22 ANIMAL LIFE IN THE YO SEMITE 



CENSUSES OF BIRDS IN THE YOSEMITE SECTION 



To convey an adequate idea of the bird life of any given area, enumer- 

 ations of species are not alone sufficient; the numbers of individuals of each 

 species must also in some way be indicated. The usual terms "abundant," 

 "common," "rare," and the like, are unsatisfactory in that their meaning 

 varies both with the person employing them and with the kinds of birds 

 considered. In the latter regard, the Western Chipping Sparrow and the 

 Western Red-tailed Hawk might both be put down as ' ' common, ' ' whereas 

 the sparrow may have been observed in actual numbers ten times those of 

 the hawk. 



Counts of individual birds are fairly practicable when made in the 

 breeding season on the basis of some unit of area such as an acre. At that 

 season each adult pair is settled within a particular circumscribed locality, 

 and the male is in song. But as soon as the young are out, and from 

 then on throughout the year until the beginning of the next nesting season, 

 most species of birds are moving about incessantly. Counts of individuals 

 are then very difficult to make and furthermore are likely to be misleading 

 because of their great variation in any small area from hour to hour and 

 from day to day. And so, in our field work in the Yosemite region, we 

 put into effect the following different method. 



Instead of using a unit of area, we used a unit of time. Birds were 

 listed, as to species and individuals, per hour of observation. In a general 

 way this record involved area, too. Our censuses were practically all 

 made on foot, and the distance to the right or left at which the observer 

 could see or hear birds did not differ, materially, in different regions. The 

 rate of the observer 's travel did, of course, vary some ; for example, when 

 climbing a steep trail, or going through chaparral, progress was slower 

 than when hiking straightaway along open ridges. Also, in some places, 

 the greater density of the vegetational cover acted to limit the range of 

 sight. But for each of these adverse features of the method there were 

 certain compensations. 



For recording a census, a piece of cardboard and a pencil were carried, 

 the names of the various species of birds jotted down, and their numbers 

 checked, as they came to notice. The presence of no species was assumed ; 

 but probabilities were given consideration in making identifications. In 

 cases where birds were seen or heard, but their identity was not established 

 with certainty, provisional names were entered, each followed by a question 



