HOW AND WHY DO BIRDS TRAVEL? 179 
But experience has, nevertheless, crept into some 
of the birds, and many that doubtless formerly went 
south with their tribe now remain and endure our 
winters. Others that could well stay still go yet, 
while with some others still the old tendency seems 
stronger at some seasons than others, and they may 
go or they may stay, according to some fancy that is 
not often apparent. Thus all the flickers or redheads 
may leave a certain region one winter, though it be 
milder than the one previous, when they stayed ; and 
it is certain (in some regions at least) that robins, blue- 
birds, and others remain all the year around (or tend 
to remain) much more frequently than formerly when 
they find a friendly haunt. 
There is another element in migration that is in 
direct opposition apparently to that just noticed. 
Birds of evident southern origin, having most of 
their species resident south, as humming birds, tana- 
gers, etc., and showing by their nest, as noted (Chap- 
ter X VIT), their hot-weather habits, also come north 
to rear their young, whence they have never been 
driven by an ice-cap. We can readily see why all 
birds should flee at the hint of cold weather, but why 
should these (as it grows warmer) make the northern 
trip? If they were ever of northern origin when the 
arctics were tropical, it seems probable that for a long 
time their ancestors have resided permanently in the 
south, and their northward tendencies seem quite re- 
cent. Only a few of them have acquired the habit. 
Here, however, may come in a factor in nest 
building which also influences the northern bird as 
