108 Trans. Acad. Sci. of St. Louts 
the end of ten days the Lark left the nest normally, but the 
Cowbird was still far behind the Lark in development. The 
following day the nest was empty and the ultimate fate of the 
Cowbird is not known. Space does not permit an extensive 
speculation as to why the Prairie Horned Lark is not more fre- 
quently parasitized but a few points may be noted: 
1. The Lark is not as small as the usual host and there was 
but little diserepaney in egg weights (Cowbird 3.12, Lark 2.65 
grams). 
gram ¥, 
bal 6B, 
Fiqure 16 
OEE Ueeemacver cemueesnee eee A Lee AN eee 
daye 2 3 Pe . ~ P rv 13 
June Prairie Horned Lark nestlings with curves of the meat 
daily temperatures of the same periods (nests Bo, April 21 “4 
May 2, inclusive, Ithaca; C;, May 6 to 15, inclusive, Ithaca; and 
No. 20, June 8 to 17, inclusive, Evanston). 
2. Fully half of the nesting season of the Lark is prior to 
sexual maturity of the Cowbird in the spring. 
. _The incubation period may not be such as to favor 
s : 
4. The exposed condition of the nest may be detrimental to 
the scantily downed Cowbird. 
