Fruit Consumption Promotes Sociality in Purple- throated
Fruitcrows
The Purple- throated Fruitcrow (Querula purpurata; plate 10- 8)
feeds on both insects and fruits, but David Snow showed that its
frugivorous habits may have been instrumental in the evolution of
its intricate social behavior. Not a true crow, this species is a member
of the highly frugivorous cotinga family (chapter 15). Purple-
throated Fruitcrows live in small, closely related communal groups
of three or four individuals that roam the forest together in search
of preferred species of fruiting trees. Within the social group there
is virtually no aggression, and all members of the group appear to
cooperate in feeding the nestling bird (they have clutches of only
one). Given the degree of relationship among the group, this form
of cooperative nesting behavior has the potential to benefit even
those individuals that are not parents of the nestling bird. The nest
is in the open and is vigorously defended by the entire group.
The group organization of Purple- throated Fruitcrows likely assists them in both locating and defending trees laden
with fruit. Because the fruit that the birds locate is normally sufficient for all of them, there is little or no competition
among the birds, a factor likely important in the initial evolution of their social organization.Plate 10- 8. This male Purple- throated Fruitcrow
is displaying to another member of his group.
Photo by Sean Williams.The Cost of Fig Dispersal
It takes lots of patient and persistent fieldwork to
unravel how nature works. Pedro Jordano measured
fruit volume and seed dispersal from a single fig tree in
a lowland deciduous forest in Costa Rica. The estimated
total crop was approximately 100,000 figs, all of which,
because they were produced synchronously, were
taken within five days either directly from the plant
or after they had fallen to the ground. During the first
three days alone, 95,000 were consumed. Birds were
the principal feeders, eating 20,828 figs each day, about
65% of the daily loss. Mammals, most of which ate fruits
that had fallen to the ground, were the other source of
loss. Parrots, which are fundamentally seed predators,
accounted for just over 50% of the daily total of figs.
The most efficient seed dispersers (those that flew away
from the tree and therefore dropped seeds outside of
the seed shadow) were orioles, tanagers, trogons (plate
10- 7), and certain flycatchers. However, these birds took
only about 4,600 figs per day. Approximately 4,420,000
seeds were destroyed each day, mostly from predation
by wasps and other invertebrates. Parrots were
estimated to account for 36% of the seed loss. Only 6.3%
of the seeds taken from the tree each day were actually
dispersed and undamaged— a good indication of the
high cost of seed dispersal.Plate 10- 7. The Blue- crowned Trogon (Trogon curucui) is
considered a good seed disperser. Photo by Sean Williams.158 chapter 10 tropical intimacy: mutualism and coevolution