The best dressed are less stressed: associations between colouration and body condition in a North American owl

Capsule: Plumage colour of Northern Saw-whet Owls Aegolius acadicus was strongly associated with body condition and may be used to distinguish the highest quality individuals. Relationships between eye colour and body condition were more complex and deserve further study. Aims: We explored the association of colouration with body condition of Northern Saw-whet Owls during their autumnal migration across Pennsylvania, USA from 1999 to 2012. Methods: We used fat and keel scores of female owls to index body condition. Since feathers are laid down during pre-migration moult, we hypothesized that facial white plumage would be more strongly associated with long-term condition (keel scores) whereas eye colour should indicate short-term condition (fat scores). Results: Facial white plumage and eye colour were largely uncorrelated, but were strongly associated with both fat and keel scores. Contrary to our hypothesis, owls with more facial white plumage had both higher fat and keel scores, indicating that facial white was strongly associated with both short- and long-term condition. This appears to be because facial white was highest in individuals most capable of maintaining good condition in both scores (the highest quality owls). Relationships between condition and eye colour were more complex, since owls with highest fat scores but lowest keel scores had lightest eyes, possibly resulting from trade-offs with pigment function and immunocompetence. Our results also demonstrated environmental forcing (cyclic prey availability) of colouration and body condition, although not the relationship between them which remained consistent between years and for different ages. Conclusion: Facial white, but not eye colour, was a robust predictor of short- and long-term body condition, permitting detection of individuals in the best and most consistent condition. Further study of colouration and condition are needed to elucidate the extent of genetic control and environmental factors in feather melanization.