105 
S 
Burrill, Bird Migration. 
waves. Even disallowing this factor, the claim might still be 
made that the speedy reappearance of so many delicate insects 
with springtime makes such a marked change in the complexion 
of the Northland that birds which are subject to local migrations 
in the South would unwittingly move Northward as ttie abund¬ 
ance of insects lured them on. This presupposes that most species 
have some advance guards of value as scouts. 
The fact that the advance birds do not return to the main 
flocks in the South, as the instance of the ducks fluctuating North¬ 
ward with the warm and South with the cold days of an Oklahoma 
winter would suggest that the birds’ instincts are readily appealed 
to to follow, after the absence of older birds is noticed. This pre¬ 
supposes also, that the birds have at least a slight knowledge as 
to the makeup of their flocks to the extent of following the lead 
of experienced individual birds rather than the young and erratic 
ones. I think there is little question on this score, that birds 
acknowledge leadership and miss leaders which disappear. Per¬ 
haps the disappearance Northward of other species, consociates 
on the Southern feeding grounds, would be noticed at this rest¬ 
less mating time of the year. It is realized that there are import¬ 
ant exceptions to this argument, just as to every argument thus 
far advanced, also that there are several species of birds, especially 
water birds, which make such long trips over night that the abund¬ 
ance of some specially delectable insect will have no influence in 
solving the migration stimulus. 
It has been shown in literature by members of the Biological 
Survey that the robin and other species accelerate their Northward 
progress in unison with the accelerated advance of spring in the 
Northern half of the continent. This evidence is as useful in 
proving the synchronous appearance of animal food suited to 
robins as it has been made useful in proving the entire dependence 
of birds on weather in the spring migration. 
As weather has been generally disproved as influential in the 
autumn migration, it should be abandoned for spring migration. 
To this tenet I can conceive of only one exception in favor of 
climate. That is, there has been little work done to show if the 
autumn migrations coincide with the falling off of the average 
weekly heat increment during the summer, a climatological fact 
starting about the time the first warblers start southward. 
