436 
to improve upon them if we were to continue or repeat the survey. 
Their special value was in the substitution of precise data, recorded in 
figures and hence available for accurate comparison and capable of 
being expanded by subsequent addition, for the vague and variable ex- 
pressions of degrees of abundance and scarcity now commonly used. 
The principal features of method to which we would call atten- 
tion are: 
1. A careful selection of the sample tracts surveyed, with a view 
to making them as nearly as possible fairly representative of the whole’ 
area from which they are chosen.* 
2. The accurate recognition and complete enumeration by two 
observers of all the birds present on long strips or belts of uniform width, 
one observer recognizing and counting the birds seen on each kind of 
habitat, and the other recording the distances traveled over each.f 
3. The form of field notes written on uniform slips day by day 
for each trip, the slips being numbered consecutively for convenient 
reference. 
4. The method of tabulation of the observations in a way to make 
them available for consolidation in various ways and for complete dis- 
cussion. 
5. A species index of the numbered notes such that all the data 
for each species may be readily assembled. 
6. The grouping and tabulation of “residence classes’. 
7. Use of the tables thus formed in comparing the composition of 
the bird population in different seasons and especially in different stages 
of the fall and spring migrations, and the tracing in detail by this means 
of the successive steps of each migration. 
GENERAL SPECIES LIST 
The following list of 195 species of which 166 have been statistically 
treated, comprises all the birds seen and recognized in the entire course 
of the Illinois survey. The letters in the first three columns of the 
table indicate a residence classification of the species based primarily on 
our own observations but taking into account also the published and 
unpublished observations of others available to us. As our main ob- 
ject is ecological, we have attached relatively little importance to the 
exceptional occurrence or comparatively insignificant numbers of a spe- 
cies outside its usual range or season of residence, but have classified 
it only when and where it was found in numbers sufficient to give it 
some appreciable significance as a feature in the bird life of the time and 
place. 
* We have repeatedly made mention of the fact that our method of enumera- 
tion limited us to birds of more or less open country, excluding us from aquatic 
situations and from dense forests or lofty trees. 
7 To make sure that practically all the birds were seen by these observers, 
several trial trips were made with a third person walking between and some 
distance behind the two; with the result that the number of additional birds 
thus flushed was altogether negligible. 
