In Britain the Common Crossbill Loxia curvirostra population has increased markedly in abundance and range with the maturing of the post-war forestry plantations. However, little published information exists on the breeding ecology of the species in these plantations. Crossbills were more abundant in years of high Sitka Spruce Picea sitchensis cone production in the upland forestry plantations of South Wales. The breeding season began early in the year with a median laying date of 13 February and declined after this peak with few nests initiated after April, when the availability of Sitka Spruce seeds declined. The modal clutch size was 3 eggs and an estimated 35% of nests produced fledged young. Most nest failures were associated with adverse weather. Our observations indicate ephemeral occupancy of Sitka Spruce forests as a consequence of variable seed availability, a peak in breeding activity determined by photoperiod and food supply and a low level of breeding productivity.