BLUE-HEADED VIREO 295 



light brown, dark brown, reddish brown, or bhickish. The measure- 

 ments of 50 eggs average 19.5 by 14.4 millimeters; the eggs showing 

 the four extremes measure 21.8 by 14.7, 19.6 by 15.3, 17.0 by 14.1, and 

 18.1 by 13.4 millimeters. 



Incubation. — F. L. Burns (1915) says that the period of incubation 

 is 10 to 11 days, but this must be based on insufficient information, for 

 the other vireos require a somewhat longer time ; no further data on 

 the subject seem available. Mr. Saunders (1938) remarks that his 

 experience "indicates that the period of incubation is at least 11 days 

 long, but, up to date there has been no opportunity to determine its 

 length definitely." 



Both parents share the duties of incubation and are devoted to the 

 care of the young. They change places on the nest at regular intervals 

 regardless of the near presence of human observers. Sometimes when 

 we drive the female from the nest the male will take her place within 

 a few feet of us. They are usually very close sitters and often must 

 be lifted off the nest if we want to examine the eggs. Mr. Saunders 

 (1938) says : "Changing places is accompanied by a call by which the 

 incubating bird notifies its mate. The mate answers with the same 

 call, and comes immediately. Tlie call is a soft, rather, low-pitched 

 trill, running downward in pitch at the end, like 'tiprrrrrr.' This call 

 is interpolated in the song by some individuals." 



The male sings regularly v.hile incubating, and probably the female 

 does to some extent; Mr. Saunders (1938) remarks: "It is impossible 

 to distinguish sexes by plumage or by habits about the nest, and my 

 observation that the female sings is based on observing two birds, both 

 in song, one singing while incubating and the other answering from 

 near-by trees." 



Dr. Cornelius Weygandt (1907) writes: 



The comradeship of the two during;- incubation had been very winning. As one 

 sat upon the eggs the other would come flying swiftly to a dead limb above and 

 then drop to the little branch from which the nest swung, lauding not a foot away 

 from it. Here the incoming bird would mew, ever so caressingly, and the bird on 

 the nest would answer in the same low tone. Sometimes the interchange of 

 greetings would be followed by interchange of positions, the sitting bird first 

 unsettling itself gently from the eggs and then flitting off to alight beside its 

 mate. The incomer would lift itself into the nest as deftly and then after 

 a few more mutual mews the relieved bird would be off to the oak-tops. Once 

 the sitting bird, this time I suppose the male, sang while brooding on the nest 

 when the other returned. 



Young. — Mr. Saunders (1938) says: 



Singing takes place all through the incubation and while the young are still 

 very small, but it gradually ceases as the young grow larger, and is heard not at 

 all after they are five or six days old, and is only resumed after they have been 

 out of the nest nearly a week and are able to shift for themselves. * * * 



When feeding young, the adults are rather more concerned than they are 

 when incubating, perhaps not for themselves, but they do not entirely trust 



