Breeding Northern Harriers, Circus cyaneus, and their principle prey, the vole Microtus pennsylvanicus, underwent synchronous fluctuations in New Brunswick between 1980 and 1984. Microtines were abundant in 1980 and 1983 and were significantly tracked by the number of nesting harriers (r = 0.90), the number of polygynous males (r = 0.89), the number of harem females (r = 0.90), and the mean clutch size (r = 0.94), but not the reproductive success of successful females (r = 0.72). Male nest defence likewise exhibited a strong relationship (r = 0.99, n = 3) with prey abundance, but nest predation did not. An unexpected association with prey abundance was the greater proportion of young females (≤ 2 years) breeding at vole lows; the reverse was true for yearling males. Young females that did breed at vole highs were significantly more productive than were old females breeding at highs. The difference arose principally through nest predation. Successful females also consistently reared significantly greater proportions of their hatchlings when voles were increasing than when they were decreasing. Our results suggest that New Brunswick harriers were affected by prey fluctuations in most aspects of their reproduction and population dynamics. Significant correlations between male food provisioning rates and clutch size and reproductive success over 3 years provide a proximate mechanism through which fecundity may vary annually. They may also provide a proximate pathway mediating for polygyny.