314 U. S. NATIONAL MUSEUM BULLETIN 211 



Because of the inaccessibility of most nests (high up and at the end 

 of limber branches) that entered into my study of the species, it was 

 possible to look into only a small number of them. Of these it was 

 feasible to make daily inspection at only 5, of which 3 sets had 4 eggs, 

 and 2 had 5 eggs each. One set of 4 eggs hatched in 12 days and all 

 the others in 13 days, reckoning the incubation period from the day 

 the last egg was laid until all were hatched. No thorough study of 

 incubation rhythms was made, but watching females at inspectable 

 nests revealed that they spend time on the eggs before the full set is 

 laid. This correlates with the fact that hatching of the young (except 

 in the set of 4, in which all hatched on the 12th day) was spread over as 

 much of 3 days and indicates that incubation may start before the 

 clutch is complete. The tangible facts ascertained from the few 

 accessible nests coincided with those inferred from parental behavior 

 at other nests. 



Although the male takes no part in incubation and may even form 

 an attachment with another female at this time, his attention does 

 not in all cases leave the incubating female entirely. Monogamous 

 males may spend much time on the guard perches near the nest. 

 Polygynous males have been known to guard at two nests if the 

 incubation periods overlap ; but if one female is in Phase 1 or 2 while 

 the other is incubating the male gives much more attention to the 

 former (if the nest of either incubating female is destroyed she very 

 quickly reverts to Phase 2). In a few cases the male has been seen to 

 feed the incubating female on the nest. 



Young. — Data on the length of the nestling period is extremely 

 scanty in the literature. In my study I was able to ascertain definitely 

 the nestling period in only three instances. In each case it was 13 

 days, calculating the period from the day the last egg had hatched 

 until all the young had left the nest under natural conditions. As in 

 the determination of the incubation period the information obtained 

 at these three nests corroborated the observations at many inac- 

 cessible nests. The male Brewer's blackbird at the river-mouth 

 colony was found to assist the female in feeding the young, both in 

 and out of the nest, in 72 out of 99 monogamous nestings and 76 out 

 of 109 polygynous ones. In the remaining cases in each category 

 the male disappeared, or the eggs did not hatch, or the nestlings died 

 before male attention could be determined, or there were not sufficient 

 observations to prove or disprove male attention. 



Although I was unable to carry out extended periods of watching 

 at any one nest, periods varying from 1 to 3 hours at various nests 

 throughout the fledging period indicate that although the female 

 usually exceeds the male in the number of trips per hour with food, 

 the male sometimes equals and even exceeds the female in such trips, 



