Molt, as discussed in our previous post, is a fascinating and essential process in a bird's life cycle, that is worthy of greater study. But bird banders and scientists don't often collect data on molt just to understand molt itself. More often, molt data is used to determine a bird's age.

In the MAPS bird banding program, age is one of the top three things we want to know about a bird when we band it, the others being species and sex. MAPS banders are primarily working with "passerines", or small, perching, terrestrial birds, and unless a bird is already banded or is one of a handful of species in which eye color changes with age, essentially only two features can give you clues about its age: its skull and its feathers.

Skulls are only useful in determining age for a limited time. Birds' skulls consist of a single layer of bone when they are young, and since their skin is also very thin and translucent, you can see a bird's skull simply by parting the feathers on its head. Examining the skull to see if, and to what extent, the second layer of bone has grown in can help banders identify birds that are less than a year old. However, this technique doesn't work for all bird species, mainly just songbirds. And since most songbird's skulls are fully formed by their first fall, "skulling" doesn't tell you much about age beyond that point.

Collecting data on molt and feathers can tell you a lot more about a bird's age. A bird's first full set of feathers, also known as its "juvenile plumage," usually differs from its second set, even if there aren't obvious differences in coloration as in gulls. Juvenile feathers are grown all at once, rather than in a more staggered or sequential fashion like during a normal adult molt. Making feathers is energetically demanding, and young birds, especially altricial species, must grow them quickly because they are "sitting ducks" so to speak as long as they are in the nest. So birds grow a set of feathers that are structurally weaker than their later feathers will be, but good enough to get them out of the nest.

Juvenile feathers are visibly different from later sets of feathers in a few key ways. Juvenile body feathers are often streakier in color than adult feathers, and are loosely textured, or "filamentous." These get molted during the preformative molt, usually at a month or two of age, and thus juvenile body feathers can only be used for identifying recent fledglings. In contrast, the flight feathers (primaries, secondaries and rectrices) and some upperwing coverts are usually retained for a full year and can be used to age birds ("yearlings") through their first breeding season, when they are considered second year birds. These feathers often differ in shape, being narrower and more tapered at the tip than the same feathers on older birds, a sacrifice in quality for quicker fledging. Last, because they are structurally weaker, juvenile flight feathers and wing coverts tend to wear and fade more quickly and look a little shabby.

Bird banders aim to distinguish juvenile feathers from those of later adult plumages. This is especially important during the breeding season because it allows them to distinguish between adult birds (also known as after-hatching-year or AHY birds) and birds that hatched in the last few months (known as hatch year or HY birds.)

Knowing a bird's age helps scientists answer many different kinds of questions. First, it allows them to understand how a population of birds is faring. Population health isn't just determined by the numbers of birds, but by their ages. A large population that is producing few young birds isn't a healthy population. Likewise, a population that produces many young is not in good shape if few of them survive to adulthood. Age data can also help scientists figure out the points in a bird's life cycle where it is most threatened. Is the population losing individuals on the wintering grounds, or are poor conditions on the breeding grounds leading to low nesting success?

Detailed age data derived by examining molt has helped scientists discover important aspects of bird ecology. For instance, populations of Black-backed Woodpeckers, which thrive in burned forests, are typically highest in the first few years after a fire but then decline within a decade. Are these declines due to adult birds moving to better, more recently burned forest? Or are young birds seeking out newly burned areas far from where they hatched? IBP scientists used molt data to determine the ages of woodpeckers and found that newly burned areas tend to be colonized disproportionately by young birds. Population declines as the time since the fire increases reflect the lifespan of the colonizing woodpeckers or their young during the first few years after fire, rather than emigration by adults.

Detailed age data may also give clues about habitat quality. When breeding territories are limited, second year birds tend to be pushed out of the best habitats into less desirable locations by older birds. An area with a very large proportion of second year birds compared to older (after second year) birds may indicate the habitat is less suitable for that species.

Careful examination of feathers and molt has helped scientists answer questions about climate change is affecting birds in Yosemite National Park. IBP scientists recently used 24 years of MAPS bird banding data to determine how warmer, earlier springs affect the timing of breeding. But how do you pinpoint when birds began breeding? Finding nests is difficult and very time-consuming. Instead, the scientists looked at when the Yosemite MAPS stations started catching hatch year birds that were fresh out of the nest. They found that birds were indeed fledging earlier, tracking the trend towards earlier springs.

Molt can even provide clues to birds' evolutionary history. For instance, when genetic analysis suggested that falcons and parrots were more closely related than previously thought, IBP biologist Peter Pyle's study of their molting patterns added more evidence for the relationship. Many falcon species were known to molt their flight feathers in an unusual sequence and observations suggested that some parrots might molt in a similar fashion. So Pyle examined flight feather molt in over 4500 museum specimens, including 80 species of parrot and 23 species of falcon. He found that the two groups molted their feathers in a similar sequence, which wasn't shared by any other group of birds. This not only helped confirm that falcons and parrots are sister groups, but that molt sequence in birds is very fixed, this unique sequence having evolved in a common ancestor to both parrots and falcons over 70 million years ago!

Molt matters a great deal. It matters to birds, as a critical and costly life process that maintains their feathers. And it matters to scientists, because examining molt and feathers can help answer so many questions in bird conservation, ecology and more. And we would not have any of this valuable molt data without dedicated and skilled bird banders, who take the time to examine feathers closely, teach their peers, and share or publish their data.