Hawks and Doves: Factors Affecting Success and Selection in Goshawk Attacks on Woodpigeons

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Abstract
A trained male goshawk was used to arrange attacks on single pigeons and flocks, at brassica feeding sites and elsewhere. Captured pigeons were compared with shot samples from the same sites, using dry weight of the pectoralis minor muscle as an index of condition. Attacks on pigeon flocks at brassica sites were more successful than when the birds were feeding on grasslands and stubbles, possibly because of variation in pigeon condition. Attacks were more successful in the hour before sunset than in the 4 previous hours. Pre-roost crop filling may have made the pigeons more vulnerable in the last hour, and the hawk might have been trying harder to obtain food as dusk approached. Attacks on single pigeons, and on birds in small flocks, were more successful than those on flocks of more than 10 pigeons. This occurred partly because single birds were in poor condition and partly because the hawk achieved less surprise as flock size increased. The hawk may also have been less likely to encounter weak pigeons, more confused, and less confident in attacks on large flocks than on small ones. Unless the hawk surprised pigeons feeding in flocks it was usually outflown. Pigeons captured from flocks which did not fly until the hawk reached them were in relatively good condition, but selection for poor condition became more marked if the birds took off when the hawk was further away and it had to chase them. The predation was selective, partly because single pigeons tended to be worse in quality and more vulnerable to attack than birds in flocks, and partly because pigeons captured from flocks were below average in condition when the hawk did not achieve complete surprise. There was selection for diseased and defective pigeons, but not for those of one particular age or sex. Goshawk predation could also select for behavior which delays crop filling until as late in the day as possible, and for flocking.