_Nesting._--The prothonotary warbler and Lucy's warbler are the only two American warblers that habitually build their nests in cavities, usually well concealed. The normal, and probably the original, primitive, nestling sites are in natural cavities in trees and most nests are still to be found in such situations today. The prothonotary is not at all particular as to the species of trees, nests having been found in many kinds of trees, although perhaps a slight preference is shown for dead willow stumps. Nor is it particular as to the size or condition of the cavity, or its location, though quite often choosing one over water or near it. The height above the ground or water varies from 3 feet or less to as much as 32 feet but there are more nests below 5 feet than there are above 10, the height of the majority being between 5 and 10 feet. The size and shape of the cavity are of little concern; if the cavity is too deep, the industrious little birds fill it with nesting material up to within a few inches of the top; sometimes a very shallow cavity is used, so that the bird can be plainly seen from a distance as it sits on the nest. The old deserted holes of woodpeckers or chickadees are favorite nesting sites; the entrances to these have often been enlarged by other agencies, or are badly weathered. In very rotten stumps, the warblers have been known to excavate partially or to enlarge a cavity. The nests built by the males in early spring, referred to above, are probably rarely used as brood nests and might be classed as dummy nests. The family nest is built almost entirely by the female, with encouragement and a little help from her mate, who accompanies her to and from the nest and in the search for material; much of the soft, green moss used extensively in the nest is often obtainable from fallen logs and stumps in the vicinity. Brewster (1878a) mentions a nest taken from a deep cavity that "when removed presents the appearance of a compact mass of moss five or six inches in height by three or four in diameter. When the cavity is shallow, it is often only scantily lined with moss and a few fine roots. The deeper nests are of course the more elaborate ones. One of the finest specimens before me is composed of moss, dry leaves, and cypress twigs. The cavity for the eggs is a neatly rounded, cup-shaped hollow, two inches in diameter by one and a half in depth, smoothly lined with fine roots and a few wing-feathers of some small bird." In Dr. Walkinshaw's (1938) Michigan nests, "moss constituted the bulk of the nesting material in nearly all cases, completely filling the nest space whether it was large or small. On top of this the nest proper was shaped and a rough lining of coarse grape-bark, dead leaves, black rootlets procured from the river-banks, and poison-ivy tendrils was added. Above this a lining of much finer rootlets, leaf-stems, and very fine grasses was used."