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Résumé :
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From April 5th to 16th 2003, a supposed hybrid woodpecker with intermediate plumage and song between the Grey-headed (GHW) and Green (GW) Woodpeckers was observed at Comblain-au-Pont (5degree34'35" E; 50degree28'42" N), 35 km beyond the regular distribution limit of GHW. This marginal localization is also distinctive of other observations of hybrids between these two species. The different elements that may have led to this hybrid woodpecker observation are reviewed : increased dispersal in marginal or isolated populations, unusual GHW influx during spring 2001 outside the known distribution area, hybridization conditions between these two woodpecker species. The latter aspect is further considered taking into account the cases observed in the Hautes-Fagnes (eastern Belgium). Four hybrid pairs (3 viridis x canus, 1 canus x viridis) were found during an 18 years period, that is 6.45% of the pairs including at least one GHQ. This high percentage is no doubt linked with the situation of this population at the north-western edge of the GHW breeding area. The higher frequency of GHW females in hybrid pairs is discussed regarding mating strategies (the females actively search for territories or males) and habitat characteristics (in the few cases observed, the hybrid pairs were formed in the preferred habitat of the male species ?). Attention is drawn on the potentially high frequency of alternative reproductive strategies, such as hybridization, occurrence of helpers at nest and intra- or interspecific brood quasiparasitism of unpaired females, that may appear at the GHW distribution edge.
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