Ardea
Official journal of the Netherlands Ornithologists' Union

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Bossema I., Roell A. & Baeyens G. (1986) Adaptations to interspecific competition in five corvid species in The Netherlands. ARDEA 74 (2): 199-210
Crows, Jackdaws, Rooks, Magpies and Jays occur sympatrically throughout the year, overlap to a larger or smaller extent in ecological demands and directly or indirectly interfere with each others breeding attempts. Since Crows are almost omnipresent and dominate the other species they have a central position in the interspecific relationships. Crow aggression consists of chasing off, molesting intruders, harassment, robbing nest material, taking over nests or damaging nest contents. Subordinate species can break through the territorial Crow's interspecific aggression in several ways. Most Jackdaws build nests in holes and often breed inside Crow territories. Hole-nesting Jackdaws are often attacked by territorial Crows, but are in their nests safe for molestation by Crows. Rooks build nests in compact colonies. Solitary nest~ can be found occasionally. Only these are sometimes taken over by Crows, which indicates that colonial nests are protected against Crows. Jackdaws, Magpies and Rooks tolerate a higher level of human activity than Crows and often breed close to human habitations. Magpie nests are usually spherical but the roof may be lacking. Unroofed nests are more often molested by Crows than roofed ones. In addition, roofed nests are more often situated near human habitations and have, on average, a better success than unroofed ones. When Crow pressure is high, Jackdaws and Rooks often forage in areas shunned by the more wary Crows. Some indications were obtained that these species profit from flocking, while foraging in Crow territories. Hole-nesting in Jackdaws, colonial nesting in Rooks, building of roofed nests by Magpies, a relatively low level of shyness in all three species, and foraging in flocks, can be considered as adaptations enabling subordinate species to feed and breed inside Crow territories.


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