They appear in Pennsylvania early in May, and remain there until the last of September. They are observed in Florida and Georgia early in March, but are not known to winter in that latitude. All the specimens in the Smithsonian collection were obtained between April and October, except one from Southern California, which was taken in December. [Illustration: _Polioptila cærulea._] Near Washington, Dr. Coues states the Blue-gray Gnatcatcher to be a summer resident, arriving during the first week of April, and remaining until the latter part of September, during which time they are very abundant. They are said to breed in high open woods, and, on their first arrival, to frequent tall trees on the sides of streams and in orchards. In California and Arizona this species occurs, but is, to some extent, replaced by a smaller species, peculiarly western, _P. melanura_. There they seem to keep more about low bushes, hunting minute insects in small companies or in pairs, and their habits are hardly distinguishable from those of Warblers in most respects. The food of this species is chiefly small winged insects and their larvæ. It is an expert insect-catcher, taking its prey on the wing with great celerity. All its movements are very rapid, the bird seeming to be constantly in motion as if ever in quest of insects, moving from one part of the tree to the other, but generally preferring the upper branches. Nuttall and Audubon, copying Wilson, speak of the nest of this Gnatcatcher as a very frail receptacle for its eggs, and as hardly strong enough to bear the weight of the parent bird. This, however, all my observations attest to be not the fact. The nest is, on the contrary, very elaborately and carefully constructed; large for the size of the bird, remarkably deep, and with thick, warm walls composed of soft and downy materials, but abundantly strong for its builder, who is one of our smallest birds both in size and in weight. Like the nests of the Wood Pewee and the Humming-Bird, they are models of architectural beauty and ingenious design. With walls made of a soft felted material, they are deep and purse-like. They are not pensile, but are woven to small upright twigs, usually near the tree-top, and sway with each breeze, but the depth of the cavity and its small diameter prevent the eggs from rolling out. Externally the nest is covered with a beautiful periphery of gray lichens, assimilating it to the bark of the deciduous trees in which it is constructed.