ECOLOGY AND SEXUAL DIMORPHISM OF EMERALD TOUCAN ETS
Cecilia M. Riley & Kimberly G. Smithmerald Toucanets are common and conspicu-
ous birds in Monteverde with large, banana-
shaped bills. They are medium-sized (31-36 cm
long) members of the family Ramphastidae, a group
that includes toucans and aracaris. Ramphastids are
primarily frugivorous, although they may eat bird eggs,
nestlings, and small vertebrates (Skutch 1944, Rernsen
et al. 1993). Their relatively large size and gape allow
consumption of a wide range of fruit sizes (Wheel-
wright 1985, Moermond and Denslow 1985; Fig. 6.6),
Although males and females are identical in color,
most species of ramphastids exhibit sexual dimor-
phism in bill size, with males having bigger bills. A
morphological analysis of museum specimens from
Costa Rica demonstrated male Emerald Toucanets were
larger (in wing and tail length) than females and that
male bills were longer and more decurved than those
of females (Riley and Smith 1992). The function of
larger bills in males and reasons for sexual dimorphism
have been debated (Moermond and Denslow 1985,
Skutch 1985a). We studied their nesting and foraging
behaviors in Monteverde in wet and dry seasons of
1985 to investigate explanations for the dimorphism.
Breeding BiologyCourtship feeding was first observed in mid-March and
was followed within a few days by nest excavation by
both adults. Thirteen nests, all in pastures or forest
edges in old snags, were monitored daily for 1-2 hr,
alternating mornings and afternoons (Riley 1986). Nest
height varied from 3 to 7.5 m with oval hole openings
averaging 7.3 cm wide x 7.7 cm long x 57 cm deep.
Females conducted 58% of the excavation and then
laid three to five eggs directly on the cavity floor. Both
sexes participated in incubation, but females incubated
more (66%). The duration of incubation sessions by
females and males averaged 11 and 15 min, respec-
tively. Nestling diets consisted primarily of fruits (79%)
from 16 species; the remainder of the diet was arthro-
pods. Food items were brought to nestlings at approxi-
mately equal rates by adults of each sex; males deliv-
ered insects more frequently than did females. Females
performed 78% of the nest sanitation, carrying fecal
sacs and regurgitated seeds away from nest sites. Over-
all, nesting success (at least one chick fledged) was
46%. Observed nest losses were the result of tree falls
and predation (Riley 1986). In southern Costa Rica,
similar nestling diets and parental division of labor
occurred, and nestlings fledged at age 45 days (Skutch
1944, 1967). Because both males and females partici-
pated in all aspects of nest construction and brood rear-
ing, sexual bill size differences are apparently not re-
lated to roles during the nesting cycle.Foraging BehaviorDiets of Emerald Toucanets consist of more than 100
species of fruits and flowers (Wheelwright et al. 1984,
Riley and Smith 1986; Fig. 6.6). Feeding primarily
from twigs on outer branches in mid-canopy, tou-
canets of both sexes eat the same fruit species, pluck-
ing fruits singly and tossing them whole down their
throats with a jerk of the head. Although the fruits that
were eaten and some foraging behaviors differed be-
tween wet and dry seasons, only one aspect of for-
aging behavior differed between sexes within a sea-
son (Riley and Smith 1992): during the dry season
(December-March), females spent significantly more
time in a single foraging position than did males. We
concluded that there were no differences between
male and female foraging behaviors or diets that could
explain differences in their bill size.
Selective pressures that could lead to sexual dimor-
phism of bills include different roles in care of off-
spring, different diets between sexes, and improved
mating success (Partridge and Halliday 1984, Jehl and
Murray 1986). The first two explanations do not ap-
ply to the situation in Emerald Toucanets (Riley and
Smith 1992). Because fruits are a patchy and clumped
resource, it is unclear how the sexes could partition
fruit resources by foraging in different microhabitats,
so sexual differences in foraging behaviors and diets
should be rare in frugivorous birds (Wheelwright
1986a, 1991, Riley and Smith 1992).
The wide occurrence of sexual dimorphism in bill
size among ramphastids remains unresolved. The
observed sexual dimorphism in bill size is likely the
result of sexual selection or other behavioral interac-
tions. Toucanets and toucans often engage in "duel-
ing," "fencing," or "billing" behaviors, in which two
or more individuals interact using their bills (Riley
and Smith 1992). Future investigations should focus
on intra- and intersexual behavioral interactions.214 BirdsE