22 Nelson North American Mainland Myiarchus. 



which crosses the Transition zone of eastern America to south 

 ern Gtinada and New Brunswick. M. laicrencei and its sub 

 species is the most widely distributed of the North American 

 species, with a breeding range, extending from the Isthmus of 

 Panama to southern Arizona and the Tres Marias Islands. The 

 species of most limited distribution is probably M. yucatanen- 

 sis, found only on the peninsula of Yucatan and on Cozumel 

 Island. 



The North American species of Myiarchus appear to have 

 but one moult a year and this occurs immediately following the 

 breeding season, from August to September or perhaps October. 

 The birds are much darker or more richly colored for a short 

 period following the assumption of the new plumage than at 

 any other period. This extreme intensity of coloration quickly 

 passes into a duller condition which continues with but little 

 change through the winter months. In spring the colors grad 

 ually fade or become bleached by the sun until in the breeding 

 season the original shades of greenish, olive and gray of the 

 back and the yellow of the under parts are almost lost in the 

 dingy browns and yellows of the frayed plumage. The upper 

 parts especially bleach to a dingy grayish brown so nearly alike 

 in several species that there is but little color difference be 

 tween summer specimens, and identification of birds in this 

 condition depends largely on size and tail pattern. This condi 

 tion becomes most marked in species living under the 

 brilliant sun of the Lower Sonoran and Arid Tropical regions 

 and is much less noticeable in species like crinitus which live in 

 regions of greater humidity and cloudiness. 



The general resemblance in color between many of the sub 

 species and even between some of the species would render a 

 description of faded spring or summer birds misleading in the 

 apparent uniformity of coloration, while the most richly 

 colored condition that is found immediately after the moult in 

 late summer or fall is so evanescent that it can scarcely stand to 

 represent the average characters. For this reason in the fol 

 lowing descriptions I have taken, when available, the unworn 

 winter birds which represent the typical colors of the various 

 species from soon after the fall moult until the wear and fading 

 of the spring and summer begin to destroy the distinctive 

 shades of color. I have designated this as the 'fresh' plumage. In 



