322 



be found convenient as a compact systematic summary of my 

 data. The frequency ratios express the comparative densities 

 of population on each kind of surface, for each species tab- 

 iilated and for all the birds of our list. Taking the ratio of the 

 number of birds found in a crop to the whole number of birds 

 as a dividend, and the ratio of the area in that crop to the 

 entire area as a divisor, the quotient is the fi'equency ratio for 

 those birds and that crop. If a species were equally distributed 

 over the entire area studied, this ratio would be 1 for all sit- 

 uations and all crops. If 40 per cent, of the area were in corn, 

 then 40 per cent, of the birds of that species would be in corn 

 fields. If, on the other hand, only 20 per cent, of the birds 

 were in corn, the density of population in corn helds would be 

 expressed liy the frequency ratio of 50 per cent. All ratios 

 below 1 indicate a density of population less than that result- 

 ing from a uniform distribution; and all greater than 1, a 

 density above that limit. 



The coefficients of preference are found by dividing in 

 succession the frequency ratios of a species for each crop by its 

 frequency ratios for each of the other crops. They are thus a 

 measure of the degree of preference of the species for one crop 

 or situation over another; and as arranged in iny tables of 

 coefficients following, they enable us to see just where the 

 preferences lie, and how they compare one with another. 

 Turning, for example, to the coefficient table for the mourning- 

 dove (Table XL, p. 327), we find at the left of the table a list of 

 the crops in which this l)ii-d is found, and a like list, in the same 

 order, at the top. At the place of intersection of the line of 

 figures for one crop with the column of hgures for another, will 

 be found the coefficient of the preference of the mourning-dove 

 for one of these crops as compared with the other. — the stand- 

 ard crop lieing the one whose name is at the head of the 

 column. Selecting, as an illustration, the column headed "corn," 

 and following it to its intersection with the line for " meadows," 

 we find there the coefficient 1.16, — the meaning of which is 

 that for every hundred mourning-doves found in a given area 

 of corn fields, 116 would be found, according to our data, in a 



