Pied flycatchers nest over other nests, but would prefer not to

Abstrakti

Pied flycatchers (Ficedula hypoleuca) are sometimes known to construct their nest on top of a previously constructed tit (Parus spp.) or flycatcher nest, a behaviour dubbed "nest take-over". Here, we tested whether flycatchers truly prefer to "take over" nests instead of choosing unoccupied cavities, and if so, whether the behaviour is due to attraction to-wards boxes chosen by others, or simply taking advantage of the pre-existing substrate. Furthermore, we tested whether flycatchers prefer building nest on the nests of conspeci-fics or on those of heterospecific tits. We offered arriving flycatchers choice in a quartet of boxes with alternative contents: an unused flycatcher nest, an unused tit nest, sawdust, and an empty box. The flycatchers strongly preferred nest boxes with sawdust. In another experiment, we offered trios of nest boxes: an old tit nest, an old flycatcher nest and empty box. The flycatchers strongly avoided the empty boxes but did not show any preference for either the tit or the flycatcher nest. When offered to construct their nest in an empty box, flycatchers had to gather more nesting material by weight. Our results show that nest take-overs in pied flycatchers result from taking advantage of a pre-existing substrate to reduce the building effort required, rather than from scarcity of nesting sites or from any direct social or competitive interaction. The results also show that pied flycatchers avoid taking over nests when a similar material advantage is available by other means, presum-ably to avoid costs due to ectoparasites and/or competition.