Boat-tailed Grackle: Iris color in Texas
DUNCANVILLE, TEXAS — Texas is one of only two US states (Louisiana is the other) that has breeding populations of three grackle species: Boat-tailed (Quiscalus major), Common (Q. quiscula), and Great-tailed (Q. mexicanus). Common Grackle is distinctive, but the two larger species overlap in many morphological characters and can present an identification challenge.
And here’s my thesis: It’s even harder than you think.
Most field guides and websites blithely assure their readers that where the two species overlap on the western Gulf Coast*, Boat-tailed Grackles have dark brown eyes and Great-tailed Grackles have yellow eyes:
Easy, right? Yeah, too easy — it’s an oversimplification.
Some Boat-tailed Grackles on the western Gulf Coast have light irises.
Joseph Kennedy snapped this photo of a singing male grackle at Brazos Bend State Park south of Houston last month:
This bird has a light iris, but it is a Boat-tailed Grackle. Joseph reports that it was singing the distinctive Boat-tailed song. The head shape (see additional shot here) and the contrast between purplish head and green-blue back are both suggestive, though both features are quite variable. That this bird is a Boat-tail also squares with local knowledge about grackle populations and behavior in the area.
Notice that the bird in Joseph’s photos shows a thin band of yellow around the outside of a dusky iris. This is markedly different from the clear, brilliant yellow-white iris shown by Great-tails. Compare the photo above with this one:
I was excited to see Joseph’s pictures. I myself have seen two light-eyed male Boat-tailed Grackles singing along the Gulf Coast in Texas, but I didn’t get any pictures, and I hadn’t been able to find much discussion on this feature. Spurred on by these pictures, and by a note from an experienced Texas birder who says he also has photographed light-eyed Boat-tails in Texas, I did some more digging.
Sure enough, I found this statement in an online excerpt from Louisiana Birds:
The eye of the male Boat-tail sometimes shows a narrow rim of yellow around the perimeter of the iris, and occasionally it may even appear to be a dull yellowish brown. … [B]ut as H. Douglas Pratt has so adeptly expressed the matter, if there is ever any doubt as to whether a grackle has the eye a bright enough yellow to belong to a Great-tail, then the bird in question is not a Great-tail. This statement applies, however, only to Louisiana and other northern Gulf Coast populations** ….
Even more interesting, I managed to find a copy of “Analysis of Sympatry of Great-tailed and Boat-tailed Grackles” (PDF) by Robert K. Selander and Donald R. Giller. It was published in The Condor in 1961.
At that time (only eight years after the publication of the watershed articles on DNA in Nature), scientists believed that Great-tailed and Boat-tailed Grackles were different forms of the same species and that they intergraded in Texas and Louisiana. Selander and Giller’s work provided important evidence that that was not true. Despite their similarity, the birds do not interbreed. They retain their morphological (and, we now know, genetic) distinctiveness despite their sympatry (range overlap).
The paper includes a section on variation of iris color (“C.” in this chart stands for Cassidix, an old generic name. The birds are now included in Quiscalus.):
You can see here that Boat-tailed Grackle eye color varies widely but never reaches the bright, clear yellow shown by Great-tails. Great-tails do not have as much variation. It’s also interesting to note that (at least of the birds they sampled) female Boat-tails show light eyes much less often than males do. Why? This makes me think of Brewer’s Blackbird — after all, Euphagus is sister to Quiscalus. (There’s lots more stuff to discover in the full paper. Have a look through it.)
So, scientists have known for at least 50 years that Boat-tailed Grackles in Texas and Louisiana can show light irises. Why then, I wonder, is this so little discussed and (apparently) so little known?
If you’re birding along the Gulf Coast this spring, take a second look at the grackles. And have your camera ready. Let’s document these birds!
* Great-tailed Grackle and Boat-tailed Grackle overlap in coastal marshes from the central Texas coast to western Louisiana. This zone of overlap has developed in the last 100 years as Great-tailed Grackles have expanded their range dramatically. [back]
** Boat-tailed Grackles on the Atlantic coast have yellow irises. In Florida (the only place where the species ranges significantly inland), they have brown irises. In coastal Alabama and Mississippi, their irises are “dirty yellow” (David Sibley). I don’t know anything about variability within these populations. Anyone care to comment? [back]