THE FUNCTIONAL-SIGNIFICANCE OF SEXUAL DISPLAY - STONE CARRYING IN THEBLACK WHEATEAR

Male black wheatears, Oenanthe leucura, carry on average almost 2 kg o f stones to cavities inside caves before each clutch is laid. This cos tly behaviour may play a role in post-mating sexual selection, if fema les adjust their reproduction to the amount of stones carried by their mates, an hypothesis tested in two field experiments. First, breeding sites with many new stones also contained many old stones from previo us breeding seasons. In a stone removal experiment, in which all old s tones were removed from half of the territories, but not from control territories, males carried the same amount of stones following the two treatments. Reproductive success was similar in the two treatments, a nd the number of old stones carried was thus an unimportant feature of the display. Second, either the number of new stones present or those carried could be the important cue of the display; this was tested by manipulating stones during the display period. When the number of new stones was doubled every second day, males reduced their stone-carryi ng activity, and when the number of new stones was halved every second day, males compensated for the removal. Laying date and annual reprod uctive success were positively affected by the treatment and the numbe r of stones carried, but not by the number of stones present, suggesti ng that females pay attention only to the number of stones carried. (C ) 1996 The Association for the Study of Animal Behaviour