et al. 1979, Drilling and Thompson 1988, Jakobsson 1988, Nur 1988a), a causal relationship between an individual bird's failure to produce young and an increased probability of that bird's subsequent dispersal or mortality has not been demonstrated experimentally. In this study, I examine three hypotheses to explain low return rates by birds that nest unsuccessfully. The low quality hypothesis posits that birds with low reproductive success are low-quality birds that are more likely to suffer mortality between breeding seasons and therefore are less likely to return to their previous breeding site (Coulson 1968, Lambrechts and Dhondt 1986, Pugesek and Diem 1990). The renesting stress hypothesis states that nesting failure increases reproductive effort by causing birds to renest; this energetic stress increases the probability of mortality. Lastly, the prior experience hypothesis states that birds use a "decision rule" based on prior experience to select nesting sites. Individuals that experience low reproductive success are more likely to disperse to another breeding site, whereas those that fledge young are more likely to breed again in the same site (Darley et al. 1977, Gratto