It is common in the skies over Galapagos waters
to see large, black birds, hovering lazily in place, looking for
all the world like kites. These are frigatebirds. Frigatebirds
belong to the family Fregatidae, which contains five species world-wide.
In the Galapagos there are two species: the great frigatebird
(Fregata minor) and the magnificent frigatebird (Fregata
magnificens). Of the two, the great frigatebird has the greater
world-wide distribution, being found primarily throughout the
tropical Pacific and Indian Oceans, while the magnificent frigatebird
is found in the Caribbean and on the Pacific and Atlantic coasts
of the Americas. The Galapagos population of magnificent frigatebirds
is considered to be an endemic subspecies. In the Galapagos, the
two species can be seen nesting side by side on N.
Seymour, but when frigatebirds are sighted in the air, they
typically are magnificent frigatebirds, because great frigatebirds
tend to forage much farther out at sea. As with the three similar
species of booby, essentially similar species avoid competition
by feeding in different locales.
As members of pelecaniformes, frigatebirds have
the key characteristics of all four toes being connected by the
web, a gular sac, and a furcula that is fused to the breastbone.
Although there is definitely a web on the frigatebird foot, the
webbing is reduced and part of each toe is free. Frigatebirds
produce very little oil and therefore do not land in the ocean.
In 10 years of observing frigatebirds, I have only seen two individuals
in the water and this was accidental: two juveniles crashed into
each other and fell in. With some difficulty they were able to
get out. The gular sac is used as part of a courtship display
and is, perhaps, the most striking frigatebird feature.
Male frigatebirds are black with a patch of red
skin at the throat that is the gular sac. During courtship display,
the male forces air into the sac, causing it to inflate over a
period of 20 minutes into a startling red balloon. As males tend
to display in groups, the effect is magnified. Then the males
sit quietly in the low shrubs watching for a female to fly overhead.
At this, the males waggle their heads from side to side, shake
their wings and call. If the display is attractive enough, then
the female will land and sit beside her amour.
The two species of frigatebird are very similar
to one another, the males being the most difficult to distinguish.
The call is one of two ways to distinguish the males of the two
species from each other. Great frigatebirds make a "gobbling"
sound, not unlike a turkey, while magnificent frigatebirds make
a rattling or drumming sound. A second, more subtle way to distinguish
the nearly identical males is to look at the scapular feathers,
long feathers that cover their shoulders. Although the feathers
are black, they are irridescent and produce different colors when
they refract sunlight. In magnificent frigatebirds the irridescence
is purple while in great frigatebirds it is green. It is easier
to distinguish the females and juveniles. A summary of differences