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 differentiation of the northern colony. 

 As expected, the Yucatan form, Chordeiles acutiperwds micromeris^ 

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

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

 otherwise modified by isolation and its more arid habitat. The 

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

 vironment, is most nearly allied to ChordeUes acutipennis micro- 

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

 tion from the parent stock. The Lower Califomian ChordeUes 

 acutipennh inferior,'^ also a desert bird, is merely a depauperate form 

 of Chordeiles acutipenms 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 ChordeUes acutipennis differs greatly 

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

 species, two, ChordeUe^ acutipennis acutipermis and Chordeiles acuti- 

 penms exUis, seem to be entirely sedentary. Another, ChordeUes 

 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 acutipenms 

 micromer*is 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 ChordeUes aoutipenMis 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 ChordeUes acutipennis resembles 

 ChordeUes 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. 

 » See p. 100. 



