BIRDS OF THE CAMBRIDGE REGION. 



319 



needles. This explains, no doubt, why Black and White Creepers are chiefly 

 confined in summer to the rougher and wilder portions of the Cambridge Region. 

 There are few ledges and no very deep accumulations of dead leaves in such 

 woods and thickets as remain in Cambridge and in the eastern parts of Water- 

 town and Belmont. 



After the young Creepers have become strong of wing and quite able to shift 

 for themselves they, with their parents, join the troops of small insectivorous 

 birds which, having laid aside family cares and the bitter sexual jealousies that 

 alienate the males from birds of their own kind and sex during the breeding 

 season, are now free to roam the woods together and to enjoy, to the utmost, 

 the few short weeks which remain before most of them must start on the long 

 and perilous journey southward. 



These ' mixed flocks,' as they are called, have been frequently described by 

 writers ; it may be well, however, to say a few words about them in this connec- 

 tion, for they are among the most interesting of all bird gatherings, and in the 

 Cambridge Region they are made up chiefly of species which are still to be con- 

 sidered in the present Memoir. They vary greatly in size and composition, at 

 different times and in different localities. Beginning to form early in July they 

 are likely to include, by the first of August, practically all the small, forest- 

 haunting birds which are to be found in the immediate neighborhood. Woods 

 less than twenty acres in extent seldom harbor more than a single flock, but in 

 those covering one hundred or more acres several different flocks may be seen 

 in the course of a single day. Most of the flocks contain one or more families 

 of Chickadees with perhaps a few White-bellied Nuthatches and almost certainly 

 a Downy Woodpecker or two. These species often constitute the resident and 

 more or less permanent nucleus of a flock which, although subject to frequent 

 and at times very considerable changes as to numbers and make-up, is not com- 

 pletely and finally disbanded until the following spring. Its shifting or tempo- 

 rary members fall into three classes: (i) summer residents — most of which 

 depart for their winter homes in the south before the first of September; (2) 

 migrants bred further to the northward and bound further to the southward — 

 few of which remain after the last of October ; and (3) winter visitors from the 

 north — some of which arrive as early as the middle of September. 



The largest and most interesting ' mixed flocks ' are found in August when 

 they are likely to include representatives of most of the smaller wood-frequent- 

 ing birds which are summer residents of eastern Massachusetts, although the 

 majority of these leave for the south earlier than is ordinarily supposed and 

 sometimes before the end of July. Towards the close of August there is usually 

 a marked and perhaps very considerable influx of migrants from further north, 

 some of which linger for days in succession, others for but a single day. Thus 



