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Flocks of insectivorous birds in the understory of lowland wet forest in
Surinam and eastern Ecuador regularly included two species of antshrikes
in the genus Thamnomanes (ardesiacus and caesius )
and four species of antwrens in the genus Myrmotherula (in Surinam,
axillaris, menetriesii, longipennis and gutturalis; in
Ecuador, the first two plus hauxwelli and ornata )
(Formicariidae), as well as a number of other species particularly in the
families Furnariidae and Dendrocolaptidae.
Each flock included only a pair or occasionally a small group of each
species. Although the individuals in a flock often spread over an area 30
m in diameter, the Myrmortherula, Thamnomanes and several
other species moved cohesively through the forest. Each flock persisted
throughout a day and, over periods of a least a week, recurred within the
same largely exclusive range in the forest.
The two Thamnomanes species had distinctive vocalizations that
provided the primary signals for flock cohesion and alarm calls.
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Although most of the species of Myrmotherula had loud
vocalizations, these were uttered too infrequently to contribute to flock
cohesion and played no role in reactions to predators.
In Central America, on the other hand, where Thamnomanes does not
occur, M. fulviventris produces loud vocalizations, that resemble
those of Thamnomanes, during disturbances to a flock.
The species of Myrmotherula in each locality segregated
ecologically in two dimensions, height of foraging above ground and
preference for foraging in live or dead foliage. A shift in the foraging
height of axillaris depending on the presence of absence of
longipennis in the same flock suggested that competition for food
has favored ecological segregation of foraging height by these species.
Thamnomanes-Myrmotherula flocks attain an unusual degree of
cohesiveness and integration of foraging specialties in comparison with
other mixed aggregations of animals.
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