CENSUSES OF BIBBS 25 



there in midwinter as in early summer. Return of summer species is 

 already much in evidence in the list made on April 29. 



In the census at Mono Lake Post Office on May 31 a 'wave' of migration 

 is indicated in the numbers of species and individuals of warblers seen, 

 which are in excess of what would be present there a month later, in the 

 height of the nesting season. The census on the Big Oak Flat Road in 

 December exhibits the congregation, in a favorable situation, of berry- 

 eating species such as the Townsend Solitaire and Western Bluebird. Had 

 it not been for the berry-laden mistletoe in the golden oaks on the talus 

 slope (pi. 16fl) this census in all likelihood would have been no larger 

 than the one taken at the same season on the floor of the Valley. 



The census-taker is struck by the variation in his records from hour 

 to hour during the day, irrespective of kind of territory covered and of 

 his own degree of alertness. This fluctuation is due in large part to the 

 fact that there are two daily jjeriods of marked activity on the part of 

 birds, namely, in the early morning, within an hour or so after sunrise, and 

 in the late afternoon, about two hours before sunset. Of these two periods, 

 that in the morning is the most impressive; in other words the observer, 

 by selecting the earlier hours for his census-walk, will make the highest 

 score and also the most representative one. It is quickly apparent that in 

 comparing the enumerations for different days and for different localities, 

 allowance should be made for this daily double fluctuation in the visibility 

 and audibility of birds. 



It is the earnest recommendation of the authors that observers in a 

 position to do so will get into the habit of taking bird censuses. The method 

 here advocated is a practicable one ; we believe it can be adopted to advan- 

 tage by anyone possessed of a fair acquaintance with bird species. A 

 'collection' of census records will afford basis for much future satisfaction. 

 On the one hand, is the pleasure of recalling to mind pleasant days afield 

 spent among the most attractive things in nature; and on the other hand 

 is the intellectual enjoyment derived from comparing bird populations 

 in kind and size from place to place and season to season, and from 

 endeavoring to account for the fluctuations which are shown, on the basis 

 of all the factors known to control the birds ' existence. 



