BREEDING DENSITY IN THE PEREGRINEFALCO PEREGRINUSAND RAVENCORVUS CORAX

Abstract

SUMMARYThe relative stability of breeding density in the Peregrine and Raven in certain inland districts is examined in relation to population limiting factors. Problems of cenus‐making are considered, particularly the use of alternative nesting places. Breeding populations of the Raven have not fallen below 15% of the maxima since 1945. While the Peregrine now shows signs of local decline, it had numeral stability equal to the Raven during the preceding period, and this is the state under consideration.Of possible controlling factors, food supply is unlikely to be directly limiting. For breeding to occur, cliffs suitable in altitude, vertical height, cover and presence of ledges are necessary. Alternative cliffs and ledges are used if available but are not essential to ensure regular occupation of a territory. Availability of suitable cliffs controls nesting density at the lower levels, but does not impose an upper limit.The Golden Eagle is a dominant competitor for nesting cliffs and perhaps for food, and when nesting space is limited Peregrines and Ravens may compete with each other for cliffs. Deliberate persecution can deplete adult populations temporarily and reduce breeding success, especially in the Peregrine, but does not limit breeding density over a period, except in unsuitable terrain. For both species, breeding success and/or migration are sufficient to maintain numbers in the study regions. When breeding success is high, no increase in nesting populations follows. The Raven has non‐breeding surplus populations during the breeding season but this is less certain for the Peregrine.Although direct evidence for its existence as defence of an area is scanty, territorialism is regarded as the factor controlling maximum nesting density by producing a “proximity tolerance limit” between adjacent nesting pairs. It is believed that such territorialism is not itself an ultimate limiting factor but has evolved in relation to food supply so that the numbers of predators are permanently balanced against this factor.