BEHAVIOR OF VERTEBRATES 455 



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much too specialized to be considered as the immediate outcome" 

 of any form of physiological excitement. They obviously have 

 a long and complicated evolution behind them and as they can 

 only be performed by two birds together there is nothing to 

 account for them as they now stand but some such process as 

 I have just sketched under the name of mutual selection. The 

 second part of the paper gives in detail some of the material 

 worked up in the first as well as some notes on various points 

 not connected with the main interest of the study. 



Pearl (26) gives us, in the seventh paper of his series, a dis- 

 cussion of the brooding instinct which is very much changed 

 possibly under domestication and certainly very much curtailed 

 under the methods employed at the Agricultural Experiment 

 Station. He concludes, however, (1) that it occurs with greater 

 or less regularity following periods of egg laying. (2) that it 

 varies in intensity at different times in the same individual. 

 (3) that it is not necessarily connected with any season and may 

 occur out of breeding season. (4) that it is ordinarily but not 

 necessarily preceded by the laying of a clutch of eggs. (5) 

 that it is apparently closely connected with the functions of 

 the ovary although the precise nature of the connection has 

 not yet been analyzed. 



The flocking habits of migratory birds is the subject of Trow- 

 bridge's (36) paper. He analyzes the automatic protection 

 which a large flock affords as follows: A single bird might be in 

 error from (a) confusion with regard to proper direction of flight, 

 (b) effect of heavy winds or thick fogs acting as a temporary 

 confusing factor while migrating, (c) gradual deviation from 

 the course due to unequal wing power. A large flock eliminates 

 these causes of error to a large extent and the origin of the 

 flock is probably due to the fact that it is protective. The 

 errors are averaged by numbers. He does not attempt to ex- 

 plain the 'sense of direction' but rather the mechanism to avoid 

 getting lost. Night migratory calls are discussed and the pro- 

 tective form of certain flight formations as e. g., the echelon. 

 This he thinks is not taken to prevent any interference but 

 so that each bird may see both forward and to the side at the 

 same time. Birds instinctively follow one another. 



Some experiments in feeding humming birds which were 

 continued for seven summers are described by Sherman (32). 



