During the process of bird banding, several bits of data are collected on each bird captured and banded. Species is recorded, of course, and measurements are taken on how much fat it’s carrying and the length of the wing. Sex is noted, if possible. And the age. Ageing birds has always been my favourite part of the banding process; I find it fascinating that it’s possible at all to determine how old a bird is. Not only is it possible, it’s actually not all that hard.
The theory of ageing birds is based on molt patterns. Many birders are familiar with the concept of graduated plumage from gulls and eagles, but passerines (songbirds) can and do show a type of graduated plumage, too. For most species of birds, the end-of-summer molt undertaken by a young-of-the-year bird will be different than that for an adult, an individual that was a parent that summer. Young birds grew a full set of feathers while in the nest, of course. Feathers are very energetically costly to grow, so if the bird can avoid having to grow a whole nother set of feathers so soon after the first set, it will. As a result, hatch-year birds will only replace a subset of their feathers during their end-of-summer molt. How many and which ones varies from species to species, but very few species will replace all of their feathers as a hatch-year bird.
The adults, on the other hand, are wearing the same feathers they’ve had since the last year’s end-of-summer molt (some might have replaced their body feathers back in the spring, but very few will also replace wing feathers in the spring). After a year of use, of being subjected to wind and sun and thick vegetation, they’re a little worse for wear. At that end-of-summer molt, the adults will replace all of their feathers. And while they replace them sequentially, the timing is close enough together that they appear to be relatively the same age.
Compare that to the feathers of the hatch-year bird. Feathers grown as a nestling tend to be poor-quality: they’re grown all at once, and very rapidly – you can guess just how energetically costly this would be. To try to minimize the costs, the feathers grown are usually slightly narrower and shorter than those grown by an adult bird, more tapered at the tips, are less richly coloured, have fewer barbs, and are weaker such that they fade and wear faster than adult feathers. The feathers replaced in the end-of-summer molt, because the bird has more time to grow them, typically look like those of adults. Even though it may only have been two to three months between when the nest feathers are grown and when the end-of-summer molt is completed, there is often a noticeable difference between feathers of the two ages. This becomes even more pronounced by spring.
The bird in the top photo replaced the topmost tertial (the tertials being the three layered feathers that line up all in a row when the wings are folded) during its end-of-summer molt last year, but not either of the two lower tertials. The difference is subtle, but visible if you know what you’re looking for. The feathers are a richer, darker brown, with richer, wider edging. There’s a little bit of wear at their edges, but that’s to be expected with feathers that are exposed all of the time, and the wear along the outer edge isn’t as great on the upper tertial as it is on the lower two. This visible difference in age, called a molt limit, tells us this bird was hatched last summer.
The bird below is the same age; it was also hatched last summer. But the reason we know this is different. For some reason, this bird has had to replace all of the tertials and some of the inner greater coverts (those shorter feathers at the top of the tertials) on its left side. This might have been a run-in with a predator, maybe a fight with another bird, perhaps feather mites or some disease like that; I don’t know. Whatever the reason, it’s had to replace these feathers independently of its usual molt – we know this because all molts, whether hatch-year or adult, occur symmetrically on both wings. These feathers are relatively fresh. Dark, broad, richly-edged, these are adult-type feathers and it’s pretty easy to see the contrast between them and the hatch-year feathers on the right wing. Having them side-by-side like this makes the differences especially apparent. You can even see how the hatch-year feathers are shorter than those of the adult.
It’s obviously a lot harder to see molt limits on live birds in the field than it is while they’re in your hand, but in a few species it might be possible to spot them on birds at your feeders, for instance. The second bird here would undoubtedly stand out. It’s kind of neat to be able to look at a bird and know its age.