Band-backed Wren / Tigerzaunkönig (Campylorhynchus zonatus) is a large wren distributed from Mexico to Ecuador where it also frequents high altitudes up to 3000 m. It breeds cooperatively, which means that breeding pairs get help raising their offspring from (mostly) genetically related individuals. Hence, this species can often be seen in noisy (family) groups year-round.
In a flock of five or six Band-backed Wrens in Santa Ana, Gutamala in February 2020, I noticed differences in eye color between individuals. While there were just single birds with a vivid reddish-brown iris, several birds in the group had light brown eyes. It seemed that last year’s juveniles (with browner eyes) were still in one flock with their parents. Unfortunately I wasn’t able to observe them building a nest, to see which or if the youngsters played a role.
M. T. Wells, who worked on the species in Costa Rica, kindly confirmed that „adults have a dark red iris, while fledglings have a blue iris that slowly changes to copper“. In the set of photos below you can see an adult with dark reddish brown iris and several younger birds with light brown eyes.