Habitat Shift of the Willow Tit Parus montanus in the Absence of the Marsh Tit Parus palustris

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Abstract
The Willow Tit and the Marsh Tit are sibling species and occupy largely different habitats in Central Sweden. While the Marsh Tit prefers deciduous woods, the Willow Tit mainly occupies coniferous forests. In the nearby Aland Islands the Marsh Tit is absent, presumably due to its lack of dispersal propensity, and there the Willow Tit was found to have expanded its use of deciduous habitats significantly. None of the other six species in the tit guild exhibited different habitat preferences in the two areas, which is taken to indicate that differences in habitat per se were not important. Instead, relaxed interspecific competition is a more likely explanation of the habitat expansion of the Willow Tit in allopatry. On the mainland we recorded interspecific hostility during the breeding season, but territories of the two species sometimes overlapped. Willow Tits have not evolved specific morphological adaptations to its more frequent use of deciduous trees in the Aland Islands, probably because also there most of it foraging takes place in coniferous trees. Furthermore, the Willow and Marsh Tits are so similar in their morphology that it is unlikely that morphological differences alone restrict their habitat utilization in central Sweden.