106 Trans. Acad. Sci. of St. Louis 
receive the food; the younger, in the rear, starve, slowly, to 
eath. Later, when food is abundant, there is a sufficient ex- 
cess to pull the younger through. Then, too, more frequently 
in the later season, the set is often completed before incubation 
is begun. 
Parasites were noted in one case only. This was a nest heavily 
infested with mites. The young here developed normally how- 
ever. 
The Cowbird and the Lark—One case of Cowbird parasit- 
ism occurred in early June at Evanston (Plate XVII, Fig. 2; 
rm 
ss 
20 4 
PJ 
10 
64 
Figure 15 eon Oa kaa 
dey 7 3 ) ; z 7 ; 3 * af 
Fig. 15. Growth in weight of a June Prairie Horned Lark nestling 
and its Cowbird nestmates (nest No. 20, June 8 to 17, inclusive, 
Evanston, Il, 1926). 
Plate XXXII). The nest contained three Lark eggs and two 
Cowbird eggs. In this case one Lark hatched a day before the 
first Cowbird, the second Cowbird hatched on the third day and 
the remaining Lark eggs failed to hatch. This one-day advan- 
tage in age in the case of the Lark may explain why it developed 
so normally in spite of the parasitism (see Table 20 and Figure 
15). But perhaps also the Lark may not be a proper host. At 
