Avian mating systems                                                                            (Switzerland, 1990-1995) . Environmental effects on mating systems can best be studied in species living under clearly different ecological conditions. Therefore, we compared the breeding biology of Water Pipits (Anthus spinoletta) between alpine valley sides with different microclimate, food availability and predator density. The social mating system (i.e. who lives with whom?) was related to male quality, measured by age, territory size and prey bio- mass within the territory: quality increased from bachelors through polyandrous and monogamous to polygynous males, with monogamy being the dominating system. The genetic mating system (i.e. who produces offspring with whom?) deviated from the social one: several nests contained young of extra-pair paternity, extra-pair maternity or with no relatedness to either parent. These cases were not linked to individual quality of males or females. They probably reflect chance opportunities for extra-pair copulations and intra-specific brood parasitism that arise from environmental conditions such as distance of nests from shared feeding grounds. Dischma Valley, near Davos, Switzerland