PREFACE. 



The present work includes, as far as known, all species and sub- 

 species of birds which occur in Illinois and Wisconsin, the total num- 

 ber being 398, with descriptions of their various plumages, nests 

 and eggs, and geographical distribution, together with more or less 

 brief biographical notes concerning them. 



The keys to families and species are practically the same as those 

 which first appeared in the author's Birds of Eastern North America, 

 revised to meet their present needs. These should enable the stu- 

 dent to readily identify a bird, the species belonging to the different 

 families being arranged according to the length of the wing (measured 

 from carpus to tip) . A key to the eggs of our more common species 

 is also included. 



Careful comparison of large series of specimens has shown that 

 while adult birds of the same species differ considerably in length, 

 the wing measure is very constant, the variation in a large number 

 of specimens being so small that, allowing for possible extremes, we 

 may safely arrange our birds in groups, using the length of the 

 wing as a basis. The identification of any species then becomes a 

 comparatively simple matter, as usually the birds contained in each 

 group are so few in number that characteristic differences are easily 

 indicated. 



Let us, for example, assume we have a bird before us which we 

 wish to identify; we should first learn to which family it belongs. 

 Turning to the Key to Families, we find this a simple matter (as the 

 families are few and the illustrated differences in the bills and feet 

 very characteristic), we discover our bird to be a duck. Having as- 

 certained the family to which the bird belongs, we turn to the Key to 

 Species. We have, of course, measured the wing and found it to 

 be 5.90 inches long, measured from the carpus (bend of wing) to 

 tip. (See illustration, " How to Measure a Bird," on page 20.) 



We now turn to the Ducks, and discover they are divided into 

 subfamilies, the Bay and Sea Ducks having a flap or lobe on the hind 

 toe, and the fresh-water ducks, or River and Pond Ducks having no 

 large flap on the hind toe. Our bird has a flap on its hind toe, and is 

 evidently a salt-water duck, belonging to the subfamily Fuligulinaz. 



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