Age-dependent differences in iris colouration of passerines during autumn migration in Central Europe

Abstract

Avian eye colour changes with age, but many aspects of this transition are still insufficiently understood. We examined if an individual's sex, age, species and body condition are related to the iris colour in common migratory passerines during their autumn passage through Central Europe. A total of 1,399 individuals from nine numerous species were ringed and examined in late autumn in northern Poland. Each individual was sexed by plumage (if possible) and assigned to one of three classes of the iris colour-typical for immatures, typical for adults and intermediate. We found that the iris was typical in 97.7% cases of immatures and in 75.8% cases of adults and this difference was significant. Species, sex and body mass index (BMI) had no significant influence on the iris colour. We show that iris colour in passerines in late autumn is strongly age-dependent and thus can serve as a reliable feature for ageing in field studies, especially in species difficult to age by plumage.

Keywords: Age determination; Autumn migration; Body mass index (BMI); Eye colour.

Figure 1. Three classes of iris colour used in this study.

A = class 1 (immature type grey iris) in Robin Erithacus rubecula, B = class 2 (adult type brown iris) and C = class 3 (iris intermediate between grey and brown) in Goldcrest Regulus regulus. Photos by Jacek Rogoziński.

Figure 2. Comparison of the three classes of the avian iris colour used in the study.

A = class 1: immature type grey iris, B = class 2: adult type brown iris, C = class 3: iris intermediate between grey and brown. Drawing by Tomasz Cofta.