THE FUNCTION OF AGGRESSIVE CHASES BY BREEDING BLACK AND RED KITES MILVUS-MIGRANS AND M-MILVUS DURING THE POSTFLEDGING DEPENDENCE PERIOD

Black and Red Kites Milvus migrans and M. milvus chase other raptors a pproaching their nests. The study of this behaviour during the post-fl edging period suggested that it reflects mainly, but not only, anti-pr edator behaviour. The frequency of vigilance and aggressive chases dec reased through the post-fledging period as predicted by theoretical mo dels of nest defence. Although predation risks were similar, Black Kit es invested more time chasing intruders than did Red Kites. Black Kite s, unlike Red Kites, chased away intruding juveniles, which may be int erpreted as a behaviour to avoid investment in unrelated fledglings. B lack Kites usually nest in loose colonies where the risk of, and selec tion pressures against, accidental investment in unrelated fledglings is likely to be greater than for Red Kites. Differences in aggressive chases by Black and Red Kites are better related to this than to diffe rent predation risks.