319 



The wide differences of their numbers in these several situations 

 can not be taken to demonstrate corresponding differences in the lo- 

 cal or ecological preferences of these birds, although they do indicate 

 something of the effects which birds may be producing on equal areas 

 in these crops. If sparrows resort to orchards largely for resting 

 places and for protection against the wind, they would tend to ac- 

 cumulate there in much greater numbers to the unit of area in a 

 country containing only scattering small orchards than in one where 

 many large orchards were within their reach; and if horned larks de- 

 cidedly prefer bare ground to a grassy turf, there will be a larger 

 number of them in plowed fields to the square mile when but few 

 fields have been lately plowed than when the larger part of the agri- 

 cultural area has just been broken up. 



Ratios of Frequency and Preference. 



Bearing in mind the necessity thus shown for an intelligent 

 analysis and interpretation of certain of the facts, the following ta- 

 bles of frequency ratios, and coefficients of preference may be found 

 convenient as a compact systematic summary of my data. The fre- 

 quency ratios express the comparative densities of population on each 

 kind of surface, for each species tabulated 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 fre- 

 quency ratio for those birds and that crop. If a species were equally 

 distributed over the entire area studied, this ratio would be i for 

 all situations 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 fields would be expressed by the fre- 

 quency ratio of 50 per cent. All ratios below 1 indicate a density 

 of population less than that resulting from a uniform distribution; 

 and all greater than 1, a density above that limit. 



The coefficients of preference are found by dividing in succes- 

 sion 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 an- 

 other ; and as arranged in my tables of coefficients following, they en- 

 able 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. 324), we find at the left of the 

 table a list of the crops in which this bird is found, and a like list, 



