Factors Affecting Breeding Success of Peregrines in South Scotland

SUMMARY (1) During 1974-82, peregrines in south Scotland produced an annual average of 1 15 young per territorial pair, between 0-60 and 1.45 in different years. Heavy rain in May lowered productivity, by reducing hunting success and by soaking exposed nests. Early clutches, begun before mid-April, were larger and more successful than later ones. (2) All pairs nested on cliffs; the few large cliffs were preferred, so that, as the population grew, more pairs used small cliffs. Cliff height and accessibility to human climbers greatly influenced nesting success, as did exposure. Those nests in recesses were most successful. (3) Performance improved as females aged; laying became progressively earlier and clutches progressively larger, from 1 to 5+ years. (4) Despite some slight eggshell thinning (attributed to DDE), organochlorine residues had at most a small impact on productivity in the years concerned (in contrast to earlier years). (5) The overall density of breeding peregrines more than doubled during the 9-year study, and also varied between different parts of the study area. But over the range of densities observed, no density-dependent depression of breeding success occurred.