Tuesday, August 9, 2011

Eastern vs. Spotted Towhee

All spring and summer, an Eastern Towhee called near my banding nets. A few times a male or female peered inquisitively at my net. Today I finally banded the male. Listen to the call of the Eastern Towhee, who says, "Drink your tea;" then listen to the Spotted Towhee's drawn-out "chweee." (These files are shared with permission of Thayer Birding Software and their great DVD, Birds of North America.)
As you can see in the photograph above (a bird from Northfield, Minnesota), except for their white wing patch, Eastern Towhees have unspotted backs and wings. Compare this bird with the bird below, a Spotted Towhee from Aberdeen, South Dakota. These species' biology is complicated. Eastern Towhees breed from southernmost Manitoba, through Minnesota and Iowa, and southward. They are absent from much of extreme western Minnesota. Spotted Towhees breed in the western United States, east to extreme western areas of the Northern Plains (ND, SD, NE, and on south). Areas of hybridization lie along the Missouri and Platte river drainages as well as along the Souris River across North Dakota and southern Manitoba. Towhees do not breed along the James River in North and South Dakota, or along the Cheyenne River in eastern North Dakota.
Normally Ornithologists conclude interbreeding birds indicate that the populations involved are simply races, not separate species. Until relatively recently, Eastern and Spotted towhees were both considered to be a single species, the Rufous-sided Towhee. However, in the range of hybridization, most of the birds you see in the western states are Spotted Towhees. Eastern Towhees predominate along the Platte River, in southeastern South Dakota, and in Manitoba. Biologists conclude that assortative mating is taking place, and that the two species do not interbreed except when no alternative exists. In the hybrid zone you often see pure Spotted or Eastern towhees. This discrimination leads to a gene barrier, albeit a leaky one, between the two populations.  

During my 30 years of banding birds in Aberdeen, in northeastern South Dakota, I caught towhees only as migrants.  Although I caught a few Easterns, most of the birds were Spotted Towhees.  Once in southern South Dakota, just west of the Missouri River, I found a Spotted Towhee singing a perfectly clear Eastern Towhee song. In fact, I have never seen an unspotted towhee in western South Dakota. Thus the biology of these forms is likely a bit more complicated than I described in this post.

1 comment:

  1. Fascinating. We have Eastern Towhees in our forest and sometimes they'll come to feed on the seed on the ground at the bird feeder. I love the way they fly and nest on the ground and their song is excellent -- a very attractive bird.