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

Michał Polakowski,Joanna Śliwa-Dominiak,Magdalena Remisiewicz,Krzysztof Stępniewski

doi:10.7717/peerj.9188

Michał Polakowski, Joanna Śliwa-Dominiak + Show 2 more

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https://doi.org/10.7717/peerj.9188

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Journal: PeerJ	Publication Date: Jun 8, 2020
Citations: 5	License type: CC BY 4.0

Affiliation: University of Szczecin, University of Gdańsk

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.

Highlights

The colour of bird’s eyes is an important, sometimes even striking or diagnostic, part of overall colouration
If a bird was aged first according to other traits, the chance was 1.84 times greater that the iris would be judged to be intermediate than if it was aged by the iris colour first
Our results show that iris colour is a reliable character for determining age in a large set of passerine species on autumn migration, especially for immatures

Summary

Introduction

The colour of bird’s eyes is an important, sometimes even striking or diagnostic, part of overall colouration. Most birds have dark eyes, irises range from red and blue through yellow to white. A large inter- and intraspecific variety of eye colours exist in birds, sometimes connected with sex but mostly with the age of an individual (Newton & Marquiss, 1982; Sweijd & Craig, 1991; Bortolotti, Smits & Bird, 2003; Wilson & Hartley, 2007; Nogueira & Alves, 2008; Mero & Zuljevic, 2015). Immature birds usually have dark irises, which turn paler or brighter as they mature. This phenomenon has been described and used for age determination in several families, including penguins, How to cite this article Polakowski M, Stępniewski K, Śliwa-Dominiak J, Remisiewicz M.

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