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

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.

Does the sex and age of birds and the size of human settlements affect recapturing of the Great Tit (Parus major) at bird feeders?

Urban and rural habitats provide different conditions to wintering birds mainly due to different access to bird feeders. Returning to the food sources, even under the stress related to trapping, could play an important role in the energetic budget of wintering birds. We studied the duration of period between the first and the second capture of the Great Tits (Parus major) caught and ringed at bird feeders. We expected that recapturing of birds, which could be connected with their experience, would depend on their sex, age and on the size of human settlements (urban vs. rural areas), which could modify the behavior of wintering birds. We found that the length of the period was the shortest for immature females and the longest for adult males (the difference being 3.8 days in average). In contrast to more experienced adults, more frequent visits in case of immature tits, which increased probability of being trapped, could be affected by their weaker condition and smaller size, which resulted in feeding whenever it was possible. At the same time we did not find any differences between urban (duration of 29.5 days in average) and rural (28.5 days) areas. Differentiation in bird densities, access to feeders and various environmental factors seems to be the reason why this issue awaits further, more detailed studies including influence of weather on the behavior of birds.