matched than expected by chance; the mean degree
of frequency matching in the songs of established
teams was greater than that of artificial teams gener-
ated by randomly combining males' song contribu-
tions (J. Trainer and D. McDonald, unpubl. data). Be-
cause manakin partners are not genetically related
(McDonald and Potts 1994), the high degree of match-
ing in such teams may have come about by conver-
gence of the two males' song contributions as the part-
nerships were forming. Although a major benefit of a
cooperative partnership to the younger, nonrepro-
ducing males appears to be the eventual assumption
of alpha status (McDonald 1993a; see McDonald,
"Cooperation Between Male Long-tailed Manakins,"
pp. 204—205), a corollary benefit may be the oppor-
tunity to develop competent performance of well-
coordinated songs preferred by females. A next step
is to learn how males' songs become coordinated
by studying vocal development during partner
formation.Figure 6.16. (above) Rate of female visitation to Long-
tailed Manakin display arenas versus frequency matching
in the joint "toledo" song.
DO SUBORDINATE SPECIES HAVE AN ADVANTAGE? TESTING THE
POINTER HYPOTHESIS WITH TROPICAL HUMMINGBIRDS
Harry M, Tiebout IIIhe hummingbirds of Monteverde compete
for a limited standing crop of floral nectar
(Feinsinger 1976, 1978), yet the region sup-
ports a high diversity of nectarivores (Feinsinger
1977). A few species have specialized on subsets of
flowers by having unusually curved or long bills
which enable them to extract nectar from flowers
unavailable to the other "generalist" species. Com-
petition is potentially most intense among the nu-
merous generalist hummingbird species with short,
straight bills. To determine how these generalist spe-
cies are able to coexist, I focused on the costs and
benefits of different foraging modes (or community
roles; sensu Feinsinger and Colwell 1978).
One pair of hummingbird species has a marked
divergence in their foraging modes and hence a highly
asymmetrical competitive relationship. The mid-sized
Steely-vented Hummingbird is a behaviorally domi-
nant territorialist which monopolizes rich nectar
sources. In contrast, the small Canivet's (Fork-tailed)
Emerald is subordinate and forages as a low-reward
trapliner. This hummingbird is usually excluded from
the high-reward flowers defended by Steely-vented
Hummingbirds, and meets its energy demands by vis-
iting flowers that are too widely dispersed to be eco-
nomically defensible by territorialists (Feinsinger and
Colwell 1978). In this competitive diad, the behavior-
ally dominant foraging mode clearly benefits from
unrestricted access to the best feeding sites. The ben-
efits of being a species with a subordinate mode (if
any) are not as obvious. One hypothesis is Canivet's
Hummingbird, with its relatively long wings, has a
lower energetic cost of flight and can profit more from
visiting widely dispersed flowers than can the terri-
torialist (Feinsinger et aL 1979), although the exper-
imental evidence is mixed (Tiebout 1991a, 1992,
1993).
Another explanation for the coexistence of this pair
is that subordinate species may be better able to dis-
cover novel sources of food. This notion has been
dubbed the "pointer hypothesis" (Tiebout 1996), in
recognition of hunting dogs, whose superior ability
to locate prey is taken advantage of by their dominant
human companions. Black Vultures exploit Turkey
Vultures in a similar fashion. In the case of competing
hummingbirds, superior food-finding ability might216 BirdsT