NTJTTALL'S WHITE-CROWNED SPARROW 1303 



must adjust to the capricious winter and spring climate of California. 

 The migratory races that nest in the far north where the country 

 becomes suitable for nesting close to the same date each year do not 

 show such wide annual fluctuations in the date of first egg laid. 



The number of eggs per clutch varies from 2 to 5. The average 

 for 215 sets is 3.27 eggs per completed clutch. The most frequent 

 clutch size is 3 eggs. The percentages are as follows: 2 eggs per 

 clutch 4.2 percent; 3 eggs per clutch 65.6 percent, 4 eggs per clutch 

 29.8 percent, 5 eggs (one clutch only) 0.4 percent. The average number 

 of eggs per clutch increases from March through May, after which 

 it declines: for March the average for 15 clutches was 3.00, for April 

 3.22 for 122 clutches, for May 3.48 for 50 clutches. In June the 

 average of 21 clutches declined to 3.24, aad in July, the average for 

 7 clutches was only 3.14. 



The average for 24 first clutches was 3.04 eggs, whereas 10 second 

 clutches averaged 3.60, 5 third clutches 3.00, aad 3 fourth clutches 

 also 3.00. I have records of all the clutches laid by one female for 

 2 years. In 1934 when she bred for the first time she laid four clutches 

 of 3, 2, 2, and 3 eggs respectively; in 1935 she laid four clutches of 

 2, 4, 4, and 3 eggs. 



Incubation. — Incubation is by the female alone. She sits on the 

 eggs with her back flush with or below the nest rim, her tail raised 

 at a sharp angle and her chin resting on the edge. She incubates 

 for periods averaging about 20 minutes each, then leaves the nest 

 to forage, uttering the same strident "eep" note she used while building 

 the nest. Hopping about on the ground hurriedly, she covers a large 

 area in a few miautes: This rapidity of movement is so characteristic 

 that I have often identified an incubating female by this behavior. 

 When the female returns to the nest she again utters a series of "eeps." 



At the start of incubation the male achieves the highest develop- 

 ment of territorial behavior. He guards his area by loud, almost 

 continuous singing and patrol. Often, but not invariably, he appears 

 to call the female off the nest. At such times he approaches the 

 nest and sings, whereupon the female leaves and the pair forage 

 together. The male therefore does not guard the nest in the absence 

 of his mate, but he may be involved to some degree in her return. 

 Often he flies to a perch near the nest and sings, whereupon the 

 female gradually moves nearer and nearer and finally resumes in- 

 cubation. At other times the female is the first to stop foraging 

 and to fly toward the nest, followed by hor mate. 



In Nuttall's sparrow the beginning of incubation in relation to 

 egg-laying varies; 5 females began on the day before the last egg was 

 laid, 5 on the day the last egg was laid, and 1 the day after. Of 5 

 females with sets of three eggs, 3 started on the day the second was 



