Ardea
Official journal of the Netherlands Ornithologists' Union

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Schipper W.J.A. (1978) A comparison of breeding ecology in three European harriers (Circus). ARDEA 66 (3): 77-102
Supplementary to previous studies on differences in food and feeding behaviour between the three sympatric harrier species Montagu's Harrier Circus pygargus, Hen Harrier Circus cyaneus and Marsh Harrier Circus aeruginosus, this paper deals with interspecific differences in breeding habitat and factors governing the breeding results. In our study localities in the Netherlands the vegetation of the breeding habitats of Circus pygargus and Circus cyaneus showed little differences, though on an average Circus cyaneus nested in more open vegetations (Table 2). Nests of Circus pygargus were on average at shorter distances from extensive open areas like crop areas, grasslands and saltings, than those of the other species (Table 3). In agreement with this it seems that in Europe Circus pygargus nests more frequently in crops than Circus cyaneus. Circus cyaneus nests more frequently in bogs and open spaces in forests. The differences in habitat are probably related to hunting properties of the species concerned. Circus aeruginosus almost invariably nested in reeds. According to literature data, this applies to its whole nesting range. In general Circus aeruginosus was the earliest breeder, Circus cyaneus started some days later, and Circus pygargus much longer afterwards (Table 4). The ecological implications of these differences have been discussed elsewhere (Schipper 1973). In our Netherlands' study areas spring temperatures affected the start of egg laying in all three species; Circus aeruginosus started egg laying at lower temperatures than Circus cyaneus, this again at lower temperatures than Circus pygargus. Correspondingly, a perusal of the literature revealed that local differences in date of first egg laying in some parts of the western Palaearctic probably are related to differences in spring temperature (Fig. 2). A relation between the breeding results and the first date of egg laying was found (Fig. 3). Early nests produced more young than late ones. In Circus pygargus and Circus aeruginosus the fledging results were negatively affected by precipitation during the nestling period. Unfortunately, the examinations on the influence of prey densities on start of egg laying and breeding results could not be complete. An expected negative influence of polygyny on the breeding success is discussed. Possibly, specific temperature demands, in addition to prey and breeding habitat demands, determines the differential northern boundaries of the nesting ranges.


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