Gap crossing decisions by reed warblers (Acrocephalus scirpaceus) in agricultural landscapes

To meet the need for research on the requirements for corridors for marshland birds, this study set out to quantify gap crossing decisions made by reed warblers moving through the landscape. In three experiments, reed warblers were released into landscape situations with different gap sizes and their movement towards reed patches fringing a watercourse were monitored. In all experiments, most birds flew over the smallest gap towards the nearest reed patch. In the experiment with two gap sizes, the probability of crossing a gap was a function of the ratio between distances to the reed patches. In the experiment with increasing gap sizes, most birds crossed the smaller gaps frequently. Near the bigger gaps, birds did not cross the gaps; instead, they only crossed the watercourse repeatedly. In the third experiment with more realistic landscape configurations.. the birds preferred nearby non-reed landscape elements to more distant reed patches. It is concluded that reed warblers were reluctant to cross gaps wider than 50 m. The results suggest that the presence and size of gaps in reed patches affect reed warblers' local gap-crossing decisions: when given a choice, the birds prefer to cross the smallest gap. Furthermore, reed warblers may be directed towards suitable marshlands by creating corridors of reed vegetation with gaps no wider than 50 m. The surrounding agricultural landscape and the presence of trees and ditches could decrease the reluctance to cross gaps in corridors.