In 2004, 39 pairs of Montagu’s Harriers were located in The Netherlands, i.e. 29 in Groningen (mainly East), 5 in the Frisian part of Lauwersmeer and 5 in Flevoland (distribution similar to previous years, numbers slightly higher; Figs. 1, 2). On average, 2.9 young/successful nest were produced. Adverse weather in June and July (mainly rainfall) resulted in partial losses among nestlings; overall breeding succes and nestling output was better than average following high vole numbers (prey list in Appendix 1). Both radio-tagged adult males in Groningen turned out to be bigamous, with nests 4.8 and 1.4 km apart; the 4 females raised 3 fledglings each. Other case of bigamy were recorded in Flevoland (females 3.1 km apart, each with 4 chicks), and in Heinitzpolder (Germany) with females 2.8 km apart. Without radio-telemetry it would have been difficult to note (or prove) bigamy, casting some doubt on the alleged low frequency of occurrence. Breeding of a 2nd calender-year female with a 3rd cy-male was proven (ringed bird captured, 2 chicks). Reading (colour)rings and capturing ringed birds revealed natal fidelity and dispersal up to 152 km from the natal site (Table I), including exhange with Germany (where much fieldwork was initiated in 2004).