Wednesday, April 17, 2013

Subspecies of the American Tree Sparrow

     One of the things I most enjoy about bird banding and having the opportunity to see birds up close in the hand is that I have the chance to learn something new with each bird. The American Tree Sparrow (Spizella arborea) above is a case in point. When we had this bird at the banding table one of the first things that was commented on was how pale the bird looked. You can see from the photos that there is a large amount of gray around the head and nape. The spot on the breast is faint and the all over color appeared washed out compared to the other birds we handled that day.

A check with the Pyle manual showed that there are two recognized subspecies of American Tree Sparrow with the western subspecies being more pale and gray. Measurements for the bird (chord, tail, etc.) all fell within the described measurements for both subspecies. As far as we banders know, we had never handled an example of the western form before. So the question we had was "is this a sparrow that would qualify as the subspecies S. a. ocliracea?
Turns out that this is not a simple question. Not only is the variation between the subspecies subtle there is also, near our location, a zone of intergradation (overlap) that covers the Great Plains. Throw in the normal variation within the species and we realized we were over our head in assigning this bird to a subgroup.
So it comes to this - I am going to ask anyone who is familiar with the western form of ATSP to comment on this bird.
Decided to add a photo of the tail just for yucks. We aged and sexed this bird as an ASY-U. One person asked if we took any wing shots and unfortunately we didn't. Most of our ATSPs are headed north (finally).

Does it look like the western subspecies?
Is it possible to separate the groups in winter?
Do these photos show enough to make a determination?
Is this a Fool's Errand that I am pursuing?

Any insights from western banders would be most welcome. Fortunately, with birds now headed north for the summer this is not an identification that I will have to deal with until next winter. However, if there is a way to separate these subspecies it might give us insight into which birds winter here in Minnesota.

Thanks to all who comment!

1 comment:

Dan Tallman said...

Compare your American Tree Sparrow to one I recently banded:

I think you can make a case that yours is paler and with a less obvious breast spot.