Monday, January 9, 2017

Hidden truth: the mystery of the median secondary coverts

I learned something new about birds while watching a bird drawing instructional video by John Muir Laws (here).

I'll do some research and present it below. But the gist is that the overlapping feathers of the wing--where the feathers closer to the back overlap the next, then the next, and so on, all the way down to the outer primary--doesn't always apply to the median secondary coverts.

Perhaps you would only care about this when drawing the wing feathers. This particular video was about suggesting the wing feathers, rather than drawing everything you know. Feather edges tend to blend smoothly on a bird's body. Wing feathers are stiffer and more individually obvious. As Laws says, however, if you draw every feather individually you'll end up with a bird that looks like a pine cone.

Here, take a look at the wing feathers on this Vermilion Flycatcher (magnified, so there is some pixelation to the photograph).

Flycatcher wing feathers labeled

Notice that the feathers labeled 1, 2, 3 are overlapping from top down on the folded wing? One is on top, it overlaps 2, which overlaps 3. Those are the three inner secondaries, often called tertials when obviously different from the rest of the secondaries. This pattern of overlap is repeated on the primaries and the greater secondary coverts. Watch the above referenced video to easily learn these and additional wing feathers (hidden from view in this photo).

If the greater and median secondary coverts are broadly tipped with white we call them wing bars. Wing bars are only hinted at on this Vermilion Flycatcher. Notice that the greater secondary coverts repeat the overlapping pattern of the primaries and secondaries (top down or left to right in this view). But what's going on with the median secondary coverts? They overlap from the opposite direction (right to left in this view)! Apparently this is the norm for flycatchers, but not(?) other perching birds. Laws also mentioned that the secondary coverts on hawks are more complicated.

I had no idea!

Now I need to go research this.

Where to look?

Reversed covert feathers is not discussed in my ornithology text book, Ornithology in Laboratory and Field, fourth edition, 1970, Olin Sewall Pettingill, Jr. And it's not in Molt in North American Birds, 2010, Steve N. G. Howell. Nor, is it in my go-to reference, The Audubon Society Encyclopedia of North American Birds, 1980, John K. Terres. Therefore, this information is not in any book in my personal birding library. Reversed covert feathers is not a topic in Wikipedia ("covert feathers"). If it's not in the de facto "Encyclopedia Galactica," then where is this secret knowledge kept? Time to search the index of all knowledge--accurate or not--Google.

"flycatcher median secondary coverts"

"median secondary covert variation"

"median secondary covert overlap"

Nope, it's no good. But I did see reversed median secondary coverts on a Song Thrush and Lincoln's Sparrow photo. That goes against what I heard in the above video. So now I'm confused and interested. I guess I'll be looking closely at bird photographs for a while... even if I am the only one.

I'm going to start at the Slater Museum of Natural History Wing & Tail Image Collection.

After looking at several families and orders of birds it appears that perching birds may all have reversed median secondary coverts, pigeons seemed to have matching order, and ducks appear to have some with the overlap as expected with a mid-wing change to reversed. It is very difficult to tell on many of the wing specimens--some are slightly messed up, others are fluffy so that it is hard to see the edges. I don't know, on some specimens it appears that all the median, lesser, and marginal coverts are reversed.

...

I had this post written in late December and saved to publish about the first of January. Since then, however, my copy of The Laws Guide to Nature Drawing and Journaling (John Muir Laws, 2016) arrived. On page 12 I found his mention of this mystery in the 1886 Proceedings of the Zoological Society of London in the article: "Observations on the Disposition of the Cubical Coverts in Birds" by J. G. Goodchild. Is this 130-year-old scientific journal article really the most-recent discussion of this anatomical structure in literature? ["Cubical coverts?" That term has zero Google references. But it will when this post is published and indexed!]

Again, unless you are a bird artist striving for super-detailed accuracy--or an avian systematicist, you may never need to know this. But that doesn't mean this phenomenon doesn't exist!