Relationships between patterns of incubation and predation in sympatric capercaillie Tetrao urogallus and black grouse T. tetrix

Abstract

To investigate the effect of timing, frequency and duration of incubation recesses of female grouse on predation, thermistors were placed in 29 nests of capercaillie Tetrao urogallus and 10 nests of black grouse Tetrao tetrix in southeast Norway during 1983–84. Two female capercaillie were killed by goshawks Accipiter gentilis during recesses and 15 capercaillie and three black grouse nests were depredated. The incubation patterns of 20 capercaillie and nine black grouse were measured during 165 and 75 days, respectively. There was no difference between species in how long females stayed away from the nest each day, but black grouse took more and shorter recesses than capercaillie. The recess activity of both species peaked before and after a period of low activity during the night. Although black grouse took the first recess earlier and the last recess later than capercaillie, they also took few recesses in dim light. Nest losses were not associated with the time or frequency of departures from the nest. The data presented suggest that factors other than predation risk may be responsible for the patterns of incubation recesses observed in grouse.