Wednesday, October 7, 2009

How to tell how old a bird is

Posted by Kirk
Today we taught our Fall Migration class so that means bird banding. This was the first non-raining day all week which made for a lot nicer conditions. The lead bander for the day was non other than my co-host on the podcast, Paul Smithson. I took a break during the day to see what they were catching and learn a little more about ageing birds. When I arrived, Paul was taking out the first bird of the day, a Fox Sparrow. This was a treat as it was the first Fox Sparrow at the nature center this fall. I love these big robust sparrows and their feeding behavior. Paul set to the task of gathering information about the bird to be recorded before it was let go. One of the things banders try to determine is how old a bird is. Often the distinction is between a hatching year bird (HY) or after hatching year bird (AHY). In the photo below he's looking at the outer rectrices to see how worn they are.

Here's the diagram banders use to determine the wear on the outer rectrices of a Fox Sparrow. Ideally they would look like one of the two drawings marked "Fall." In reality, that doesn't always work. In this case, the feathers looked somewhere in between the two drawings. He checked a couple of other feathers but wasn't confident enough to make a determination. It was probably a hatching year bird but without enough certain information the age was put down as unknown.
A very different technique came into play while determining the age of all the White-throated sparrows that were caught this morning. In the fall, white-throated sparrows can be aged by looking at the color of the iris of the eye. A hatching year bird will have a grayish brown to brown iris while an after hatching year bird will have a reddish brown iris. The distinction can be subtle and good lighting helps. Can you tell what color the iris is for the bird below? It was only slightly easier in person, which is to say, hard.

I thought it would be fun to include another photo here of a white-throated sparrow. Look at the difference in the heads. There are two different color morphs of the White-throated Sparrow. The one above is a tan-striped morph as the light colored stripes on the head are tan. In contrast, the bird below is a white-striped morph. (The stripes looked even whiter in person)
These are not two different sub-species. They are both represented about evenly in the population. What is really fascinating is that research suggests that the birds almost always mate with a bird that has the opposite color morph. Also, both males and females of the white-stripe morph tend to be more aggressive during mating season. The more aggressive white-striped females are preferred by both color types of males. Ironically, both types of females prefer the less aggressive tan-striped males. So, white-striped females do well and tan-striped males do well while white-striped males and tan-striped females do less well. That's all clear right?

Another fun bird of the day was a Winter Wren. It was the first winter wren caught at the nature center in 2009. I just love that little tail!

That's all for tonight. Thanks for stopping in.



Heather said...

Wow, this looks like a difficult thing to do. I'm very intrigued by the banding process, and I enjoy reading accounts of the different aspects of what happens when a bird is banded. Thanks for sharing this. And thanks for dropping by my blog. I see you write for The Intrepeter... I just purchased a copy of Alan Leftridge's "Interpretive Writing." The world becomes smaller every day!

Kirk said...

Thanks for stopping in Heather. I hope you enjoy Alan's book. The concept behind interpretive writing (similar to a good interpretive program) is really strong. I ikethe approach of universals, tangibles, intangibles, etc.