Pelecaniformes


 

 


Frigatebirds

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 are:

 Key Features  Fregata minor  Fregata magnificens
 Male Call  gobble  rattle
 Scapular Feathers  green irridescence  purple irridescence

 Female

Color of Eye Ring  red blue 
 Throat and Breast  white  black throat & white breast
 Juvenile  Face and Throat  rust-tinged  white

 

Frigatebirds build nests in low-lying shrubs and produce a single egg. Both parents take turns feeding for the first three months but then only by the mother for another eight months. It takes so long to rear a chick that frigatebirds cannot breed every year. It is typical to see juveniles as big as their parents waiting to be fed. When they sit waiting for endless hours in the hot sun, they assume an energy-efficient posture in which their head hangs down, and they sit so still that they seem dead. But when the parent returns, they will wake up, bob their head, and scream until the parent opens its mouth. The starving juvenile plunges its head down the parent's throat and feeds at last.

The name "frigatebird" calls to mind the sails of ships and, indeed, frigatebirds sail gracefully in the air currents overhead. Their wingspan is some 7.5 feet and their deeply forked scissor-like tails afford them ultimate maneuverability. Their other common name, however, the "man-o'-war" bird, reflects the way in which they use their consumate flying and maneuvering skill. Frigatebirds are pirates who harass incoming birds, especially boobies until the victim is so upset that it disgorges its catch. The frigatebird then drops with amazing speed and plucks the bolus out of the water, or even catches it before it hits! Not only do frigatebirds harass other species, but other frigatebirds as well. The disgorged bolus may pass through several beaks before it is finally swallowed. In addition to stealing food from boobies and from one another, they steal nesting materials as well. It was this kind of piratical maneuvering that caused the two juveniles to crash into each other and fall into the sea.

Their reputation for piracy, however, may be somewhat exagerated. With their great maneuvering skill and their long, strongly hooked beaks, frigatebirds can glide along the surface of the water, swing their head down, and pluck out fish and squid. In a similar manner they can pick up hatchling sea turtles, unprotected chicks, and disgorged food right off the ground. It is also common to see a frigatebird skimming over the surface of El Junco, a rare fresh-water lake on San Cristobal, dipping its beak down and sipping a drink.

Next: Brown Pelican

Return to Sea Birds



    for more info, contact Dr. Robert Rothman: rhrsbi@rit.edu