I pulled out the drawer—and blinked at the jays, rows of them; all pale blue with brown wings. I looked at the name on the case, on the tray, and the name on each specimen. They all said the same, Kitta chinensis chinensis, and it was the bird described as green, like my new specimen. It was uncanny. The new green specimens and the old blue ones were identical in size, in structure of bill, crest, feet, tail; they must be the same. And they were. The book, I found, described how the colors changed with age, and in John Gould's magnificently illustrated folio, Birds of Asia, published in 1861, he had the green hunting jay depicted both as a green bird with red wings and, in the background, a "blue" green hunting jay like our museum specimens. When alive, and when freshly killed, the birds are green. But with the passing of time the green changes to pale blue, and the red wings to brown wings. Probably my new specimen, now a year old, is less green than it was when fresh. And when twenty years old, like our museum skins, it will be blue too.
The riddle was solved, and it fits into a well-known phenomenon, "museum age" or post-mortem change. "Foxing," we call it for short. We see it in the male American merganser, where the lovely rich salmon color of the fresh bird becomes plain white. The emerald cuckoo of Africa has vivid rich yellow under parts when fresh, and this too becomes dingy white. Gray Canada jays become more brownish. Birds that are olive or other shades of green tend to become more olive; brown birds tend to become more russet or foxy (hence the term "foxing"). We keep all our specimens in dustproof, lightproof metal cases. The change is not caused by fading. Apparently it's a change in the pigment, perhaps from oxidation.
Taxonomists, the men who classify and name birds, have been fooled by it. Old skins used to represent the birds of an area may give a quite different idea of what they are like than do fresh skins, and when skins of different age are compared, the conclusions may be wrong.
Foxing is one of the pitfalls for the unwary taxonomist, and something he has to guard against.
HOW BIRDS USE COWS AS HUNTING DOGS [Ref]
The sportsman out for quail or woodcock uses dogs to drive out the birds for him. Starlings and cowbirds about Chicago use the same principle in hunting grasshoppers. Instead of dogs they use cows, though of course the cows are intent on something else and presumably unconscious of the fact that they're helping the birds.
As the cow grazes slowly across a meadow, it scares up grasshoppers close in front of it. The cowbirds and starlings take advantage of this. Instead of covering the meadow on foot, constantly alert for a sitting grasshopper, or to chase one they flush, the birds keep with a grazing cow. They take up a position by the head, or a foot, and catch the insects the cow disturbs. The cow is so much larger than the bird that it is likely to flush more insects. The grasshoppers on the wing are much easier to see than when at rest in the concealing grass, and some fly directly toward the bird. Too, the grasshoppers fleeing a cow are less likely to be alert to other dangers.
CONFIRMED BY OBSERVATION The advantages of this to the bird are obvious. But we've just assumed they were, and we had no data on the relative efficiency of the two methods of hunting. A few years ago, however, while in El Salvador, I was able to get quantitative data proving that using a cow as a beater was advantageous, as we suspected, and showing how much more effective it was, something we did not know.
The bird concerned was not the starling, which does not occur there, or a cowbird, which occurs but consorts little with cows, but was the grove-billed ani, a black cuckoo about twelve inches long of the tropics of Central and South America. Like our starling and our cowbird, it kept with cows, catching the grasshoppers and other insects that flew up. Both anis and cows were common in the grassy fields about our headquarters in San Salvador. We decided, my son Stanley and I, to watch anis with cows for a few hours, and then without cows for a few hours; thus getting the average rate for each type of feeding. We quickly found it wasn't as easy as that. Something always happened; even on the levelest and most open fields the birds were constantly disappearing behind a tuft of grass, or in a hollow, or, if nothing else, behind the cow's head or feet. Then, too, the ani we elected to watch wouldn't pay attention to the job in hand. It would wander off, or go to sleep. And sometimes, when we were about to discontinue watching a somnolent bird, it would snap up an insect. Perhaps it had been watching all the time. Finally we found we had to record observations of many short periods, of from three to fourteen minutes each, and add them together.