Plants and Hummingbirds: A
Coevolution
Hummingbirds have evolved mutualistic relationships
with plants, feeding on nectar but facilitating cross-
pollination. The impressive diversity of sizes and bill
shapes exhibited by hummingbirds (plates 10- 29– 30)
is directly attributed to the diversity of flower shapes
and patterns that offer nectar, a clear example of
adaptive radiation (chapter 8). Hermits feed heavily
on the nectar of heliconia flowers. Many heliconias
produce relatively constant amounts of nectar per
flower but one heliconia studied by Peter Feinsinger in
Costa Rica, Heliconia psittacorum, exhibits a bonanza-
blank pattern of nectar production. Some of its flowers
contain abundant nectar (bonanzas), some essentially
none (blanks). Many other tropical plants, especially
those in open successional areas, also are bonanza-
blank flowerers. Hermits must visit many flowers in
order to encounter one with high nectar content, thus
the bonanza- blank pattern presumably aids Heliconia
psittacorum in accomplishing cross- pollination.
In a study of 10 successional plant species and 14
hummingbird species at Monteverde Cloud Forest in
Costa Rica, Feinsinger documented that flowering was
staggered among plant species, resulting in a constant
nectar supply to hummingbirds. In five plant species
that were closely measured for nectar volume, the
bonanza pattern was evident. Feinsinger speculated
that plants may conserve energy by producing large
numbers of “cheap” nectarless flowers and merely a few
“expensive” bonanza flowers, forcing hummingbirds
to visit many flowers to find satiation. With the birds
visiting many flowers, cross- pollination is promoted.
Hummingbird species display a range of foraging
patterns. Trapliners go from flower to flower, in a
sequence, the way a hunter might visit a series of traps
(plate 10- 31). Some species are high- reward trapliners,
which visit but do not defend nectar- rich flowers. The
territorialists defend dense clumps of small flowers.
Hummingbirds known as low- reward trapliners forage
among a variety of dispersed or nectar- poor flowers.
Other species are territory parasites; they come in
two types, large marauders and small filchers. The
large marauders force other hummingbirds to give
ground and thus many of these are of large size (for
hummingbirds). The small filchers zip in and out,
grab a quick sip or two of nectar, and swiftly move
on, avoiding agonistic interactions with larger species.
Plate 10- 29. Hummingbirds such as this male Black- throated
Mango (Anthracothorax nigricollis) have evolved a unique
anatomy permitting them to extract nectar while hovering.
Note the tongue partially protruding from the bird’s beak.
Photo by John Kricher.Plate 10- 30. The Rufous- tailed Hummingbird (Amazilia tzacatl)
occurs in forest openings, fields, and gardens from Mexico
to southern Ecuador. Here it exhibits typical hummingbird
feeding behavior. Photo by Nancy Norman.172 chapter 10 tropical intimacy: mutualism and coevolution