We compare behavioural repertoires of breeding Circus cyaneus in a dense eastern Canadian population with published accounts of behaviours from other parts of North America and Europe. Canadian harriers exhibited behaviours previously unrecorded in North America, such as male and female mate rejection, increased courtship displays by polygynous vs monogamous males, a decline in the number and intensity of courtship displays with declining vole Microtus pennsylvanicus abundance, male initiation of nest building, parent-offspring recognition, and a high incidence of late nestling starvation due to male desertion. Populations in Canada and Orkney differed in the timing and cause of chick starvation and in the extent to which adult and immature males and females displayed.