I have documented four major direct benefits to the
beta male from his cooperation with an alpha male.
First, upon the disappearance of an alpha male, it has
always been the beta who has succeeded to the alpha
role (n = 11). Second, although very rare, beta males
do sometimes mate (4 of 263 copulations). Third, fe-
males tend to return to perches at which they visited
or mated in prior seasons. Of 27 banded females seen
at perches in two or more seasons, 16 returned to the
site at which they had previously been sighted, and
10 mated with the replacement alpha following a turn-
over. Finally, the success of alpha males was highly
correlated with that of their predecessors. While they
were betas, the current alpha males played a major
role in creating the success that they then "inherited."
By expending considerable effort in courtship display
as a beta, a male stands to gain considerable, albeit
long-delayed, benefits. Because the mean age of mat-
ing for males is 10.1 years (McDonald 1993b), young
(<8 years) males have very few options for success
anyway, so the deferment of reproduction entailed by
being beta seems to be the best strategy. An additional
element in the payoff is the age differential between
male and female reproduction. Females probably
begin nesting by age one, and certainly by age two
(McDonald 1993b). Thus, a beta male that helps at-
tract a young female stands to mate with her through-
out her prime.
Although the genetic evidence allows rejection
of the kin selection hypotheses, and the behavioral
data indicate the nature of the direct benefits to beta
males, many questions remain. Why, for example,
does this form of male-male cooperation occur in
this species and its four congeners elsewhere in theneotropics but nowhere else in the animal king-
dom? The answer almost certainly lies in the patterns
of female choice of mates. The lack of opportunities
for mating success by young males arises because
over the course of several seasons, virtually all the
females in a local population choose one or a few
males as mates, A vital component in the difference
between female mate choice in this genus and that
in all other lekking birds may be a difference in cog-
nitive ability. The hippocampus is a part of the brain
that plays a major role in processing spatial infor-
mation (Krebs et al, 1989). Long-tailed Manakins
have a large hippocampal ratio (relative to the rest
of the brain), comparable to that of birds that cache
their food, such as Clark's Nutcracker (Nucifraga
Columbian a), a jaylike bird of western North America
that has been shown to perform amazing feats of
spatial memory in laboratory tests (Kamil and Balda
1985). Although Long-tailed Manakins do not cache
their food, they may need to remember the time and
place at which fruits emerge over very large areas,
in order to survive periods of food scarcity. Given
a brain capable of acute spatial memory, it may be
easy for females to "map" the locations of the best
among a large set of pairs of displaying males. Other
manakins in wetter habitats may have more easily
located food sources and lack the highly developed
hippocampus. Females of those species maybe rela-
tively incapable of the acute discrimination, spatial
mapping, and long-term memory exhibited by Long-
tailed Manakins. With more options for present re-
productive success, males of other manakin species
may not need to cooperate with other males for fu-
ture opportunities.WHY JOIN MIXED-SPECIES FLOCKS?: A FRUGIVORE'S PERSPECTIVE
Lisa K. Valburgost mixed-species flocks are made up of
insect-eaters, with a marked lack of partici-
pation from birds that use nectar or fruit
resources (Powell 1977b). In Monteverde, however,
one of the most prominent species in mixed-species
flocks is the Common Bush-Tanager (Chlorospingus
opthalmicus), a bird known to consume a wide vari-
ety of fruits (Wheelwright et al. 1984). Previous work
on the flocks of Monteverde demonstrated that Com-
mon Bush-Tanagers maintain 0.5-ha home ranges and
participate in mixed-species flocks only when they
pass through their territories (Powell 1977b). I wished
to determine how social group composition and size
affected the proportions of fruits and arthropods con-
sumed by Common Bush-Tanagers. I studied this spe-
cies for four reasons: (1) it is an important member of
mixed-species flocks; (2) it feeds alone, in single-
species groups, and in mixed-species flocks; (3) it con-
sumes a wide variety of fruits, in contrast to the highly
insectivorous diet of most mixed-species flock partici-
pants; and (4) these birds are abundant, noisy, and
relatively easy to follow on their home ranges.
Differences in behavior between insectivorous and
frugivorous birds in flocks have been linked to dif-205 BirdsM