4.27.2013

Seabirds

Birds, vertebrates of the class Aves, are the second largest group of vertebrates on Earth, following fish. Unlike marine reptiles, seabirds do not actually live in the water; however, they depend on the sea for their food, and their bodies are highly specialized for aquatic life. Many of the seabirds who feed on animals in the coral reefs also spend some of their time in other marine areas. All of them go to shore during parts of their lives, and some migrate from one ocean area to another.
 
Seabirds are among the longest-lived birds in the world, many with life spans of 30 years or more. For these animals, reproducing is a serious investment of time and energy. Compared to terrestrial avians, seabirds produce fewer offspring and the young are slower to mature, taking an average of seven years. Many choose their mates for life, and the males and females work together to incubate the eggs.

There are many species of birds that nest near coral reefs and interact with the reef food webs. Some of the largest seabird families represented on the reef include frigate birds (family Fregatidae), tropic birds (Phaethontidae), petrels (Procellariidae), boobies (Sulidae), terns and noddies (Sternidae), and albatrosses (Diomedeidae).

Magnificent frigate birds (Fregata magnificens) are striking black birds with deeply forked tails and wingspans of about 7.5 feet (24.6 m). They are easily recognized by their gular sacs, red membranous pouches that the males inflate during courtship. Unlike other seabirds, magnificent frigate birds do not produce a lot of preening oil. These animals rarely float or paddle in the ocean, even to gather food, so their wings require little waterproofing. Instead, most of their meals are stolen from other birds using a highly effective technique of harassment, pestering the victim so much that the irritated bird regurgitates its meal. When the stomach contents are finally disgorged, the magnificent frigate bird deftly catches the mass, often before it hits the water, and whisks it away. If there are no other birds feeding in the area, magnificent frigate birds, who are capable fishers, revert back to the predator mode and catch their own fish or squid from the water.

Magnificent frigate birds (Fregata magnificens)
Magnificent frigate birds (Fregata magnificens)

Reefs also support white-tailed tropic birds (Phaethon lepturus). The adults of this species have wingspans of about 37 inches (94 cm) and can be identified by the long central streamers of tail feathers, black markings on the wings, and yellow bills. Generally feeding at twilight, the white-tailed tropic bird flies high over the ocean, then gracefully dives a distance of 50–70 feet (15–20 m) in pursuit of fish and squid. To lessen the impact of the dive, the birds have shockabsorbent air-filled pouches on their chests. In nesting season, the female lays one pink-and-brown egg on the bare ground or among rocks.

white-tailed tropic birds (Phaethon lepturus)
White-tailed tropic birds (Phaethon lepturus)


A reef bird that is capable of both diving and skimming for food is Audubon’s shearwater (Puffinus iherminieri). Its dark head and brown upper body are set off by a white belly and throat. To feed, Audubon’s shearwater flies close to the water, alternately flapping and gliding, picking up small crustaceans and fish larvae that swim near the surface. If it spots a fish or squid in deeper water, the bird dives after it. Like most shearwaters, it lays a single egg inside a hole in seaside cliffs.

Puffinus iherminieri
Puffinus iherminieri

The red-footed booby is the smallest member of the booby family, having a wingspan of 36 to 40 inches (91.4 to 01.6 cm). Its coloring is unusual among birds because individuals can vary from white to brown. The adults have torpedoshaped bodies, long pointed wings, distinctive bright red feet, and conical blue bills. Red-footed bobbies feed by diving for fish and squid. During mating season, a pair builds a nest in the tops of trees and usually lays two eggs; however, they only hatch one of the eggs, perhaps because competition for food is keen among marine bird populations.

Sooty terns, members of the group of birds known as “sea swallows,” have forked tails, long, pointed wings, and slender bills that curve downward. Adults display distinctive blackand-white plumage, but juveniles have sooty-colored feathers on their heads and chests. Favorite foods of sooty terns include small fish and crustaceans. Each spring, thousands of the birds migrate to tropical islands to form nesting colonies that cover acres of ground. Within these sprawling assemblies, new parents work together to create nurseries where all of the young birds are kept and protected.