Adults have a partial prenuptial molt in April and May, involving the tail, the wing coverts and all the body plumage, which produces the well-known brilliant plumage of the female and the duller plumage of the male. The complete postnuptial molt in summer produces the gray winter plumage in both sexes, in which the crown, back, and scapulars are "light drab" or "drab-gray," with narrow white edgings, and the upper tail coverts, as well as the under parts, are white. The sexes can be recognized in adult winter plumage by size only.
Food.—The other two species of phalaropes feed mainly on the water, but the Wilson phalarope is more of a shore bird and obtains most of its food while walking about on muddy shores or wading in shallow water. It does, however, adopt the whirling tactics of the others occasionally, concerning which Mr. Dawson (1923) says:
Instead of swinging from side to side with a rhythmical motion, as do the reds and northerns, the Wilson whirls all the way around. Moreover, he keeps on whirling, and though he pauses for the fraction of a second to inspect his chances, he goes on and on again like an industrious, mad clock. One bird which I had under the binoculars turned completely around 247 times in one spot, without stopping save for instantaneous dabs at prey. These dabs were directed forward or backward, i. e., with or against the direction of the body motion. A single gyration normally contains two such minute pauses, accompanied by a hitching motion of the head; and these are evidently the periods of maximum attention, since they are followed by, or rather flow into, the prey stroke, if game is sighted. "Game" is not always abundant nor certain, and I have seen a bird whirl a dozen times without a single stroke.
The method of feeding on mud flats or in shallow water is well described by Roland C. Ross (1924), as follows:
When feeding along the shallows with least, western, and red-backed sandpipers, they differed from them not only in size and color, but in their habit of steady, energetic walking and the constant "side sweeping" with the bill. Occasionally they picked objects from the surface with their needle bills, but this was not very actively pursued. In deeper water they fed among the northern phalaropes, knots, and dowitchers, wading along until they swam in places. However, they were able to wade where the northern swam. At such depths they feed with the head clear under and the energy of the feeding operation was indicated by the motion of the tail. They commonly walked steadily back and forth through the deeper sections of the ponds, and in such deep places they moved as headless bodies, evidently feeding as usual in the surface mud. From the vigorous side moves of the tail it would seem they were feeding in their usual manner as well; that is, "side sweeping." When the birds were standing to feed in the deeper places the tail was again much in evidence, and indicated the manner of feeding. This would seem to be a probing motion performed with some rapid vibration which was communicated to the tail as a series of quivers. It is rather a droll sight, and arresting as well, to see a certain area marked out by headless gray bodies buried in the water up to the bend of the wing, the vibrating tail indicating the vigorous operations being carried on down below. It seemed their best feeding was in the deeper waters.
The feeding habits of this and the other phalaropes are almost wholly beneficial. They live very largely on the larvae of mosquitoes. They also eat crane-fly larvae, which are often very destructive in grass lands and wheat fields. Predaceous diving beetles, which are a nuisance in fish hatcheries, are eaten by them. Dr. Alexander Wetmore's (1925) analysis of the contents of 106 stomachs showed that the food of the Wilson phalarope is mainly insects, of which various flies made up 43.1 per cent, aquatic bugs 24.4 per cent and beetles 20.1 per cent. The remainder of the food included brine shrimps, amphipods, eggs of water fleas, and seeds of various aquatic plants.
Behavior.—Much of the interesting behavior of the Wilson phalaropes has been described under different headings above. In all its movements it is light, airy and graceful. Its flight is much like that of the lesser yellow legs, with which it is often associated; but, when suddenly alarmed, it sometimes flies hurriedly away in a zigzag fashion. On its breeding grounds it often hovers, almost motionless in the air, as the upland plover sometimes does. It swims lightly and buoyantly, but apparently does not dive. It walks about on land actively and daintily, where it is said to resemble the solitary sandpiper. It mingles freely on its feeding grounds with various other species of shore birds. Toward the close of the nesting season the females become very gregarious; as early as June 18, in southern Alberta, we saw them in large flocks, mixed with lesser yellow legs, flying about the marshy lakes.
Voice.—The only note I have recorded is a soft, nasal grunt or subdued quack. Dr. Walter P. Taylor (1912) describes a peculiar nuptial (?) call note "as oit, oit, oit, somewhat resembling the croak of a toad during the breeding season. At the instant of utterance of the note the bird which is calling raises its head somewhat, pauses momentarily in its flight, and its throat bulges slightly." Mr. Saunders calls it a low note sounding like croo, croo, croo.
E. S. Cameron (1907) writes: