Figure 1.

Experimental setup. A symbol was randomly assigned to the tit nest, and the alternative symbol was attached to the adjacent empty box. These two boxes portray tutor choice, and tit territoriality prevents settling there. Two empty boxes with the alternative symbols were offered for arriving flycatchers approximately 25 m away, adjacent to each other. The two boxes fall within the small territory defended by male pied flycatchers, forcing the female to choose in which to build its nest. The tutor nest and its accompanying empty box were always visible from the flycatcher boxes. Replicate setups were at least a kilometre apart to ensure independence.