Figure 6.4. Black-faced Solitaire at its nest on the trunk of a tree in the Monteverde Cloud Forest
Preserve. Photograph by Kathy Winnett-Murray.watching in the nonbreeding season (July-February).
One can spend long stretches of time seeing few or
no birds and then suddenly be surrounded by a host
of flitting shapes and varied calls. These flocks fall
into three categories: (1) army ant-following flocks,
(2) frugivorous flocks, and (3) insectivorous flocks that
do not follow army ants. The first two flock types are
drawn by a common resource, either abundant insects
flushing in front of a swarm of army ants (see Sec.
4.6.4) or fruits. The insectivorous flocks not associ-
ated with army ants form around vocal groups of birds
and move through the forest in search of dispersed
food.
In Monteverde, army ant-following flocks are gen-
erally not as large as in the lowlands, either because
ant swarms are smaller or because they are rarer and
less dependable food sources. In Zones 1 and 2, North-
ern Barred-Woodcreepers and Ruddy Woodcreepers
(occasionally joined by Collared Forest-Falcons) are
the principal army ant followers (Mays 1985, B. Young,
unpubl. data). In Zones 3 and 4, army ants are rare
and swarms are usually unaccompanied by birds. In
Zones 5 and 6, Immaculate, Bicolored, and Ocellated
Antbirds, and less frequently Plain-brown Wood-
creepers, are the major components of the army ant
flocks (B. Young, unpubl. data).
Flocks of frugivorous birds can form around any
fruiting tree in any part of Monteverde. Some frugi-
vores (e.g., Silver-throated, Spangle-cheeked, and
Common Bush-Tanagers) join or form the nucleus of
flocks of mostly insectivorous birds in Zones 3-5. In
Zones 5 and 6, large flocks of tanagers (especially the
genus Tangara), honey creepers, and other frugivores
move in a coordinated fashion through the forest.
These frugivorous flocks are restricted to mideleva-
tion forests on the Caribbean slope and are virtually
unstudied.
The dynamics of flock formation and behavior are
better understood in the insectivorous flocks that
occur throughout Monteverde. The "nuclear species,"
around which other species form flocks, are Golden-
crowned Warblers and Lesser Greenlets (Zones 1 and
2), Common Bush-Tanagers and Three-striped War-
blers (Zones 3-5), and Scarlet-rumped Caciques (Zone
6) (Powell 1979, Tramer and Kemp 1980, Valburg
1992c). In Zone 3, flocks peak in size in the morning
and late afternoon and are much larger (by a factor of
3-4) during the nonbreeding season (Powell 1979).
Flocks that form without a nuclear species are un-
stable and disintegrate rapidly. For many of these
species, individuals of a given species drop out when
the flock reaches their territory boundary and are re-
placed by individuals from the neighboring territory.
Average flock speed is 5.4 (± 2.4) m/min; larger flocks
move more slowly than smaller flocks. Physical and
social factors influence flock dynamics; increasing
wind intensity and precipitation decrease flock size
(Powell 1979, Shopland 1985). In Zone 4, flocks con-185 Birds