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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