Peregine Falcons laid clutches at 75% of temtories annually and fledged young from 58%. Sixty-seven per cent of pairs fledged young each year. Brood size at fledging was 2.16, equivalent to 1.44 young per pair, or 1.23 young per temtory. For the population persistent rain and low temperatures in the three to four months between egg laying and to a week or so after hatching had an adverse effect on breeding success; in years when the number of raindays was high fewer pairs bred, more clutches failed to hatch and some very young nestlings may have died. July-September raindays accounted for 60% of the variation in young fledged per territory while mean maximum temperature between August and October accounted for 74% of the variation in the percentage of territories from which young fledged and raindays in those months accounted for 58%. On the other hand, brood size was larger in years when it was wet during the nestling period. Overall, severe drought enhanced the breeding success of the population because more pairs bred successfully. Almost all of the adverse effect of rain could be attributed to nest quality: (1) in cold, wet years Falcons with a choice of nest, or with a well- drained, well-sheltered nest, bred as successfully as they did in dry years, while those without such nests performed poorly; and (2) the average number of young fledged from temtories with a choice of nest was about 50% greater than that from territories with only one nest. The advantage of holding a territory with a high quality nest(s) as oppposed to a poorer nest, has clear selective value in the Darwinian sense.