An examination of iris color variation in Northern Saw-whet Owls ( Aegolius acadicus ) relative to sex and age

Abstract

The factors that influence avian iris color variation encompass a range of variables, including demographic characteristics such as sex, age, and body condition, as well as environmental factors including diet and habitat. Some pigments found in irises, such as carotenoids, are energetically costly to produce, especially for predators who consume little to no plant-based food resources. In this study, we investigated the influence of sex, age, and location on iris color variation in a sample of 566 Northern Saw-whet Owls ( Aegolius acadicus ) captured during banding operations stationed along the Mississippi flyway in Minnesota, Wisconsin, and Indiana between 2016 to 2022. We predicted that: (1) female owls would display lighter iris colors on average than males; and (2) hatch-year birds would display lighter iris colors than older individuals. We tested these predictions using generalized linear models and interpreted our results using predicted probabilities. We found no significant difference in iris colors between males and females when using field-assigned sex or physiological proxies, e.g., body mass. In contrast, we found that after-second-year individuals displayed darker-colored irises than hatch-year, after-hatch-year, and second-year birds. Finally, owls at the Wisconsin banding station were significantly more likely to display darker-colored irises than at other banding stations. These findings highlight the complexity in the mechanisms driving carotenoid variation displayed in avian irises. Iris color may serve as a valuable standardized index of body condition that can be utilized in addition to conventional metrics, such as fat and keel scores.

Keywords

• body condition
• carotenoids
• indiana
• minnesota
• mississippi flyway
• mist netting
• owl populations
• wisconsin