482 THE POPULAR SCIENCE MONTHLY. 



close estimate of the numbers of each species occupying a locality, 

 any given season. Thus, during May and June, 1873, I found eleven 

 nests of the cat-bird in a given area, and feel confident that I recog- 

 nized twenty-seven individuals of this species. If this is correct, then 

 there were five cat-birds not nesting, and, I should judge, all male 

 birds. It will be remarked that I overlooked the nests of these 

 " extra " birds. This I believe is impossible. By going over a given 

 space an acre at a time prying into every nook and cranny, climb- 

 ing every tall tree and searching over every small one, as well as 

 bushes and brier-patches, it is hardly possible to overlook any nest, 

 especially so large and conspicuous a one as that of the cat-bird. The 

 habits, too, of non-nesting birds differ from those then breeding. They 

 are much less restless, do not chirp and twitter, or exhibit distress 

 when closely followed, as in the case of nesting-birds. 



Having carefully examined a bird's-nest which seemed to agree 

 most nearly with the published descriptions of such nests, I then 

 noted each nest found and marked the amount of variation in the 

 construction and position. Take, for instance, the nest of our very 

 common robin (Turdus migratorius). Here we have a nest largely 

 constructed of coarse twigs and grass, lined with " a cup-shaped fabric 

 of clay or mud," this mud, again, being covered with finer grass, 

 horse-hair, and occasionally a few feathers. This nest is an excel- 

 lent one to study for degrees of variation in construction ; and here 

 note these differences. During the past spring and early summer, we 

 found thirty-two nests of the robin in an area of about four hundred 

 acres. 



Of these thirty-two nests I will speak, jn-incipally, as to their con- 

 struction, especially with reference to the care exhibited in the mud- 

 lining, and refer but incidentally to the positions of the nests. 



Eleven nests were what might be called " typical," following 

 the description given by Dr. Brewer in the latest work on North 

 American ornithology. 1 In the eleven nests the mud-lining was 

 complete, extending to within about an inch and a half of the rim, or 

 top of the nest. In fourteen, the mud-lining was more or less incom- 

 plete, although always extending over the bottom of the nest, i. e., so 

 much of the interior surface as the eggs or very young birds rested 

 upon. Without an exception, I believe, the fine grass and hair lining 

 the interior of each nest were in greater amount in proportion as the 

 mud-lining was imperfect ; so that, in some instances, the mud being 

 concealed, the nests were very similar to those of other thrushes. 

 The remaining seven nests were altogether " abnormal," and, noticea- 

 bly, each of these seven nests was in such position as a robin would 

 not be supposed to select. A careful study of the surroundings, how- 

 ever, showed that there was always some outside advantage, such as 



1 " A History of North American Birds," by Messrs. Baird, Brewer, and Ridgway. 

 Vol. i., p. 27. Boston, 1874. 



