Friday, June 12, 2009

Summer- When Aging Birds Gets Harder

When Peter Pyle published the Identification Guide to North American Birds, he forever changed the way banders look at the birds they handle. Now, instead of "ring and fling" banders find themselves looking at much more detail in order to determine age, sex and sometimes subspecies of birds they net. One of the things that banders have had to really learn are the feathers and molt patterns of birds wings. This is often key to figuring out the age of a bird.

This Black-capped Chickadee shows one of those characteristics we sometimes see - retained, older feathers that contrast distinctly with newer feathers. These brown feathers on the birds wrist help age this bird as a second year bird.

Sometimes just one feather gets replaced and contrasts with the others as in the greater coverts of this Indigo Bunting. The one blue feather in the row of coverts is the clue to see here.

Many times the clue we're looking for is much more subtle and sometimes even misleading. In the wing of this Brown Thrasher there is a contrast in the color of the tips of the greater coverts. The 2 outer coverts next to the primary coverts contrast with the rest of the greater coverts by being paler and not as white as the coverts nearer the body. This contrast would suggest a "molt limit", or difference in age of feathers, however, there is such a thing as a "pseudolimit" where there is a natural contrast in the colors of feathers not related to age. This can make some birds impossible to age. Knowing what to look for means looking at lots of birds over the years.

While this can slow down the processing of birds at a banding table, the information gained could give us a much more intimate understanding of the life histories of many of the bird we band.

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