BREWER'S BLACKBIRD 307 



by the female, and not until Phase 2 commences. Bendire (1895) 

 says that "both sexes assist" in nest construction, but at the river- 

 mouth colony the male's activity with nesting material is almost 

 entirely functionless as far as the actual construction is concerned. 



In Phase 1 there is considerable aggressive behavior and even 

 fighting for the possession of nest sites, even though actual nest con- 

 struction is still to come. This fighting is largely between females. 

 A fight between females at a nest site usually brings a response from 

 their mates, who alight nearby but do not always act belligerently at 

 first. The approach of the males may cause one or both females to 

 assume the generalized display, and the males then tend to guard 

 their mates. 



Pairs acting as a team will defend a nest site, but in such cases it 

 is more often the female rather than the male who initiates the attack. 

 Males will, however, defend the nest site without the female being 

 present. 



Although Phase 1 may start as early as the third week of January, 

 actual nest construction, copulation, and egg-laying for the first brood 

 (Phase 2) does not commence earlier than April, usually not until the 

 second or third week (the earliest observed copulation: April 6, 1945). 

 Thus Phase 1 of the first cycle may be stretched out for as long as 12 

 weeks. Phase 1 of subsequent cycles is exceedingly brief and can 

 possibly be considered absent. 



Males were both monogamous and polygynous; an individual might 

 be polygynous one year and monogamous the next. The number of 

 breeding males in the colony varied from 13 to 31; females from 14 

 to 36. Polygyny varied from one polygynous male in 1943, when 

 the population was 13 males to 14 females, to 12 polygynous males 

 in 1947 when the population was 18 males to 36 females. The number 

 of mates per polygynous male was generally two; but in 1946 six males 

 has two females each and one had three. In 1947 seven males had 

 two, four had three, and one had four females. 



When polygyny occurs it usually comes about in the following 

 fashion: When the female is incubating (Phase 3) the male does 

 not guard her constantly as he did formerly; and he takes no part in 

 incubation. Consequently he pays more attention to other females. 

 If an unmated female, or, more rarely, a female whose mate does not 

 seem to be aggressive enough to guard her, is present, the unoccupied 

 male may take this female polygynously as a "secondary" female. 

 More than one secondary female may be acquired successively in this 

 fashion. 



In most cases the male guards the secondary female as assiduously 

 as he had his primary female, and I did not become aware of the new 

 attachment until the secondary female had already started nest con- 



