Does plumage colour signal fitness in the tawny owl Strix aluco?

Abstract

Plumage colour has been hypothesized to signal fitness in its component parts survival and reproduction. In the tawny owl Strix aluco, grey individuals have been considered to be more viable and productive than brown ones in cold environmental conditions. The recent increase in the frequency of the brown morph in Finland has been explained by the climate warming. However, in recent decades there have been no trends in the general mildness of winters. Therefore, trends in the abundance of owls due to climate warming per se were unexpected. In a population near the southern coast of Finland, the abundance of grey (not brown) individuals increased significantly during the period 1986-2018, suggesting that natural selection has recently favoured the grey colour morph. During snowy and cold winters grey owls seemed to survive better than brown ones, suggesting that plumage colour signals adaptations to winter weather. The proportion of brown individuals in each year was best explained by the joint effect of snow cover and winter temperature. Extreme peaks in snow fall and cold winters seemed to affect substantially and rather similarly the abundance of both colour morphs. Between population differences in the occurrence of colour morphs were probably due to local and temporal differences in the snow cover. The number of fledglings was significantly related to the colour morph and weather conditions of the preceding winter in males but not in females. This suggests that coloration of males signals fitness in reproduction.

Body Condition in the Tawny Owl Strix aluco near the Northern Limit of Its Range: Effects of Individual Characteristics and Environmental Conditions

“…The present study extends and summarises the results of some earlier studies [ 9 , 27 , 30 ]. It examines how the annual and seasonal variations in food supply and winter weather conditions are reflected in the body condition of breeding females and males of different colour morphs (grey and brown) in a population of Tawny Owls near the boreal southern coast of Finland.…”

“…Reproduction near the northern limit of the species’ range is particularly affected by the highly fluctuating abundance of small voles, the staple prey of owls [ 19 , 20 , 21 , 22 , 23 ]. The plumage colour seems to signal fitness in survival and reproduction [ 12 , 24 , 25 , 26 , 27 ]. Grey Tawny Owls seemed to be more viable and productive than brown ones in cold environmental conditions [ 28 , 29 ].…”

“…It examines how the annual and seasonal variations in food supply and winter weather conditions are reflected in the body condition of breeding females and males of different colour morphs (grey and brown) in a population of Tawny Owls near the boreal southern coast of Finland. This population seems to live in somewhat different environmental conditions than populations studied elsewhere in Finland [ 27 ]. In this population, body condition seemed to be largely determined by the bird’s age, time of the season, and stage of the nestling period [ 30 ].…”

Significance of plumage colour for winter survival in the Tawny Owl (Strix aluco): revisiting the camouflage hypothesis

“…In this population the trends in the occurrence of colour morphs were largely opposite those found in the earlier studies (Brommer et al 2005, Karell et al 2011, i.e. the proportion of grey individuals, not of brown ones, increased significantly (Solonen 2021). The breeding habitats of owls were mainly rural, but some of the territories were situated in suburban or urban parks.…”

“…In a population in southern Finland, about 70% of both sexes were grey (Brommer et al 2005). In a nearby population, 47.7% of different individuals recorded were grey (46.2% of females and 49.3% of males) (Solonen 2021).…”