62 MEMOIRS OF THE NUTTALL ORNITHOLOGICAL CLUB. 



(3) Breeding rather commonly, but apparently only very locally, for one or 

 more seasons which have been followed by periods of total absence : Olive- 

 sided Flycatcher, Orchard Oriole, Black-throated Bunting. 



It is by no means certain that all the birds included in the first of these 

 three classes have been long established as summer residents, or that they will 

 continue to be so. Indeed there are reasons for believing that one or two of 

 them did not occur at all in summer thirty or forty years ago. No one of them 

 is generally distributed, and most of them occupy stations of limited extent more 

 or less remote from the favorite breeding ranges of their respective species. 

 Most of the birds placed in the second class appear to be of highly irregular 

 occurrence, although two of them — the Mockingbird and Cardinal — have been 

 noted rather frequently during the past few years. 



With respect to birds such as those named in the third class, as well as 

 to some of those included in the first and second classes, an interesting 

 question arises, viz., why, after having bred rather commonly or even numer- 

 ously in the Cambridge Region, during one or more periods of apparently brief 

 duration, — as the Orchard Oriole and Black-throated Bunting are known to 

 have done, — , or not uncommonly for a number of successive years, — as is 

 illustrated by the case of the Olive-sided Flycatcher, — should they have nearly 

 or quite deserted this region .? So far as we know, they were not subjected to 

 any general or very serious molestation while with us, nor were their haunts 

 materially changed until after they had departed. What, then, could have led 

 to their withdrawal .? I can answer this question only by suggesting a theory 

 which is based largely on established facts, but partly, as I am free to admit, 

 on pure assumptions. 



Many if not most birds show a marked preference for breeding in certain 

 regions, throughout which they are more or less evenly and generally distributed, 

 but within which their numbers do not seem to increase beyond fixed maximum 

 limits no matter how carefully the birds may be protected or how successful 

 they may be in rearing young. It has been assumed that these limits are deter- 

 mined by the food supply. This no doubt is an important factor at certain sea- 

 sons and in certain localities, with birds which subsist largely on seeds and fruit, 

 for food of this kind is seldom inexhaustible. But that insect-eating birds often 

 exhaust the supply of insects, especially in summer and in regions largely under 

 cultivation, I find it difficult to believe. I have observed — as, indeed, who has 

 not ! — that few birds — excepting those which, like the Swallows, Terns, Herons 

 and Gulls, are accustomed to nest in colonies — tolerate very near neighbors of 

 their own species during the season of reproduction. At its beginning each pair 

 takes possession of a definite tract of woodland, orchard, swamp or meadow, 

 which the male is ever on the alert to defend against trespassers of his own kind 



