Interspecific interference competition in three European raptor species

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Abstract
(1) These analyses were based on 366 territory-years in Common Buzzards, 134 territory-years in Goshawks and 105 territory-years in Honey Buzzards. All sets of data which could have been biased by factors other than interference competition were previously omitted. (2) Common Buzzards and Goshawks defended territories against con- and interspecifics throughout the year in the study area. The migratory Honey Buzzards also defended their territories during the breeding season. Goshawks took over several nesting territories of Common Buzzards, and have also taken young of Honey and Common Buzzards. After Goshawks settled in territories of Common Buzzards, the Buzzard pairs disappeared from these areas. This seemed to be the most important evidence for competition for territories between species and had effects on local Buzzard density. (3) Interspecific spacing of all three species was highly regular, indicating a competition for space. Spacing between conspecifics was also regular. (4) Due to interference by Goshawks, Common Buzzards and Honey Buzzards lost a proportion of young each year, which was highly correlated with the distance to the nearest active Goshawk nest. (5) Between Common Buzzards and Honey Buzzards no such effect could be detected, despite interactions observed between these species. (6) Other factors, besides Goshawk interference, which could have lowered breeding success in Common or Honey Buzzards, such as food or weather factors, intraspecific interference or habitat differences, were ruled out.