To ensure long-term persistence, organisms must adapt to climate change, but an evolutionary response to a quantified selection pressure driven by climate change has not been empirically demonstrated in a wild population. Here, we show that pheomelanin-based plumage colouration in tawny owls is a highly heritable trait, consistent with a simple Mendelian pattern of brown (dark) dominance over grey (pale). We show that strong viability selection against the brown morph occurs, but only under snow-rich winters. As winter conditions became milder in the last decades, selection against the brown morph diminished. Concurrent with this reduced selection, the frequency of brown morphs increased rapidly in our study population during the last 28 years and nationwide during the last 48 years. Hence, we show the first evidence that recent climate change alters natural selection in a wild population leading to a microevolutionary response, which demonstrates the ability of wild populations to evolve in response to climate change.

We studied variation in plumage colour and life history in a population of tawny owls (Strix aluco) in southern Finland, using 26 years of data on individually marked male and female owls. Colour was scored on a semi-continuous scale from pale grey to reddish brown. Colour scoring was repeatable and showed a bimodal distribution (grey and brown morph) in both sexes. During the study period, colour composition was stable in the study population in both sexes. The sexes did not mate assortatively with respect to their colour. Colour was a highly heritable trait and was under selection. Grey-coloured male and female owls had a higher lifetime production of fledglings, and grey-coloured male (but not female) owls produced more recruits during their lifetime than brown individuals. Selection on colour was mediated through viability selection and not through fecundity selection. Our results reveal remarkably strong selection on a genetically determined phenotypic trait.

Summary1. Understanding which factors regulate population dynamics may help us to understand how a population would respond to environmental change, and why some populations are declining. 2. In southern Finland, vole abundance shows a three-phased cycle of low, increase and decrease phases, but these have been fading out in recent years. During five such cycles (1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995), all tawny owls Strix aluco were censused in a 250-km 2 study area, and their reproduction and survival were monitored.3. Males and females showed similar dynamics, but experienced breeders recruited more offspring and had higher survival than first breeders. Offspring recruitment, but not survival of breeding individuals varied in accordance with vole abundance. 4. The population's numerical response to prey abundance was primarily due to first-breeding individuals entering the population in the increase phase when immigration was the highest. Firstbreeding birds were younger, but experienced breeders were older in more favourable vole years. 5. A stage-specific matrix population model integrating survival and fecundity showed that, despite obvious variation in fecundity between vole cycle phases, this variation had limited importance for overall tawny owl population dynamics, but that the survival of experienced breeders during the low phase is most important for population growth. 6. Model and data agreed that the vole cycle drives the dynamics of this avian predator by limiting the recruitment of new breeders during the low phase. Population dynamics hence differ not only from the classic example of the species in a more temperate region in the UK where the number of territories is stable across years, but also from the dynamics of other avian vole predators in Fennoscandia where the recurring crash in vole abundance drastically lowers adult survival thereby creating vacancies.

In colour polymorphic species morphs are considered to be adaptations to different environments, where they have evolved and are maintained because of their differential sensitivity to the environment. In cold environments the plumage insulation capacity is essential for survival and it has been proposed that plumage colour is associated with feather structure and thereby the insulation capacity of the plumage. We studied the structure of contour feathers in the colour polymorphic tawny owl Strix aluco. A previous study of tawny owls in the same population has found strong selection against the brown morph in cold and snowy winters whereas this selection pressure is absent in mild winters. We predicted that grey morphs have a denser and more insulative plumage, enabling them to survive better in cold climate compared to brown ones. The insulative plumulaceous part of the dorsal contour feathers was larger and the fine structure of the plumulaceous part of the feather was denser in grey tawny owls than in brown ones. In the ventral contour feathers the plumulaceous part of the feather was denser in females than in males and in older birds without any differences between morphs. Our study suggests that insulative microscopical feather structures differ between colour morphs and we propose that feather structure may be a trait associated with morph‐specific survival in cold environments.

Multiannual cycles in the abundance of voles and other animals have been collapsing in the last decades. It has been proposed that this phenomenon is 'climatically forced' by milder winters. We here consider the dynamics of bank and field voles during more than two decades in two localities (170 km apart) in southern Finland. Using wavelet analysis, we show that a clear 3-year cycle disappeared in the mid 1990s. However, the vole cycle returned in both localities after about 5 years despite winters becoming increasingly milder. In both localities, vole cycles were mainly determined by bank voles after the period of noncyclic dynamics, whereas field voles were dominant before this irregularity. Wavelet coherency analysis shows that spatial synchrony temporarily broke down during the period of noncyclic dynamics, but was fully restored afterwards. The return of the cycle despite ongoing rapid climate change argues against 'climatic forcing' as a general explanation for loss of cycles. Rather, the population-dynamical consequences of climate change may be dependent on the local species composition and mechanism of delayed density dependence.