BIRDS OF THE CAMBRIDGE REGION. 63 



and sex, although he often seems quite willing to share his domain with birds of 

 other and perhaps closely related species. The extent of the area thus monopo- 

 lized varies exceedingly with birds of different species. An apple orchard which 

 affords sufficient room for — let us say — two pairs of Yellow Warblers, two 

 pairs of Orioles, three or four pairs of Chippies and four or five pairs of Robins, 

 seldom or never harbors more than a single pair of Kingbirds or Crested Fly- 

 catchers. 



I know a tract of woods which has been regularly frequented for many 

 years by a pair of Great Horned Owls and a pair of Hairy Woodpeckers. They 

 usually rear their broods, but the number of breeding birds remains always the 

 same. Apparently there is not room for more than a pair of each kind, although 

 the woods exceed one hundred acres in extent. What becomes of the young.? 

 As I do not often find them in neighboring woods, I have little doubt that they 

 remove to considerable distances to breed — perhaps sometimes as far as twenty 

 or thirty miles. Such birds as the Golden Eagle and Duck Hawk require even 

 more ' elbow room ' when breeding. As a rule the species which roam over the 

 most ground in the course of their daily wanderings claim and maintain the 

 broadest preserves, while those of sedentary habits often content themselves with 

 very modest freeholds. Whatever the extent of the domain, the birds who 

 occupy it as a summer home evidently regard it as exclusively their own. The 

 readiness and celerity with which trespassing birds are accustomed to retire when 

 attacked or even merely threatened by the established tenants, has seemed to me 

 to indicate that the claims of the latter to temporary ownership are recognized 

 and respected by all right-minded birds. It is probable, also, that with birds, as 

 with human beings, the simple fact of possession counts nine points in the law. 

 In my opinion the desire for exclusive possession so conspicuously shown by the 

 male, and often by him alone, is usually the direct result of sexual Jealousy. 

 This, as is natural, makes him intolerant, during the breeding season, of the 

 near presence of rival males. If his concern were chiefly in respect to the food 

 supply, it would be equally manifested at every season and towards all birds 

 who subsist on the same food that he and his mate require — which is certainly 

 not the case. 



What I have just said has made it plain, I trust, that, from the view point 

 of the birds, a locality or region may be fully populated by them, when to us 

 it seems but sparsely tenanted. Whenever it becomes overpopulated, the sur- 

 plus birds (usually the younger or weaker ones, no doubt) are evidently driven 

 from their natal haunts and forced to seek freeholds elsewhere. In other 

 words there is an overflow to regions or localities more or less distant. As the 

 waters of a pond or river return to their natural bed or channel when, after a 

 period of flood, their volume is reduced within normal limits, so, it may be 



