92 BULLETIN 86, UNITED STATES NATIONAL MUSEUM. 



States. In the last mentioned region there is no evidence that it 

 formerly occupied a greater area than now ; while on the other hand 

 it seems at the present time to be, even if slowly, extending its range, 

 particularly in Texas and California. For some reason not apparent 

 it no longer breeds in Central America southeast of Guatemala, which 

 hiatus may have aided in the diiferentiation of the northern colony. 

 As expected, the Yucatan form, Chordeiles acutipennis microineris^ 

 is most closely allied to the parent race, Chordeiles acutipennis acuti- 

 pennis, retaining the same tail markings but becoming larger, and 

 otherwise modified by isolation and its more arid habitat. The 

 northernmost race, Chordeiles acutipennis texensis, also of desert en- 

 vironment, is most nearly allied to Chordeiles acutlpenms micro- 

 meris,'^ but still more modified, presenting the extreme of differentia- 

 tion from the parent stock. The Lower Californian Chordeiles 

 acutipennis inferior,^ also a desert bird, is merely a depauperate form 

 of Chordeiles acutipennis texensis, thus modified by semi-isolation 

 and that peculiar but not well-understood influence which produces 

 in southern Lower California so many small forms. 



Migration. — The migration of Chordeiles acutipennis differs greatly 

 from that of Chordeiles virginianus. Of the five races of the present 

 species, two, Chordeiles acutipennis acutijyennis and Chordeiles acuti- 

 pennis exilis, seem to be entirely sedentary. Anotlier, Chordeiles 

 acutipennis inferior, withdraws only from the more northern portions 

 of the Lower California peninsula to its southern end. Although the 

 comparatively few available specimens of Chordeiles acutipennis 

 micromeris do not permit any careful analysis of its migratory move- 

 ments, we know enough to show that its route lies through Central 

 America to its winter home in Costa Eica and Panama. 



The northern Chordeiles acutipennis texensis has the greatest 

 migration range of all the forms of the species. It passes from the 

 southwestern United States to extreme eastern Panama. Its route 

 both southward and northward seems to be entirely overland, 

 through Mexico and Central America. It appears in spring in the 

 southwestern United States usually some time in April, though 

 during some years in certain localities not until the middle of May, 

 but exceptionally as early as March 21. It leaves this country ordi- 

 narily in September, but lingers on rare occasions as late as October 

 23. The normal migration movement thus extends over about two 

 months during both spring and autumn. 



Habits. — In many of its habits Chordeiles acutipennis resembles 

 Chordeiles virginianus. It is, however, more crepuscular, does not 

 have the high, booming flight, and has different notes. It is fond 

 of the chaparral, groves, gravelly or rocky mesas, valleys, slopes, 



1 See p. 100. 

 a S«« p. 109. 



