How do you keep those colors shining so bright?

Here's a nice Herring Gull (Larus argentatus) shot from today, taken as the birds came back in from the lake to the marina:

Herring Gull (Larus argentatus), North Point Marina, Lake Co, IL 1/20/2014

An interesting thing to point out here is the brown mottling on the head and neck. That's an indication that the bird's in basic plumage, which results from a pre-basic molt, replacing all (or most) of the bird's feathers. Some birds (not gulls) only have this one molt each year, but most have at least one more, known as a pre-alternate molt. Sensibly, the plumage this results in is known as alternate plumage. (In the vast majority of birds that have alternate plumages, the pre-alternate molt is partial, typically some head and body feathers, while the long tail and wing feathers are only molted once.) The terminology was worked out by Humphrey and Parkes in 1959. (1)

Here's a couple of other examples -- Mallards (ducks follow the same pattern but do some odd things with the timing):

Mallard (Anas platyrhynchos) male in alternate plumage,

Lincoln Park Zoo, 11/30/13

Mallard (Anas platyrhynchos) male in basic plumage,

Waukegan Beach, Lake Co, IL 8/6/2013

 

and Common Yellowthroats:



Common Yellowthroat (Geothlypis trichas) male in alternate plumage,
Wadsworth Savanna Forest Preserve, Lake Co, IL 5/4/2013

Common Yellowthroat (Geothlypis trichas) male in basic plumage,

Illinois Beach State Park, Lake Co, IL 9/14/13

This terminology has always bothered me a bit. When I first learned it, it seemed truly problematic -- why call one plumage alternate, as if it was something the bird just put on for the summer. Given that alternate plumage is usually the one the bird wears for breeding, it seemed that it was rather too important to be designated "alternate". Then I did some reading, and found out that while they do label the plumages and molts this way, the terms basic and alternate aren't actually derived from the plumages. Rather, they refer to the molt strategies -- the basic strategy being to replace all of your feathers once a year, and the alternate strategy being to add a pre-alternate molt. The advantage to this, of course, is that you can have bright feathers for breeding and dull feathers for hiding when you don't need to impress anyone. The disadvantage is that it takes energy to replace feathers.

But there's still the question of why the "basic" strategy deserves the name. Humphrey and Parkes argued that evolution generally proceeds from simple to complex, and therefore that the simplest strategy should be the ancestral state. I do have a problem with this approach -- specifically, the determination of the ancestral state is based on an assumption that turns out to be seriously wrong. (We've had the basic facts here since the 1860's, but it wasn't until 15 years ago or so that we really started to realize how wrong we were.)

I could concede that the ancestral strategy is expected to be the simplest, if the most recent common ancestor of modern birds was close to the earliest feathered critters. But it clearly wasn't. Based on molecular clock methods, Michael Benton argued that this ancestor lived approximately 100 million years ago, in the early Cretaceous. But he himself pointed out that this was 30 million year older than the most recent fossil evidence we have. (2) So it seems we can confidently assign a Cretaceous age to the last common ancestor of modern birds. But Archaeopteryx lithographica, the earliest bird we have a fossil of (at least traditionally), lived in the early Jurassic, 150 million years ago, and Hu Dongyu et al. reported finding well developed feathers even earlier. (3) Protofeathers have also been found in numerous non-avian theropod lineages, and possibly even ornithischian dinosaurs, all of which push the origin of feathers back long before the modern birds.

The reason this is important is that those early feathered dinosaurs and ancient birds must have molted their feathers as well, since feathers would have worn out the same way then that they do today. At what point organized molt strategies started to emerge is anyone's guess, of course, but given how widespread the alternate molt strategy is in modern birds, it's hard to imagine that it took most of the 70-100 million years that were available since that last common ancestor. Since we have at least 50 million years, and possibly closer to 100 million years, between the origin of feathers and that earliest modern bird, it seems likely that there were several different molt strategies already in place by the time that modern birds emerged. Given that the alternate strategy is more common in today's birds than the basic one, it's at least possible that the alternate strategy is the ancestral one, and basic strategies evolved by the loss of the alternate molt.

I realize that the current system isn't likely to change anytime soon, and I wouldn't want it too. Whether the assumptions it's based on are correct or not, it's widely used today, and changing it would cause more confusion than it would be worth. (The same reason why we continue to Latinize scientific names, I guess.) But just because we use the terms doesn't mean we shouldn't keep examining the assumptions hiding behind them.

(1) Humphrey, Philip S., and Kenneth C. Parkes. "An approach to the study of molts and plumages." The Auk 76.1 (1959): 1-31.

 

(2) Benton, Michael J. "Early origins of modern birds and mammals: molecules vs. morphology." BioEssays 21.12 (1999): 1043-1051.

 

(3) Hu, Dongyu, Hou, L., Zhang, L., & Xu, X. "A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus." Nature 461.7264 (2009): 640-643.