Populations of forest grouse – capercaillie (Tetrao urogallus), black grouse (Lyurus tetrix) and hazel grouse (Tetrastes bonasia) - have been declining through all of Europe. Habitat loss, fragmentation and degradation are recognized to be the most important ultimate causes behind this trend. In Fennoscandia, there is a general consensus that forestry practices have a primary role, even though the mechanisms are still not fully understood. Nest predation is generally thought to be an important proximate cause of the declines, but how nest predation relates to habitat changes remains poorly understood. I combined long-term data provided by the Natural Resources Institute Finland (LUKE) from inventory studies, both for grouses and predators, with an artificial nest experiment. I investigated a) how predation rate varies with forest age and landscape structure; b) what is the possible role of non-native mesopredator species as predators; c) how nest predation rate relates to larger scale reproductive success. In spring 2021, I placed 141 nests with two hen eggs each, in the regions of Kainuu and North Karelia for 14 days with camera traps. The nests were equally divided between mature forests (>80 years), young forests (<40 years) and edges of mature forests (in a mature forest 5m from a clearcut or a field). I found that the overall predation rate was low (~13 %) and similar in the three sites, but predation time was faster in mature forests, suggesting that when these are scarce, they can act as an ecological trap by increasing nest detectability. However, nest predation decreased with the increasing of mature forests in the landscape around the nest, supporting the hypothesis that on a larger scale forestry may increase generalist predator densities. Areas with higher predator densities suffered higher nest losses. The main predators were pine martens, badgers and magpies, followed by bears and ravens. No nests were predated by raccoon dogs or American minks. There was no correlation between areas with higher nest predation and areas where grouse had lower reproductive success which may result from other factors, e.g., chick predation. My results add to the diverse outcomes of several studies of grouse nest predation in Europe, which together indicate large variation in nest predation, no consistency in predatory species, and weak effects of landscape composition on nest predation
