1976] 
A /cock — Populations of Centris pallida 
127 
the interval between marking and last sighting for those males 
captured while hovering was only 2.62 days. Included in these 
results are four males that were hovering when first captured but 
which later switched to become patrollers. (A small minority of 
all marked males, N = 20, exhibited the capacity to patrol and 
to hover, frequently doing both on the same day, patrolling in 
the early morning and later hovering.) Thus unless hovering 
males experienced a much higher rate of mortality than patrollers 
(which seems unlikely), one must conclude that these males were 
more likely to move long distances from day to day. This is not to 
say hoverers completely failed to exhibit site tenacity. Hovering 
males were continuously present at their aerial stations for several 
hours, while patrolling males often appeared only at irregular 
intervals in their searching ranges. Moreover some hoverers did 
return to a general area, and often a specific site, over a period 
of days. One male appeared at a specific location among the 
branches of a palo verde for four days running; another main- 
tained a low aerial station by a creosote bush for five consecutive 
mornings. These males were, however, definitely the exception 
to the rule. 
Discussion 
Males of the bee C. pallida are unusual in exhibiting two very 
different techniques of mate-location (patrolling and hovering), 
although one or the other of the two patterns is associated with 
a great many species of Hymenoptera (e.g. Evans, 1966; Frison, 
1917; Linsley, 1965; Rozen, 1958; Shinn, 1967) including various 
other species of Centris (Frankie & Baker, 1974; Raw, 1975). 
The coexistence of these two divergent strategies in a single species 
appears related to the great variation in size of males of C. pallida. 
Size influences the ability of males to claim digging sites in com- 
petition with other males (Alcock et al., in press). Thus large 
males patrol and dig, small males hover. As I have shown in this 
paper, these two activities are linked with different tendencies 
to defend space against conspecific intruders and to return to a 
particular location. These differences are discussed below. 
The pattern of short-term (several hours) residence at an aerial 
station or perch associated with apparent defense of the area (or 
at least non-overlapping distributions of males) is not unique to 
C. pallida. These traits occur in such unrelated solitary Hymen- 
