The Birds’ Calendar 
being generally effected by the female, but fre- 
quently with the assistance of her mate. The pe- 
riod required for incubation varies with the size 
of the bird, from ten days or less for the small- 
est species up to about eight weeks for the os- 
trich. 
Most species produce two sets, and a few 
three sets of eggs each year. The entire nest- 
ing season is longer than commonly supposed. 
While for the majority of birds it is comprised 
in about six weeks—from the middle of May to 
the end of June—the great horned owl lays its 
eggs in March, sometimes even in February, 
other owls and hawks (sometimes the song 
sparrow) in April, whereas the second set of 
many species is not produced till July. Indeed 
the cedar-bird and goldfinch commonly wait 
till July before laying the first set, and the 
goldfinch even delays sometimes till August. 
(The period is even longer than the foregoing 
for the entire country, extending from January, 
for some of the birds of prey, to the end of 
September. ) 
The full set of eggs varies in number. 
Among thrushes, warblers, finches, etc., the set 
contains four or five, which is perhaps the gen- 
eral average. But eagles, whippoorwills, hum- 
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