EASTERN BLUEBIRD 243 



quired to build the bulky nest and lay five eggs. The bird had about 

 half incubated the first three sets, but the last two sets were perfectly 

 fresh. Thus, in about 76 days the birds had built five bulky nests and 

 laid 25 eggs. 



Arthur T. Wayne (1910) had an experience that almost equaled the 

 above record. At Mount Pleasant, S. C, he took three sets of white 

 eggs from a single pair of birds in one season, on March 30, April 12, 

 and May 6; this bird laid another set late in May, and these were 

 allowed to hatch. The interval between March 30 and April 12 was 

 a short time in which to build a nest and lay four eggs. 



The measurements of 50 eggs in the United States National Mus3um 

 average 20.7 by 16.3 millimeters; the eggs showing the four extremes 

 measure 22.9 by 15.8, 20.3 by 17.8, 17.8 by 16.0, and 22.4 by 15.2 

 millimeters. 



Incubation. — The period of incubation is generally conceded to be 

 about 12 days, though in some cases it may be a few days longer. 

 The young birds remain in the nest 15 to 18 days, according to various 

 observers, but probably the former figure is near the average. Both 

 of these periods are evidently more or less varipble according to 

 circumstances. Mr. Smith (1937) noted that incubation "usually 

 began with the completion of the clutch, but one instance was recorded 

 where it began with the laying of the fourth egg in a complement of 

 six. Of nine successful incubations of the two broods, the period 

 consumed 14 days in four instances, 15 days in three, and 13 and 16 

 days respectively, in two instances." 



Incubation is performed mainly by the female, but the male assists 

 in this duty to some extent. Mr. Smith (1937) says: 



In one instance the male was seen to take his mate's place upon the eggs three 

 times in the course of three hours. The male of No. 2 pair fed his mate at intervals 

 and maintained the semblance of a watch during her absence for food. Often 

 the male would fly to the box, or a near-by limb, uttering rapid call-notes, where- 

 upon the female would fly out and away for feeding. The male did not always 

 remain near until his mate's return, but not infrequently he left shortly after the 

 departure of the female. Absences, from meager observation, varied both in 

 frequency and in regard to length of time. One nest was under observation from 

 3.15 to 5.30 p. m., and schedule is as follows: Female left at 3.27, returned at 3.35; 

 remaining on the nest until 3.52. At 3.57 the male entered the nest and incubated 

 until 4.17, when he left. At 4.18 the female returned to stay until 4.25. The male 

 returned to the nest at 4.26, staying until 4.33 and returning again three minutes 

 later for another period on the nest, which lasted until 4.48. The female entered 

 the box at 4.59, and remained until 5.16. After four minutes absence, she came 

 back and was still on the nest at the close of the observation period, ten minutes 

 later. The male of No. 1 pair was not seen to take any part in the duties of incu- 

 bation, although considerable time was spent in observation, three and a half 

 hours being spent at one sitting. 



