The Norwegian grouse-nest survival lottery

During incubation, hens in gallinaceous species are faced with trade-offs between self-maintenance and metabolic requirements of the eggs. The hen also has to minimize the probability of detection by predators, by nest site selection and nesting behaviour. I survived daily number and length of recesses, incubation constancy and the timing of recesses of 70 capercaillie (Tetrao urogallus), 11 black grouse (Lyrurus tetrix) and 18 willow ptarmigan (Lagopus lagopus) hens by the use of camera traps during 2009-2015. The differences in nesting behaviour between species were small. Capercaillie, black grouse and willow ptarmigan took 1.84, 2.06 and 2.25 daily recesses averaging 29.10, 25.90 and 26.64 minutes, resulting in incubation constancies of 0.96, 0.96 and 0.95, respectively. Predators detected nests only when the female was sitting on the nest. 31 of 69 (31%) nests were robbed the 2 hours after midnight when the incubation constancy was 0.97. During 04:00 – 06:00 capercaillie took most recesses, but was still sitting 95% of the time on the nest. The difference in nest predation between species cannot be attributed to differences in the observed nesting behaviour. Julian date, number or length of the recess, incubation constancy or interactions between the factors did not explain differences in nest survival of capercaillie hens. I suggest that variation within females normal incubation behaviour does not affect the probability of nest predation, and that predators generally detect grouse nests when the hen reveal her nest when escaping the approaching predator. The survival of a grouse nest is a lottery where the hen usually has little influence on the outcome