Dating Game, Passerine Style

Here's a find from today:

Brown Creeper (Certhia Americana), Van Patton Woods FP,
Lake Co, IL, 1/11/14

 

So, is it a male, or a female?

Well, obviously the birds figure it out, since they're fairly common across the entire US, but even if you were holding the bird (as banders do), you probably wouldn't be able to tell the difference. There are no plumage differences, birds have no external genitalia, and even structural measurements (wing length, bill length, etc.) all overlap nearly completely. (During the breeding season it's possible, which is how we got those measurements on known birds.)

Here's another example of a species that's very difficult to tell. 

Black-capped Chickadee (Poecile atricapilla), Bear Creek Nature Center,
El Paso Co, CO, 1/1/14

When I was working with them, we used wing length to tell males from females, since plumage features don't seem to work. So, how do the birds themselves figure it out?

There's a lot we don't know -- we can guess that behavior, especially vocal behavior, accounts for a lot of it. But a close relative of the Black-capped Chickadee illustrates another possibility, one that I suspect our chickadees use as well. Blue Tits (Cyanistes caerulea) are monomorphic (males and females look alike), or at least they look like it to us. But, when you examine them in near UV light, males have obvious crown and chest patches that females don’t. (1) In other words, they are sexually dimorphic, when you look at them the way that other Blue Tits see them! (Yes, Tits and most other birds can see into what we call ultraviolet light.)

This pattern appears to be quite common among birds (2,3) , which means that many of our ideas regarding sexual selection in birds are being revised, since they were based on an important misunderstanding of which birds show dimorphism. (4)

There is a more general point to be made here, with regards to studies on animal behavior. If you’re trying to determine how a bird or any other animal is reacting to the information available to it, it’s important to know just what information is available! A classic example, to my mind, is the mirror test – if you present a puppy with a mirror, they often act aggressive towards the puppy they see in the mirror. I’ve rarely, if ever, heard anyone point out how important scent is to a dog’s view of the world, though, and I have yet to hear of a mirror that smells like a puppy!

Sometimes scientific revolutions (even small ones in limited fields, like here) don't come about from people focusing just on the really big questions. More often, they come from people asking about details -- why do we find Mesosaurus fossils in both South America and Africa? Why do the mockingbirds of the Galapagos Islands all look just a bit different than the ones on the mainland? Why doesn't the movement of the earth seem to affect our measurements of the speed of light?

The answers to those questions all prompted major changes in the way we think about the world, just as the question "What does a Blue Tit look like in UV?" changed the field of avian behavior. Just something to keep in mind, the next time you're watching that little Brown Creeper working over the pine tree in your backyard.

(Answers: Because S. America and Africa used to be joined. Because all of them evolved from a S. American common ancestor. Because... well, this one's a little hard to answer briefly. The first question was an important part of Wegener's evidence for continental drift, the second one a key moment for Darwin's views on evolution, and the last was Einstein's starting point for his special theory of relativity.)

(1) Andersson, S., örnborg, J.& Andersson, M. (1998) Ultraviolet sexual dimorphism and assortative mating in blue tits.   Proc. R. Soc. Lond. B 265 (1395) 445-450  

(2) Eaton, M. D. (2005). Human vision fails to distinguish widespread sexual dichromatism among sexually “monochromatic” birds. Proceedings Of The National Academy Of Sciences Of The United States Of America, 102(31), 10942-10946.

(3) Mullen, P., & Pohland, G. (2008). Studies on UV reflection in feathers of some 1000 bird species: are UV peaks in feathers correlated with violet-sensitive and ultraviolet-sensitive cones?. Ibis, 150(1), 59-68.                    

(4) Burns, K., & Shultz, A. (2012). Widespread cryptic dichromatism and ultraviolet reflectance in the largest radiation of neotropical songbirds: implications of accounting for avian vision in the study of plumage evolution. Auk, 129(2), 211-221.