24 THE AUD 'U-B.OW? BUTS Eerie 

obvious to everyone, and it must be equally obvious, consequently, 
that until we know how abundant, on an average, the various species 
are in the various parts of the country and throughout the country 
at large, we can make little definite application, either scientific or 
strictly practical, of the knowledge we now have. Our present informa- 
tion in this field is like a chain, one of the links of which is missing and 
has been replaced by a piece of twine. To substitute iron for cotton at 
this point is the object of the studies now in progress in Illinois on the — 
local distribution, average numbers and ecological preferences of the 
various species of Illinois birds.” 
The general method of procedure followed in acquiring the data is 
presented in the following quotations from a paper entitled “The 
Orchard Birds of an Illinois Summer” by Stephen A. Forbes and Alfred 
O. Gross, published in 1921 in the Bu/letin of the Illinois State Natural 
History Survey (Vol. XIV). The expressions “plankton method” and 
‘“‘nlankton net,” when used, refer to a net and methods for securing the 
minute floating and free-swimming organisms from the open water of a 
lake or river and determining the exact and relative numbers of each 
kind. 
Having realized for many years the urgent need of numerical data 
concerning the species of birds in the State as indispensable to their 
valuation as ecological, and especially as economic agencies, it occurred 
to the senior author in 1905 that an equivalent of the plankton method 
might be used in the ornithological field by putting in place of the 
plankton net, two men who should walk in parallel lines a definite 
distance apart, should identify and count all the birds flushed by them 
or crossing their track on a strip of a given width—say 150 feet—and 
should make at the same time a precise record of the kinds of surface 
and situations which they were traversing, of the distances traveled 
over each successively, and of the kinds of birds seen and the numbers 
of each kind on each such section of the 150-foot strip. The product of 
such a series of expert observations would be like that of a huge net a 
hundred and fifty feet wide, drawn in straight lines across every kind 
of crop or other surface vegetation,” by which all the birds found there 
should be caught and held until they had been identified and counted. 
The data so obtained would evidently be quite as useful for their purpose 
as those of the plankton net, and the results of their collation and 
analysis would be quite as dependable. 
A satisfactory test of the method having been made during the 
summer of 1905 on a 400-acre grain and stock farm belonging to the 
University of Illinois, two assistants were engaged, both students of 
the University at the time—one the junior author of this paper, who 
* Forests of tall trees were avoided since the birds there could not be listed exhaustive- 
ly; and in orchards, the more open woods, patches of close shrubbery, and the like, the 
strip surveyed was usually narrowed to sixty feet. 
