Communal Roosting by the Eastern Bluebird in Winter

Although Bent (1949) notes that Eastern Bluebirds (Sialia sialis) may at times roost "several together," and Forbush (1929) is somewhat more explicit, the paucity of information on the bluebird's communal roosting suggests that the habit has been only rarely observed.Thus, the literature apparently contains no precise description of the behavior or physical condition of the birds nor any but general references to the bad weather which elicits this interesting response.This paper records observations by the senior author of roosting by a number of bluebirds in ,a nest box eight feet from the window of her house near Bloomington, Indiana, on five successive nights during extremely cold weather in February, 1958.After a brief review of some of the literature, the observations are analyzed with a view toward suggesting certain behavioral and physiological factors that may have been involved.In this connection the bluebird is compared with the Winter Wren (Troglodytes troglodytes), in which the habit has been more thoroughly studied. LOCATION AND GENERAL BACKGROUNDThe senior author's house stands at 990 feet above sea level, on the top of one of the highest hills in Monroe County, Indiana, in section 21, township 9 north, range 2 west.The nearest trees are a woods some 150 yards to the south, and there are extensive ,areas of mature woods no more than 300 yards distant in any direction.The nest box in question is southeast of the house and therefore sheltered to some degree from cold winds, which are usually from the northwest.The box, 50 inches above the ground and on the west side of a bare pole ten inches in diameter, gets the last rays of the winter sun, which, indeed, shines into the one and one-half inch entrance hole.Inside dimensions are six inches in length and width and eight inches in height.The redwood boards are three-quarters of an inch thick, and the top is hinged to permit inspection of the contents of the structure.This box is one of six within one-quarter mile of the house.In February, 1958, it contained a few pieces of dry grass, but it had not been used for a nest since 1956.Other boxes had been occupied by nests in 1957, however, and throughout the fall of the latter year small flocks of bluebirds had engaged in their typical desultory exploring of all the boxes.Earlier in the winter, prior to the experience about to be related, as many as 22 bluebirds had been counted at one time on the wires and fence posts about the hilltop.Until February the winter's weather was not especially rigorous.The first four days of that month were marked by snow on the ground, but this cover began to melt on Feb. 5, and disappeared in a few hours.It may be important that throughout the period of the communal roosting the sky was clear and the ground bare, and there were no known conditions rendering it difficult for bluebirds to find and gather whatever food supply the area afforded.Zero and near-zero night air temperatures began on Feb. 8-9; on the nights of 14-15 and 15-16, Vol.