Ardea
Official journal of the Netherlands Ornithologists' Union

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Schipper W.J.A., Buurma L.S. & Bossenbroek P. (1975) Comparative study of hunting behaviour of wintering Hen Harriers Circus cyaneus and Marsh Harriers Circus aeruginosus. ARDEA 63 (1-2): 1-29
Hunting methods of wintering Hen Harriers Circus cyaneus (abbreviated to c.c.), and Marsh Harriers C. aeruginosus (abbreviated to c.a.) were studied in Flevoland (Netherlands) and the Camargue (France) (Table 1). The results are compared with those obtained on spring and winter food (Schipper 1973b). The study is especially aimed at questions of interspecific and sexual (intraspecific) differentiation. The choice of hunting habitat appeared to be connected with hunting methods, which were mainly directed towards surprising the prey. The 'Surprise effect of the encounter predator/prey seems to be determined by vegetation and terrain structure, flying speed and flying height and the conspicuousness and manoeuvrability of both the predator and the prey. In addition ground bunting of c.c. and c.a. was observed in areas with abundant voles. Some of the hunting characteristics mentioned appeared to be influenced by wind speed and wind direction relative to the direction of flight. Over higher vegetations the flight was proportionally higher, presumably to maintain sufficient, insight into the vegetation. The hunting method of the c.c. ?? seemed to have the largest possible surprise effect, in contrast to c.a. ?? (= birds in ? plumage), which showed the strongest preference for carefully scrutinizing higher and more structured vegetations (see Table 7). Of the species and sexes compared c.c. ?? have the lightest underparts and c.a. ?? the darkest. These colour differences are assumed to result, in differing surprise effects on prey. The inter- and intraspecifically different hunting methods mentioned above enhance the surprise effect differences related to the conspicuousness of the underparts. The differences found agree (1) with the conclusions of Nieboer (1973), who, basing himself on the structure of the flying apparatus, considered Cc. to be more manoeuvrable than Ca., and ?? more manoeuvrable than ?? and (2) with conclusions drawn from the food composition during the nesting season (Schipper 1973ib). The same structural characters that lead to interspecific and sexual differences in manoeuvrability may explain certain differences in migratory habits between Cc. and Ca., and between ??and ??of these species. In sympatrically nesting harriers the significance of the sexual size dimorphism, in which the ? is the smaller sex, can only be understood on the basis of differences in size and agility of the prey, the division of duties of ? and ? during the nesting season, and to differences in breeding season and choice and extent of hunting habitat.


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