ALE. Verrill on the Distribution of N. American Birds. 249 
Art. XXXV.—Note on the Distribution of North American Birds ; 
by A. E. VERRILL. 
confirmed by many additional facts, and the boundaries have 
the distribution of birds in latitude, I have found that the essen- 
tial limiting cause is the average temperature of the breeding 
season, which for the majority of our birds may be taken as 
April, May and June. <A line drawn upon the map of eastern 
North America representing the mean temperature of 50° F., 
during these three months, will coincide with the southern bound- 
ary of the Canadian Fauna, as previously determined from the 
examination of the birds breeding in that subdivision. Another 
ine representing the temperature of 65° will represent the south- 
ern boundary of the Alleghanian Fauna as distinguished from the 
Louisianian ‘or Gulf State Fauna, which has also been admitted 
by Prof. Baird and others, from the characteristic birds of the two 
Tegions, This line commences on the Atlantic coast near Ports- 
and Kentucky, and up the valley of the Mississippi, probably 
4 alot the lower part of the dno 
__ 4€ad us to suppose that th ? 
_ Tegions at least, although it may be less apparent. Thus it has 
Am. Jour. Scr.—Seconp Series, Vou. XLI, No. 122.—Mancn, 1866. | 
oa 
