Breeding territory fidelity in a partial migrant, the American dipper Cinclus mexicanus Holly A. Middleton, Christy A. Morrissey and David J. Green Middleton, H. A., Morrissey, C. A. and Green, D. J. 2006. Breeding territory fidelity in a partial migrant, the American dipper Cinclus mexicanus .  / J. Avian Biol. 37: 369  / 178. American dipper Cinclus mexicanus populations are frequently composed of resident individuals that occupy permanent territories year round and migratory individuals that overwinter with residents but migrate to breeding territories on higher elevation creeks each spring. Between 1999 and 2004 we examined how migratory strategy (resident/migratory) and sex differences influence breeding territory fidelity of American dippers occupying the Chilliwack River watershed, British Columbia, Canada. Counter to expectation we found that the migratory strategy of American dippers did not influence whether birds breeding in one year were found on their former breeding territory in the next. Migratory strategy also did not affect the probability that known surviving dippers occupied the same breeding territory in the following year. Males and females were equally likely to be found on their former territory in the following year (females 43%, males 41%) and known survivors had similar levels of breeding territory fidelity (females 74%, males 68%). However, breeding territory fidelity of males and females varied in response to different factors. Surviving female dippers were more likely to be found on their former breeding territory in the subsequent year following a successful breeding attempt than an unsuccessful breeding attempt. Prior reproductive performance did not influence whether surviving male dippers were found on their former breeding territory. Male dippers were more likely to be found on their former territory and, if they survived, have higher breeding territory fidelity when their mate also returned to that same territory. Mate retention also influenced whether females were found on their former territory in the following year but had no effect on the breeding territory fidelity of known survivors. We argue that sex-specific dispersal decision rules in American dippers are driven by sex differences in the predictability of breeding performance between years and sex differences in how mate retention influences subsequent reproductive success. H. A. Middleton and D. J. Green (correspondence), Centre for Wildlife Ecology, Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6 CANADA. E-mail: djgreen@sfu.ca. C. A. Morrissey, Canadian Wildlife Service, 5421 Robertson Road, Delta, BC V4K 3N2 Canada. Strong breeding territory fidelity has been observed in many species of birds (Greenwood 1980, Greenwood and Harvey 1982). Individuals are thought to benefit from returning to a previously occupied breeding territory because familiarity with a territory can promote foraging efficiency or predator avoidance, reduce con- flicts with neighbours and facilitate mate retention which in turn enhances reproductive success (Hinde 1956, Hoover 2003). Nevertheless, there is considerable varia- tion in the levels of fidelity to a breeding territory both within and among species (e.g. brown thrasher Toxos- toma rufum 13  /30%; Haas 1998, bobolink Dolichonyx oryzivorus 45  /51%; Bollinger and Gavin 1989, protho- notary warbler Protonaria citrea 34  /96%; Hoover 2003). Studies examining factors that influence variation in levels of fidelity to a breeding territory in bird species have frequently found that males are more site-faithful # JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 37: 169  /178, 2006 JOURNAL OF AVIAN BIOLOGY 37:2 (2006) 169