CLASSIFICATION 



141 



follows: toes four, three in front and one behind, except 

 in the vultures ; all toes armed with strong, sharp, curved 

 talons (tfil'iinz); bill with a cere (ser : Latin, sera, wax) 

 or covering of skin at its base through which the nostrils 

 open, very stout and strong, the upper mandible tipped 

 with a sharp pointed hook. 



In addition to this classification by structure, which is 

 essential for a careful study of birds, they are also classi- 

 fied by their habits. For example, birds are divided into 

 four classes based on 

 their migratory habits. 

 Birds like the downy 

 woodpecker and English 

 sparrow are permanent 

 residents throughout 

 their range, that is, they 

 can be found within 

 given limits at any time 

 of year, while bobolinks 

 and humming birds are 

 summer residents, mi- 

 grating southward at 

 the end of the season. 



Birds like wild geese, fox sparrows, and the like, arc 

 transients, stopping along their migratory route for rest 

 or food or to escape unfavorable weather; while such 

 birds as the snowy owl, great northern shrike, and red- 

 poll are winter visitants which have migrated to us from 

 the North when the cold became excessive and the food 

 supply diminished. 



Birds are classified also by their nesting habits. Some 

 birds, like the meadow lark and bobolink, nest in the open 

 field, and their nests are made inconspicuous rather than 

 inaccessible; other birds, like certain hawks and eagles, 



Figure 144. — The Robin. 

 Sometimes a winter resident. 



