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SUBPHYLUM VERTEBRATA: 



this book. Any adequate consideration of identifica- 

 tion must treat species. For this purpose, there are 

 many field books or guides. The present discussion 

 is Hmited to certain aspects of the biology of birds. 



RELATIONSHIPS 



Perhaps the most obvious and definitely the most 

 distinctive external characteristic of birds is the body 

 covering of feathers. Feathers are modified reptilian 

 scales. Therefore, the development of feathers and 

 the presence of scales on the legs and feet indicate 

 the close relationship between birds and reptiles. In 

 fact, in one way of looking at relationships, birds are 

 more closely related to crocodilians than crocodilians 

 are to other living reptiles. 



EXTERNAL FEATURES 



Feathers are an obvious external adaptation to 

 flight. These structures are extremely light and 

 strong, their weight does not detract greatly from the 

 ability to fly, and feather strength is necessary for 

 flight. Other external structures show various modifi- 

 cations for ways of life other than flight. For example, 

 bills are adapted for such things as eating seeds, 

 spearing fish, tearing prey, obtaining flower nectar, 

 catching fish and insects, and boring into wood. Also, 

 feet are modified for such things as perching, walking, 

 wading, swimming, and tearing prey. However, we 

 shall confine our discussion to feathers. 



A typical feather consists of two main parts, the 

 shaft ("stem") and two flexible vanes (one on either 

 side of the "shaft). The shaft is also composed of two 

 parts, the lower calamus without vanes and the upper 

 Tachis that supports the vanes. Some birds have an 

 ajtershajl, or smaller duplication of the main feather 

 structure attached to the shaft. The vane also has two 

 parts, an outer vane which overlaps the next outer 

 feather and is narrower than the inner vane. Vanes 

 are composed of progressive branchings starting from 

 the shaft. Barbs are the first branching of a vane from 

 the rachis and each barb is like a tiny feather due to 

 branches called barbules. Some barbules have their 

 lower edges frayed into slender filaments called 

 barbicles. Finally, some barbicles have slender hooks, 

 or hamuli, which tend to hold together and support 

 the vanes. 



Adult feathers are of four types. The cnnlour feath- 

 ers form the outline or contour of the bird and include 

 the flight feathers. Down feathers are hidden beneath 

 the contour feathers and have no hamuli. Fdoplumes 

 are hair-like feathers. Powder-down feathers are modi- 



fied down that grows throughout life but in which the 

 barbs degenerate into a fine powder. Powder-down 

 is found only in a few groups of birds such as herons 

 and bitterns. These various feather types are dis- 

 tributed in one of two ways. The flightless birds, such 

 as ostriches, rheas, emus, and penguins, have the 

 feathers uniformly distributed over the body. How- 

 ever, most birds have feather tracts and areas where 

 there are no contour feathers. 



Plumage coloration is due either to the presence of 

 chemical colors in the feathers or to the structure of 

 the feathers. The main chemicals are carotenoids, 

 producing red, orange, and yellow, and melanin, 

 producing black, gray, brown, and reddish-yellow. 

 Feather structure results in white, where all light of 

 all wave lengths of the visible spectrum is reflected; 

 blue, where blue light is reflected as a result of barb 

 structure; green, where the cuticle enclosing the barbs 

 is transparent yellow and blue light is reflected 

 through the cuticle; and irridescent colors, where 

 light is reflected either through broadened and 

 twisted, overlapping barbules containing rod-shaped 

 melanin granules, or through feathers having spher- 

 ical melanin granules. 



No matter how color is produced it is assumed to 

 serve one of four purposes. Three types are con- 

 sidered protective because they may "hide" the ani- 

 mals from potential predators. Cryptic coloration blends 

 the bird into its background and is found in marsh 

 birds, sparrows, ground nesting birds, and others. 

 Counter shading, consisting of lighter coloration below 

 to counteract shadows, is considered concealing and 

 is found in shorebirds. Deflective coloration is found in 

 various birds having conspicuous colors or markings 

 during flight but no such "flash markings" when not 

 flying. The fourth type of coloration, epigamic, is not 

 easily explained. Epigamic coloration, the brilliantly 

 colored feathers of one sex, is sometimes believed to 

 bring the sexes together during courtship or to be a 

 means of sex recognition. 



Birds go through a sequence of feather changes, or 

 molts. After the adult plumage is attained, molts fall 

 into an annual cycle. The molts and plumages from 

 hatching to adult and the adult annual cycle are as 

 follows: The first, or hatching, plumage is called 

 natal down. This completely covers young hatched at 

 an advanced stage of development such as shorebirds; 

 it is scarce on the immature hatchlings, such as are 

 found in songbird species. The postnatal molt pushes 

 natal down to the tips of developing juvenile plumage. 

 Juvenile plumage first appears as stiff quills that soon 



