228 
Psyche 
[Vol. 89 
Osmia californica 
Almost 500 nests were recovered in the four-site years of sampling 
(Table 4). Nest utilization was unchanged at Mendon during both 
years, but a substantial increase was recorded at Faust from 1979 to 
1980. 
The proportion of one-year cells declined significantly at both 
sites from 1979 to 1980 (Fig. lb. Table 4; Faust X 2 = 221.5, P < 
0.001; Mendon X 2 = 30.0, P < 0.001). In agreement with results for 
O. montana, the proportion of one-year forms produced at Faust 
was significantly higher than that produced at Mendon during both 
years (1979, X 2 = 92.5, ?< 0.001; 1980, X 2 = 5.7, P < 0.025). 
As with O. montana, there was an association between sex and 
number of years to complete development (Table 4). For all site- 
years (except Faust 1979 for which insufficient numbers of two-year 
forms were available for statistical tests) there was a higher propor- 
tion of males among one-year forms than among two-year forms 
when cells from mixed nests only were considered (Faust 1980, X 2 = 
4.3, P < 0.05; Mendon 1979, X 2 = 20.2, P< 0.001; 1980, X 2 = 10.2, 
P < 0.005). When all cells were considered, the sex ratio of one-year 
forms was always higher than that of two-year forms; but only one 
of three comparisons was significant (Faust 1980, X 2 = 2.4, P > 
0.10; Mendon 1979, X 2 = 1 1 .8, P > 0.001 ; 1980, X 2 = 2.0, P > 0. 10). 
Between-year differences in the proportion of males and females 
among one- and two-year forms at each site were less evident than 
for O. montana (Table 4). At Mendon the combined sex ratio of 
offspring declined significantly from 4.2 (1979) to 1.8 (1980) (X 2 = 
30.0, P< 0.001) but no such change was evident at Faust (X 2 = 0.0, 
P > 0.90). The decline in the sex ratio at Mendon was due to a 
significantly greater proportion of female progeny produced in 1980 
for both one-year (X 2 = 15.5, P < 0.001) and two-year forms (X 2 = 
7.5, P > 0.01). These results are the reverse of those found for O. 
montana. 
The distribution of one- and two-year forms within mixed nests 
were similar to results obtained for O. montana (Tables 2, 4). Thus, 
nests having double switches were most numerous and transitions 
from one- (inner cells) to two-year forms (outer cells) were inter- 
mediate. 
As with O. montana, a substantial proportion of two-year forms 
were ‘’doomed” (range 37.0-56.5%, Table 3) because cells contain- 
ing one-year forms were often constructed lower in the nest. The 
