Sex ratios of 368 fledglings from 145 broods of Montagu's Harrier over the period 1987-93 are analysed. The sex ratio of fledglings was biased toward males, and this bias was close to statistical significance (203 males and 165 females, P = 0.052). Published data confirm this bias (N = 591, P = 0.003). The effects of several parameters on the sex ratio are then examined: food abundance, egg laying date, egg rank order, harrier density, breeding success and harrier colony size. We found significant effects of egg rank order, the first eggs being female biased, the others male biased, and to a lesser extent, colony size. The other factors did not affect sex ratios significantly. We discuss these results, compare them with other published harrier and raptor sex ratios, and show that there is no congruence amongst them.