ily Viscaceae (Sargent 1994). The four include three
species of Euphonia and one member of the closely
related genus Chlorophonia. About 25 species of eu-
phonias and four species of chlorophonias are found
in the Neotropics, where they are considered mistle-
toe specialists (Sutton 1951, Isler and Isler 1987), The
degree of mistletoe specialization varies considerably
within the two genera. For example, in Mont ever de,
the Blue-hooded Euphonia is a relatively strict mistle-
toe specialist, with over 80% of its diet consisting
of Viscaceae fruits, whereas the Yellow-throated Eu-
phonia has a more generalized fruit diet including figs
(Ficus spp.) and other sweet juicy fruits, with only
about 20% of its diet from Viscaceae. A more diverse
set of 10 bird species, including flycatchers, vireos,
and pigeons, consume the fruits of the Loranthaceae
and Eremolepidaceae in Monteverde (Sargent 1994).
The relatively strict specialist in this group of
birds is a small tyrannid flycatcher, the Paltry Tyran-
nulet. Approximately 80% of its diet is composed of
Loranthaceae-type mistletoe fruits.
One species of Loranthaceae, Gaiadendron punc-
tatum, is exceptional among mistletoes in that it
grows on the ground, where it parasitizes the roots of
neighboring plants (see Morales "Plants on Leaves,"
p. 80). Its seeds have very little viscin (not needed to
stick onto host twigs), and its fruits differ from those
of other species of Loranthaceae, They are juicy and
have a high sugar content. They are never eaten by
Paltry Tyrannulets or other typical Loranthaceae-
feeding bird species, They are eaten by chlorophonias,
which handle the fruits in the same way they handle
other sweet, juicy fruits such as figs. They squeeze the
fruits with their bills, ingesting the juice and some
pulp, discarding the skins and seeds.
Why is it that birds that readily consume fruits of
mistletoes in the family Viscaceae do not eat those
in the Loranthaceae (Gaiadendron excepted) and
vice versa? Different birds handle and digest fruits
in different ways. The euphonias and chlorophonia
squeeze Phoradendron fruits by shucking off the
tough skins, which separate from the pulp, and swal-
lowing the contents. Anywhere from 8 to 60 rnin later,
they defecate the seeds, often doing a quick wipe-and-
sidestep motion to detach them from their cloacas and
attach them to the perch. During the short trip through
the gut, the sugars in the pulp are rapidly absorbed.
Euphonias and chlorophonias have unusually short
and unrestricted digestive tracts; they completely lack
a gizzard and associated sphincters (Forbes 1880). The
euphonias' use of mistletoe fruits might be related to
their simplified digestive systems (Wetmore 1914).
Other birds may not be able to eat these fruits because
of their more complex digestive tracts; they may be
unable to handle and process these viscin-laden fruits
quickly enough to make them profitable. Alterna-
tively, processing in a gizzard may release toxins from
the relatively unprotected seeds. Research is needed
to clarify the processes that prevent other birds from
eating these fruits.
Loranthaceae-eating birds, such as the Paltry
Tyrannulet, swallow Antidaphne, Struthanthus, and
Oryctanthus fruits whole. Five minutes after swallow-
ing a fruit, the bird regurgitates the seed and, with a
quick dip of the head, attaches the seed to its perch.
The pulp plus skin are separated from the seed in the
bird's gizzard. While the seed (with viscin) is sent
quickly back out the mouth, the pulp and skin spend
a longer time going through the digestive tract where
the fats are digested slowly. It takes skill to dispose of
the seeds correctly; I have watched young birds struggle
to rid themselves of seeds and stringy viscin strands.
The interaction between mistletoes and their avian
seed dispersers is an unusually intimate and special-
ized one in Monteverde as in other parts of the world
(Ali 1931, Docters van Leeuwen 1954, Godschalk 1983,
Reid 1986, Snow and Snow 1988). Monteverde pre-
sents an exciting system to study specific coevolution-
ary adaptations on the part of plants and avian frugi-
vores that disperse their seeds (Reid 1991).SEED DISPERSAL AND SEEDLING RECRUITMENT IN
A LADRACEOUS TREE SPECIES
J. P. Gibsonany tropical trees produce fruits that are
eaten by foraging birds. Depending on the
species, a bird tends either to remain in that
tree or to fly to another perch to digest the fruit. The
seeds, which birds regurgitate or defecate, are unharmed
so birds serve as seed dispersers. Aspects of this rela-tionship in the Lauraceae (avocado family) have been
studied extensively in Monteverde (Wheelwright et al.
1984, Wheelwright, 1988b, 1991, 1993, Wheelwright
and Bruneau, 1992). However, little information exists
on patterns of seed dispersal and the resulting patterns
of seedling recruitment in lauraceous trees.289 Plant-Animal InteractionsM