Spacing of Sparrowhawk Nesting Territories

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Abstract
Large differences exist in the density of sparrowhawk [Accipiter nisus] nesting territories in 12 areas of Britain, and there are environmental factors associated with them. In all these areas, sparrowhawks nested only in woodland of a certain structure (growth stage), and used the same restricted nesting territories in different years. The distribution of such territories had remained almost constant in each area for many years, and during the study period it changed only slightly, as certain woods entered and others passed beyond the stage at which they were suitable. In continuously suitable nesting habitat, nesting territories were regularly spaced, but at different distances apart in different regions: from 0.46 km in the South Solway plain (Cumberland) to 2.06 km in Mar Forest, on Grampian hillsides (Aberdeenshire). This is equivalent to densities of 473 to 24 nesting territories/100 km2; in practice such densities were found only over thoe (sometimes small) parts of each area which were rich in suitable woodland. Over the study areas as a whole, and including other habitats, densities of 14-96 nesting territories/100 km2 were found. Where nesting habitat was restricted, no evidence was found for clumping of nesting territories. Availability of nesting habitat, coupled with spacing behavior, was thus a major factor limiting the overall density of territories in all areas. Comparing areas, 79% of all the variation in the mean distance between nesting territories in continuous nesting habitat could be explained in terms of altitude alone (positive correlation, P < 0.001), and 69% in terms of land productivity alone (negative correlation, P < 0.001). This distance between nesting territories increased by 0.1 km for every 25 m rise in altitude or for every 0.36 points (on a 1-10 scale) drop in land productivity index. Probably these environmental factors were not important as such, but only through their influence on prey populations. In 3 areas where censuses were made of woodland songbirds (the main prey), the density differences found were in the same direction as in sparrowhawks but were not as great. In most areas, some 60-85% of all sparrowhawk nesting territories were occupied each year; but occupancy varied from 32% in 1 yr in Speyside (Inverness-shire) to 91-97% in 4 yr near Windsor (Berkshire).

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