BLACK-HEADED GROSBEAK 61 



construction," the birds "heaped up a mass of green willow leaves, 

 plucking for the purpose the terminal twigs of the youngest trees, 

 and wedging them to a height of nine inches in a convenient crotch. 

 In the top of this mass, kept cool by reason of evaporating moisture, 

 they set the conventional root-line cup." 



J. Stuart Kowley writes to me: "1 have found many dozens of 

 nests of this bird throughout southern California. Along the west- 

 ern slopes of the Sierra Nevadas in Tulare County, this grosbeak is 

 an abundant nester. Most of the nests I have found have been in 

 raanzanita bushes or in willows. The nests are so thinly made on 

 the bottoms that frequently the eggs can be seen from the ground 

 when looking up through the bottom of the nest." 



Weston (1947) says that such thinness was not observed in any 

 of the eight nests that he studied in Strawberry Canyon. 



Eggs. — The usual set of the black-headed grosbeak consists of 

 three or four eggs. Weston (1947) records the numbers of eggs in 

 192 sets. There were 18 sets of tvv^o, 96 sets of three, 75 sets of 

 four, and only 3 sets of five eggs. 



The measm-ements of 50 eggs average 24.7 by 17.7 millimeters; 

 the eggs showing the four extremes measure 38. 2 by 17.8, 24.4 by 

 19.6, 21.8 by 17.8, and 22.9 by 15.8 millimeters. 



Incubation. — This is shared by both sexes alternately during the 

 day and is done by the female alone at night, according to Weston 

 (1947). He says that incubation starts with the laying of the next 

 to the last egg. "On an average day the eggs are incubated about 

 99 percent of the time, about 40 percent of the time by the male 

 and 60 percent by the female. The average length of each incuba- 

 tion period of the male is close to 20 minutes, of the female 25 to 

 30 minutes." Both sexes occasionally sing at u-regular intervals 

 while on the nest, and are thus helpful in locating the nests. "Al- 

 though the male sings while alone at the nest, the female usually 

 sings only while the male is in the near vicinity. * * * The eggs 

 begin hatching on the twelfth day of incubation. In each of three 

 nests containing three eggs each, the last egg hatched twenty-four 

 hoiu-s after the others." In some other cases, the eggs hatched 

 "within a few hours of one another." 



S. F. Rathbun wi-ites in his notes for June 3, 1893: "This morn- 

 ing I found the nest of a black-headed grosbeak. The nest was 

 built in the fork of a willow sapling at a height of some 10 feet, and 

 the male bird could be plainly seen on the nest. I shook the sap- 

 ling lightly, expecting to see the bird fly off, but such proved not 

 the case, and neither did it occur when it again was shaken, so I 

 took my knife and carefully cut the sapling, lowering it to the level 

 of my face, not more than a foot away; and only when my free hand 



