NEWTON I (1980) Summaries theme day Humans and birds of prey Densities and breeding succes of Sparrowhawks in Britain. LIMOSA 53 (4): 140-140.

Sparrowhawks Accipiter nisus in Britain declined in numbers in the late nineteen-fifties, and disappeared completely from some south-eastern districts. This decline was associated with the wide use of organochlorine pesticides, especially aldrin and dieldrin. Since that time, the use of these chemicals has become progressively more restricted, and over large parts of the country Sparrowhawks have recovered in numbers. However, the birds are still absent from some south eastern districts, where pesticide use was heaviest. In 1971, I began to study what was limiting numbers and breeding in 12 districts, in different parts of Britain.
      In all 12 districts, the hawks nested only in woods of a certain structure, and avoided woods that were too thick or too open. So their breeding population in any one district was limited partly by the amount of suitable woodland available. In each district, birds also used the same restricted nesting places in different years, but usually built a new nest each time, so that such places were easily recognised by the groups of nests which lasted for years. In wellwooded districts, nesting places were regularly spaced, but at different distances apart in different districts. Thus the average distances between nesting places (measured from the centres of the groups of nests) varied in the 12 districts from 0.5 km on fertile land to 2.1 km on more barren land. This was equivalent to woodland densities of 45 territories/km2 at one extreme to 0.2 pairs/km2 at the other. For the 12 districts, the average distances between nesting places in continuous woodland were highly correlated with altitude above sea level and with soil fertility, being farther apart. on higher and poorer ground. Sparrowhawks presumably did not respond to altitude or soil as such, but to the densities of their small-bird prey, which became sparser on higher and poorer ground. Hence, in anyone district, Sparrowhawk breeding numbers were limited both by the amount of suitable woodland, and also by the spacing within the woodland, in turn dependent on prey supplies.
      Radio transmitters were attached to birds to find their home ranges. Males were found to occupy individual home ranges spaced out through the nesting habitat. Each male built a nest near the centre of its home range, thus accounting for the regular nest spacing within woodland. The home ranges of the males varied in size between districts, being larger where prey were scarce, and thus accounting for the variable nest spacing between districts. The ranges of the males were almost mutually exclusive near the start of breeding, but expanded later in the season, to overlap considerably between individuals. Except when they were on nests, females had much larger home ranges than males, again overlapping widely between individuals. The range of anyone female might overlap three or more male ranges. As in the males, however, the ranges of females were much larger in areas where prey were scarce than in areas where prey were plentiful.
      By trapping and ringing the occupants of particular nesting places in successive years, it was found that more than half the females changed nesting places between one year and the next. Females were much more likely to change in poor habitats than in good ones, and more often after a breeding failure the year before than after a success. Most movements between breeding places were less than 5 km, but some were longer, and one bird nested in successive years at places 27 km apart. Less information was obtained for males, but they showed much greater fidelity to nesting places from year to year than did females.
      Details of breeding success, and eggs for chemicals analysis, were obtained from the same 12 areas as were used to study nesting density. Comparing areas, there was a strong relationship between breeding success and DDE levels in eggs, with the greatest production of young in areas where DDE levels were lowest. DDE is the main metabolite of the insecticide DDT. It reduces breeding output mainly by causing eggshell thinning (and thus egg breakage), but it also kills developing embryos, so that many incubated eggs fail to hatch. PCBs were also found to cause embryo deaths, thus further reducing breeding success. Nonetheless variations in DOE levels accounted for most of the differences between areas in breeding success. This chemical is still a major factor reducing the breeding success of Sparrowhawks in some parts of Britain.

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