Monday, January 10, 2011

The old hermit that lives in the woods

Hermit ThrushHermit Thrush, Hagg Lake, Washington Co., Oregon on 17 December, 2010 by Greg Gillson.


Here is another photo of the Hermit Thrush from last week's post.

This bird has more to tell us.

If you click on last week's photo and bring up the largest version from the pBase photo gallery, you can see a dark spot on the lower eyelid. I believe this is probably a tick. Birds can host numerous insect ectoparasites, including feather lice, fleas, mites, and ticks.

But I want to concentrate now on the opposite end of the bird--its tail.

Below I have created a larger view of just the tail. What do you notice? There are two things to see. Look close.



Did you notice that the tips of the tail feathers are very sharp-pointed, not blunt?

Did you notice that there are light and dark bars on the tail?

Both of these features indicate that this bird is less than about 15 months old. Due to the December date, we can say this bird is 5 or 6 months old. Why can we say this?

The pointed tips on the juvenile tail (and wing) feathers are common to many birds. Juvenile feathers are the first real (non-down) feathers a bird grows. The wing and tail of most smaller birds are kept for a year and molted in the fall. Adults have blunt ends to the wing and tail feathers. To have juvenile tail feathers in winter, this bird has to be about 6 months old.

Those light and dark bars on the tail? They are growth bars. Most adult birds molt their wing and tail feathers a few at a time, in sequence, symmetrically. Besides allowing the bird to keep flying through molt, it is less stressful to molt a few feathers than many. [Ducks are an example of birds that molt all their wing and tail feathers at the same time. They become flightless for several weeks after the breeding season.]

These growth bars are present on juvenile wing and tail feathers and not adult feathers, likely because of the nutritional stress incurred growing all those feathers at once.

According to Steve N. G. Howell in the 2010 book "Molt in North American Birds":
The dark bands represent feather material grown during the day, whereas the light bands indicate nocturnal growth. Thus, one pair of bands represents a day's growth and the total number of pairs indicates the number of days taken for the feather to grow. This phenomenon, which is analogous to that of growth rings in trees, is termed "ptilochronology."

There is your vocabulary word for the day. Remember to use "ptilochronology" several times today as you carry on conversations! And for extra credit, use "ectoparasites" and "ptilochronology" in the same sentence. I just did--and you can too!