Habitat variation and population regulation in Sparrowhawks

Abstract

In a study area in south Scotland, Sparrowhawks did not occupy the available nesting places at random, but more often used those places where breeding success was highest (here called high‐grade places). Most birds stayed on particular nesting places for only one year, but others stayed up to 8 years. Some birds moved from low‐ to high‐grade places as they aged. Continued occupancy of certain places was thus produced by many different individuals occupying such places in rapid succession, but most staying for only one breeding season.On the most used (high‐grade) nesting places pairs produced more than enough young per breeding attempt to offset the average annual mortality, but on the less used (low‐grade) places they produced too few. Low‐grade nesting places therefore acted as a sink, whose occupancy could be maintained only by continual immigration. Over the study area as a whole, the population was in balance, with reproduction matching mortality.Habitat changed over periods of 15–30 years, as woodland matured. Nesting places in young woods, with small densely‐growing trees, showed the highest occupancy and nest success. Both aspects of performance declined as the woodland aged, and trees became larger and more widely spaced. Long‐term stability in nest numbers and success in the study area as a whole was associated with a system of rotational forest management, which ensured a continuing availability of young woods.It is proposed that spatial variation in habitat quality is involved in the regulation of breeding numbers. Removal experiments confirmed the presence of non‐breeders, which could attempt to breed when high‐grade nesting habitat was made available to them, but otherwise remained as non‐breeders despite the presence of vacant low‐grade habitat. This situation, involving an interaction between habitat quality and bird quality, probably occurs in some other raptors too.