Thursday, May 7, 2009

How old's that thrasher?

Posted by Kirk
It was a beautiful Thursday at the Lee & Rose Warner Nature Center where my co-host of the Twin Cities Naturalist Podcast, Paul and I work as naturalists. One of the fabulous things about being a naturalist is that you get to spend time outside on beautiful days. Many new migrants showed up today and the school group coming out took the Spring Birds class so that means banding! Fairly early on, the banders caught a beautiful large Brown Thrasher. What a gorgeous bird. Check out that gold eye! I usually think of thrashers as desert birds since that's usually where I see them. There are Brown Thrashers at the nature center every year but for some reason I never seem to stumble upon them. It was a thrill to see it up so close. The photo doesn't even begin to do this bird justice.

The next photo gives you a real idea for the size of a Brown Thrasher. This particular bird had an interesting feature that can be used to age the bird. Banders need to know all kinds of tricks to figure out how old a bird is. Look carefully at the tail of the thrasher. Notice anything?

Sometimes banders look at the condition of tail feathers, the fresher and less frayed, the newer. This tail is a little worn but that isn't the important thing to notice. There's a faint light colored band on all of the feathers about an inch or so from the tip of the tail. Can you pick it out? A variation in color on a feather is not uncommon. Sometimes there are series of bands that correspond to feathers growing at night or during the day. In the case of this thrasher though, there is only one band and it was likely caused by a change in diet while the tail feathers were growing. Most likely there was a minor deficiency in nutrients. You see this from time to time on single feathers. The key thing to note, however, is that the band appears on all of the tail feathers in the same location. For this band to appear at the same place on all of the feathers, they would have all had to form at the exact same time. Adult birds don't molt all of their tail feathers all at once or it would be very hard to fly. Unless there is some freak accident where a bird is attacked and loses all of its tail feathers, the only time all of the tail feathers grow in at once is when the bird is born. Since the band appears at the same place on all of the feathers we can tell that they all grew at the same time. Since the only time that happens is at birth, we know that these are the original tail feathers this bird grew. That tells us this is a first year bird that was born last summer.

This young male was banded and released. Hopefully he'll go on to have a long successful life. From now on, even when he gets new tail feathers, if anyone catches him again they'll know when we was born because of his band.