BLACK-TAILED GNATCATCHER 377 



and at a height of about 3 feet in a small laurel sumac. The four 

 eggs were hatched on or about May 21 and the young left on June 5. 

 A third nest with four well-grown young, undoubtedly belonging to 

 the same pair, was encountered on July 12, about 100 yards from the 

 second and at the same height. 



The duty of incubation is divided between the two parents, the 

 eggs seldom being left uncovered even for a moment; the anxiety of 

 one of the males to resume his duties would cause him sometimes to 

 almost shove his mate from the nest. While sitting, this same indi- 

 vidual maintained a vigilant watch, frequently peering over the edge 

 and closely scrutinizing the ground beneath. In another case it was 

 found that the male occupied the nest more than half the time during 

 the day, being relieved by his mate at intervals of approximately one 

 hour. Soon after sunset, however, the female would take her place 

 there and presently, with head tucked in, would be settled for the 

 night. 



The male, or less frequently the female, would fiercely attack other 

 birds, including gnatcatchers, which strayed within perhaps 50 feet 

 of the nest, darting at them repeatedly with rapidly snapping bill. 

 A flock of bushtits offered greater difficulties, as each one attacked 

 merely dodged back into the foliage, to emerge serenely as soon as 

 the assault was transferred to another member of the flock. 



Eggs.— The normal set of eggs consists of four. Any larger number 

 is at least very rare, but some nests contain only three. It appears 

 that the laying of sets of three may be an individual tendency, as the 

 female observed in 1921 laid two sets of three, while that of 1927 laid 

 three sets of four. Mr. Hanna (1934) gives the average weight of 38 

 eggs as 0.99 gram, with a maximum of 1.11 grams and a minimum of 

 0.82 gram. Comparing the eggs of the black-tailed and western gnat- 

 catchers, Harry H. Dunn (1906) says: "Several writers, and even as 

 good an authority as Davie, claim that there is a difference in the eggs 

 of the two species, but from a long string of sets which have come under 

 my observation I am unable to agree to this. To my mind they are 

 absolutely indistinguishable, especially a few days after being blown, 

 when the eggs of both species fade considerably, becoming a pale, 

 washed-out blue, instead of the deep green they possess when freshly 

 laid." 



The measurements of 36 eggs average 14.4 by 11.5; the eggs showing 

 the four extremes measure 15.1 by 11.5, 14.5 by 12.0, 12.9 by 10.8, and 

 13.0 by 10.6 millimeters. 



Young. — The incubation period, which I ascertained in only one 

 case, proved to be rather longer than expected, considering the small 

 size of the eggs, being 14 days from the laying of the last egg. Al- 

 though the eggs were laid on successive days and incubation was ap- 



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