PRAIRIE HORNED LARK 351 



three or four days, a marked leveling during the seventh and eighth 

 (in one case the sixth), and a gradual rise during the ninth and tenth. 

 Nestlings in May grew slightly more in the same period than a 

 nestling in June and much more than a nestling in April. This dis- 

 crepancy of growth seems closely (though perhaps indirectly) 

 correlated with the temperatures of these seasons. 



A lessening in weight growth occurs, normally, between the seventh 

 and eighth days. This is brought about by the simultaneous un- 

 sheathing and drying of most of the feathers. On the other hand, 

 growth in length shows, if anything, an acceleration at this period 

 due to the extension of the tail. Marked variations in growth occurred 

 in the various broods measured and in the different young of the 

 same brood. This was brought about by two things: The fact that 

 a slight difference in age gave the older larks a great advantage in 

 securing food from the parents; and the fact that food was more 

 plentiful later in the season than at the beginning. 



Length-growth curves show a precipitate rise during the first three 

 days, a slight leveling during the next three days, and a precipitate 

 rise during the sixth, seventh, eighth, ninth, and tenth days. The 

 cause for the intermediate leveling is not understood, but the rise 

 toward the end of the nesting period is brought about by the growth 

 of the tail. 



In the early breeding season the enemies of the young are weather 

 and a scanty food supply. The weather may result in snows that 

 bury them from sight. The scanty food supply may result in the 

 starvation of one or more of the nestlings. Starvation results from 

 the automatic feeding reaction of the adults wherein the nestling 

 nearest that part of the nest habitually approached by the adults 

 will receive the first feeding ; if the food is scanty this bird will receive 

 all or nearly all the food. Only when food is so abundant that the 

 first nestlings fed do not swallow promptly will the remainder of the 

 brood be fed. In this case food is withdrawn from the mouth and 

 put in the next and so on. The female lark rarely brings more than 

 will go into one mouth; the male may feed two or more, but never 

 four or five, at a time. Those young larks that have a few hours' 

 advantage in hatching — a full day in several cases in the early 

 spring — will have the advantage in size that will allow them to push 

 to that side of the nest over which the food always comes. They 

 survive; the others may perish. Such occurred in many observed 

 nestings in April. 



Predacious enemies cause a greater and greater loss as the season 

 advances into June and July. The optimum season for the welfare 

 of the young is shown to be May. 



