Dawn Chorus Birdblog: Molto Bene
Sat May 31, 2008 at 06:27:05 AM PDT
If the world of birds and birding was like the human world, I'd be like a society section reporter, scrutinizing everyone's attire (to see how well they're doing) and checking on who's aging well and who's not. Fortunately, in the world of birds, there's actually some justification for doing so. I can just say I'm a molt geek, and let it go at that.
Nice outfit on this Cooper's Hawk, but she has a few loose ends there, doesn't she? Teenagers... sheesh.
(I'm at our cabin, with some computer problems and some dialup issues, so pictures will be trickling in over the next little bit of time. Lots more pix from the workshop that inspired this in last week's Dawn Chorus.)
Birds need to molt to replace their feathers, which become less effective as they get worn by age and exposure. Flight feathers lose their strength and body feathers lose their ability to keep the bird dry and regulate temperature. Once or twice a year (or more, or less... it's complicated) those feathers need to be replaced.
It takes energy to grow the feathers, and it takes more energy to fly and keep warm while they're missing, so birds have evolved different "molt strategies" to keep them functioning as well as possible through the process. Knowing a species' molt strategy allows you to studying patterns of replacement and from that, one can make a more accurate determination of age of a specific bird. This is most easily done in hand, but with good optics or camera gear and/or a little luck, it can be done in the field sometimes, too. (More about why this is useful below)
Juvenile Orange-crowned Warbler with new body feathers emerging. Notice the very loose texture of the existing body feathers
Birds' first sets of feathers are cheap, made of lower quality material and with a looser texture. The most dangerous period in a bird's life is when they're in the nest, helpless and unable to fly, in the same location for weeks. There is a premium on being able to get out of there as quickly as possible. Feathers need to be grown quickly, so the most resources go into the crucial flight feathers, but even those are normally skimpier than adult feathers. After the bird survives its first few clueless days out of the nest, they begin to replace the cheap stuff with better feathers. I'm not going to get into every species (thankfully) but will describe a few molt strategies below.
One strategy shared by all species is that they molt flight feathers in the wing of tail in mirrored pairs to keep them balanced. Sometimes feathers are lost due to accidents and the "partner" won't be replaced.
In passerines and some other small birds, they begin replacing body feathers very soon after leaving the nest. These juvenile feathers are extremely loose-woven on some species and, while they do offer a bit of insulation, they're not terribly effective. On many species, you will notice that just-fledged young have spotty or streaky plumage where their parents are solid colors (juncos and robins come to mind), and that plumage is usually gone within a matter of weeks. After molting the body feathers, they will some groups of feathers in the wings, and finally (usually in their second year of life) they will molt the flight feathers in their wing and tail. In many larger birds, some feathers may be held for longer than two years.
Common Yellowthroat (juvenile above, adult below). Notice the shape of the flight feathers - especially the outer feathers - which are more tapered in the juvenile, with pointier tips, and thicker with blunter tips in the adult. You can also see a difference in the quality of the covert feathers (the feathers that cover the upper parts of the wing), as well as some of the coverts already being replaced in the juvenile's wing.
By looking at the mixture of feathers on a bird, you can usually tell if it's a juvenile (or "hatching year"), a "second-year" with a mixture of adult feathers and some retained juvenile feathers, or an "after second-year" bird with all adult feathers, some older than others. This helps us get a better handle on bird populations.
Why does it matter? What does it tell us about population or habitat?
Depending on time of year, age can tell you some interesting things about populations. On the breeding grounds, for example, the mix of second-year birds (younger adults) vs. after-second-year birds (older adults) can tell you something about the quality of the habitat. Older birds are better able to get and defend prime nesting territories, so younger birds must often make do with lesser quality sites. (Think of a really desirable apartment building in a great location, with decent security and a lot of good places to eat nearby - many of the residents will be long-term tenants, and just a few units will turn over in any given year. In a dicey part of town there's a lot of turnover, although there will be some residents who do okay and can still raise a family in the tougher neighborhoods.)
If it's springtime (breeding season) and you find that most of the adult birds are older, then it's probably pretty high quality habitat. They think it's worth coming back to year after year, probably because they've been successful in their nesting attempts. If most of the breeders are in their second year of life, then it's a sign that the habitat may be undesirable for one reason or another. It could be a short-term problem (a predator just built a nest in the middle of the patch of trees) or it could be long-term (location is marginal for food protection). If a long-term monitoring program shows a change in the mix of young-vs-old breeders, it tells us that something is changing in the habitat.
I'll add a little more in the comments, but one last thing: Next week I'll be in Yosemite for my annual hawks and owls pilgrimage, so dcoronata will be guest hosting with a nice piece on the dawn chorus. I've read a bit of it already, and it's great.
Permalink | 60 comments