Hello, Birders.
Thanks to Michael Price for posing a great question. I believe Michael's is motivated by what is referred to by evolutionary biologists as "adaptationist thinking." It's a fancy term, but it's a simple concept. Those of us with training in or exposure to natural history have an understandable tendency to look at some trait (say, molt) and to try to explain it in terms of some adaptation (say, males looking colorful so as to attract females).
Again, this mode of thinking is natural, and it is often correct. But it can sometimes be incorrect. For example, we might look at a trait like the spines of a cactus and deduce that the spines evolved to prevent herbivory. However, evolutionary biologists believe that cactus spines may have evolved as heat-shunt devices (a cactus needs to shed excess heat) and that protection from herbivores has been evolutionarily "piggy-backed" onto those spines.
Back in the bird realm, I am frequently struck by our tendency to explain vagrancy in adaptationist terms. Yes, vagrancy happens a lot. But that doesn't mean it's somehow an adaptive trait or otherwise "good." Maybe it's simply a byproduct of--maybe it's simply "piggy-backed" onto--the trait of long-distance dispersal.
And I also believe we have a tendency to think of molt in adaptationist terms. (Not all of us: I believe Peter Pyle and Steve Howell are doing great work to get us past that mode of thinking.) We look at molt, and we make adaptationist assumptions about why it happens. It's preferable, I believe, to approach the evolution of molt with a "clean slate"--without any assumptions. Molt is something birds *have* to do. The trait of molt is already in place. It is fixed. The question, then, is how adaptations are piggy-backed onto the trait. Examples: the evolution of migration, the evolution of the breeding cycle, and the evolution of bright colors. Those adaptations are best thought of us as the "cart" that is pulled along by the "horse" of molt.
And, of course, there is the danger of putting the cart before the horse.
Birds molt. Molt happens. And then other things happen--for example, the tie-in with the acquisition of bright colors for the breeding cycle. Here are some examples. (1) Ducks have come up with the elegant solution of molting into a snazzy basic (not alternate) plumage. (2) European Starlings, House Sparrows, and Snow Buntings have come up with the clever solution of a basic plumage that, in the process of getting worn out, winds up looking really snazzy, for breeding. And (3) some birds, like our poor sparrows, haven't figured out a solution, and they look totally crappy in the breeding season. (But they compensate with awesome songs.)
To be technical about it, molt is part of a bird's "Bauplan," a term coined by Stephen J. Gould and Richard C. Lewontin. It's just there. It's a pre-existing condition, if you will. There's nothing a bird can do about it. Molt may or may not be put to use, as an adapation, for breeding. If it *is*, then there may be multiple strategies: again, bright, fresh basic plumage in courting ducks vs. *worn* basic plumage in snazzy breeding starlings. But, again, sometimes molt is not put to use as an adaptation for breeding, as in, say, sparrows, Wrentits, and tubenoses. Those birds rely on other aspects of their Bauplaene for breeding--song, display, alternative mating systems, etc.
The classic paper on adaptionist thinking, widely available on the internet, is Gould and Lewontin's "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme," published in 1979 and widely available on the internet. For example:
http://tiny.cc/Brh5i. Their 1979 paper triggered a revolution in ecological studies.
But to sound a little bit like Jon Dunn on the subject of Roger Tory Peterson and Ralph Hoffmann (Jon says Peterson gets all the credit that Hoffmann deserves), I would like to point out that Charles Darwin absolutely understood the concept of "Spandrels." I'll sign off by excerpting a paragraph from Chapter 6 of the Origin of Species. It's written in 19th-century Victorian prose, not 21st-century internet brevity. So go slow with it. But, at the risk of sounding preachy, I urge you to read it. This little section alone changed the whole way I look at birds and nature. It's profound. In particular, check out the 4th and final paragraph, excerpted below. It is brilliant, and it says everything that Gould and Lewontin say in "Spandrels." When you get to Darwin's examples of woodpeckers and vultures; substitute "molt" and "breeding" in there, and you'll see. Enjoy! --Ted Floyd
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In the first place, we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not. In a former chapter I have given instances of most trifling characters, such as the down on fruit and the colour of the flesh, which, from determining the attacks of insects or from being correlated with constitutional differences, might assuredly be acted on by natural selection. The tail of the giraffe looks like an artificially constructed fly-flapper; and it seems at first incredible that this could have been adapted for its present purpose by successive slight modifications, each better and better, for so trifling an object as driving away flies; yet we should pause before being too positive even in this case, for we know that the distribution and existence of cattle and other animals in South America absolutely depends on their power of resisting the attacks of insects: so that individuals which could by any means defend themselves from these small enemies, would be able to range into new pastures and thus gain a great advantage. It is not that the larger quadrupeds are actually destroyed (except in some rare cases) by the flies, but they are incessantly harassed and their strength reduced, so that they are more subject to disease, or not so well enabled in a coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted in nearly the same state, although now become of very slight use; and any actually injurious deviations in their structure will always have been checked by natural selection. Seeing how important an organ of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swim-bladders betray their aquatic origin, may perhaps be thus accounted for. A well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in turning, as with the dog, though the aid must be slight, for the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters which are really of very little importance, and which have originated from quite secondary causes, independently of natural selection. We should remember that climate, food, &c., probably have some little direct influence on the organisation; that characters reappear from the law of reversion;, that correlation of growth will have had a most important influence in modifying various structures; and finally, that sexual selection will often have largely modified the external characters of animals having a will, to give one male an advantage in fighting with another or in charming the females. Moreover when a modification of structure has primarily arisen from the above or other unknown causes, it may at first have been of no advantage to the species, but may subsequently have been taken advantage of by the descendants of the species under new conditions of life and with newly acquired habits.
To give a few instances to illustrate these latter remarks. If green woodpeckers alone had existed, and we did not know that there were many black and pied kinds, I dare say that we should have thought that the green colour was a beautiful adaptation to hide this tree-frequenting bird from its enemies; and consequently that it was a character of importance and might have been acquired through natural selection; as it is, I have no doubt that the colour is due to some quite distinct cause, probably to sexual selection. A trailing bamboo in the Malay Archipelago climbs the loftiest trees by the aid of exquisitely constructed hooks clustered around the ends of the branches, and this contrivance, no doubt, is of the highest service to the plant; but as we see nearly similar hooks on many trees which are not climbers the hooks on the bamboo may have arisen from unknown laws of growth, and have been subsequently taken advantage of by the plant undergoing further modification and becoming a climber. The naked skin on the head of a vulture is generally looked at as a direct adaptation for wallowing in putridity; and so it may be, or it may possibly be due to the direct action of putrid matter; but we should be very cautious in drawing any such inference, when we see that the skin on the head of the clean-feeding male turkey is likewise naked. The sutures in the skulls of young mammals have been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate, or may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles, which have only to escape from a broken egg, we may infer that this structure has arisen from the laws of growth, and has been taken advantage of in the parturition of the higher animals.
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