Vol. XXVI 

 1909 



Roberts, Colony of Yellow-headed Blackbirds. 377 



deposited in a nest in which the Yellow-head had three eggs and 

 the latter laid no more eggs but incubated these four. 



All the eggs of a set are colored alike but there is considerable 

 variation in different sets. Occasionally the appearance in a nest 

 of an egg differently marked from those already there, suggested 

 the possibility of a female now and then laying an egg in a neighbor's 

 nest. 



In seventeen nests the period of incubation was completed; the 

 eggs in nine nests in which the full complement was laid being 

 destroyed before hatching. In these seventeen nests the period of 

 incubation, inclusive of the day on which the last egg was laid, to 

 the day on which the first egg hatched, was nine days in one in- 

 stance, ten days in twelve, eleven days in three, and twelve days 

 in one. Thus ten days may be considered the usual period of 

 incubation. The nine day period was in the case of the only set 

 of five eggs that hatched. 



In the seventeen nests in which incubation was completed all 

 the eggs hatched on the same day in only three nests ; in three nests 

 one egg hatched each day; in two nests two eggs hatched the first 

 day and one egg each day thereafter ; in four nests the eggs hatched 

 irregularly during three days; in two nests the four eggs in each 

 hatched during two days; in the set of five eggs one egg hatched 

 each day for three days, the remaining two on the fourth day. 

 In one instance it was two days after the first egg hatched before 

 the second hatched, this in a nest containing three eggs, one of 

 which was infertile. In each of three nests there was one infertile 

 egg. This irregularity in the time of hatching of the eggs is perhaps 

 due to individual variation in the time of beginning incubation, or 

 faithfulness to the duties of incubation on the part of different birds. 



A brief resume of the foregoing exhibits the following facts in 

 regard to the nidification of the Yellow-headed Blackbird: 



The nesting period in southern Minnesota is from the middle of 

 May to the latter part of June. 



The female builds the nest and incubates the eggs without any 

 assistance from the male. 



The male assists in the care of the young, but only to a limited 

 extent. 



The body of the nest is constructed of wet material, the drying 

 and contracting of which fixes it securely in position. 



