Camouflage may promote fitness of given phenotypes in different environments. The tawny owl (Strix aluco) is a colour polymorphic species with a grey and brown morph resident in the Western Palearctic. A strong selection pressure against the brown morph during snowy and cold winters has been documented earlier but the selection mechanisms remain unresolved. Here we hypothesise that selection favors the grey morph because it is better camouflaged against predators and mobbers in snowy conditions compared to the brown one. We conducted an online citizen science experiment where volunteers were asked to locate a grey or a brown tawny owl specimen from pictures taken in snowy and snowless landscapes. Our results show that the grey morph in snowy landscapes is the hardest to detect whereas the brown morph in snowy landscapes is the easiest to detect. With an avian vision model we show that, similar to human perceivers, the brown morph is more conspicuous than the grey against coniferous tree trunks for a mobbing passerine. We suggest that with better camouflage, the grey morph may avoid mobbers and predators more efficiently than the brown morph and thus survive better in snowy environments. As winters are getting milder and shorter in the species range, the selection periods against brown colouration may eventually disappear or shift poleward.

Dataset contains:

- Data and R code

- Pictures for online citizen science game

- Validation study pictures