A BIRD’S GEOGRAPHY AND ARITHMETIC. 191 
even some may lay while yet on their way north long 
before they reach the nesting region. 
It can not be the mere act of incubating that sus- 
pends egg laying, for many birds begin to sit so soon 
as a single egg is laid, where from four to eight are laid 
afterward. We have seen, however, that incubation 
does affect the circulation, even of the males, making 
the crop of the male pigeon secrete and scale up a 
peculiar curdlike substance. 
Outside influences, such as fright, sudden cold, or 
bad weather generally, may affect a bird so as to cause 
it to cease laying, as may be observed in any barnyard. 
It seems a little remarkable that a bird having so 
much mathematical perception should not be better 
able to know its own egg as well as its number. But 
many seem to be unable to see that they are duped or 
else are wholly indifferent to the fraud. There is the 
well-known instance, however, of the summer yellow- 
bird’s building another floor above the false egg, thus 
showing her knowledge, but she has been more fre- 
quently observed feeding the squabby cowbird’s nes- 
tling than rejecting its egg. There are times, how- 
ever, when the cowbird’s egg is found pierced or 
thrown out of the nest by various birds. But this is 
off of our topic. 
There are some facts recorded that show that, 
besides the very prompt return, almost to the day, of 
some migrants, birds have an accurate estimate of how 
many days lapse between certain events that recur 
regularly. Do they count, or just “feel it in their 
bones ” ? | 
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