Kingdom: Animalia Phylum: Chordata Class: Aves (Birds) Order: Ciconiiformes Family: Sulidae
Sulidae (Boobies and Gannets) are plunge-diving birds that are most closely related to the Cormorants (Phalacrocoracidae). They probably originated in the late Cretaceous, more than 60 million years ago. Some authors group all species of Sulidae under one genus (Sula), while others recognise three genera (Papasula, Sula and Morus). This system reflects differences in morphology and biology among the species, although their high degree of adaptation to the marine environment results in them sharing many characteristics.
Sulidae have a cigar-shaped body, long pointed wings and relatively long, wedge-shaped tail. The neck is long and thick, with well-developed muscles. The head is dominated by the stout, conical bill. The bare skin around the neck and bill is often brightly colored, and plays an active role in ritual displays. The eyes are placed at each side of the bill and are orientated towards the front, giving the birds excellent binocular vision, which is essential for fishing from air. Like most fish-eating birds, Sulids are light colored in the underparts, particularly the belly. The upperparts, especially the wings, are most often dark. The white colored underparts blend in against the brighter sky, thus rendering the predator less visible for the prey fish. As a further adaptation to their specialized fishing technique, Sulids have subcutaneous fat and well developed air sacs, which act as cushions to protect the birds from the violent impact of diving into water. Sulids lack brood patches and they incubate by sitting on their “heels” and wrapping highly vascularized webbing of their feet around the eggs. The short and stout legs are situated far back on their bodies, allowing the birds to swim well.
Habitat and distribution
Sulids live primarily at sea and do not set foot on land except during the breeding season. Boobies are found in tropical or subtropical waters, whereas Gannets favor temperate environments. The pantropical Boobies (Masked, Red-footed and Brown), are circumpolar in distribution, occurring over oceans between the tropics. Brown and Blue-footed Boobies often feed in inshore waters, while Red-footed and Abbott’s take long foraging trips over several hundred kilometers from the nearest land. A variety of sites on offshore islands and rocky outcrops are used for breeding. Several species place nests on exposed, flat ground, others do so on cliffs. The third most common nesting habitat is on top of tall tropical trees. All species nest colonially in quite high densities, which has favored the development of ritualized displays, particularly to indicate site-ownership or to obtain a mate.
Feeding and diet
Sulidae prey mostly on mobile, schooling fish that frequent open waters. In tropical waters, flying-fish and squid are also frequent prey. Once they have located their prey, Sulids close their wings and plunge vertically from great heights (10-50 m), head first, into the water. Just before entering the water, they extend their wings backwards alongside their body and this assists them in reaching greater depth. Once under water, they use their wings to penetrate even deeper (up to 25 m). Prey is caught on the way up and is usually swallowed underwater, to avoid harassment from Frigatebirds and Gulls.
Many species pair for life and reunite annually at the nest-site. Pair-bonding displays play an important role and take up a significant part of the breeding season. Boobies make extensive use of their wings and feet in displays in which the incoming bird salutes with outstretched feet just as it is about to land. All Sulids tend to nest in colonies. Sulid nests are quite rudimentary, especially those of ground-nesting species. Often, they consist only of a slight depression or an accumulation of debris, glued together with the birds’ excreta. The Red-footed Booby and Abbott’s Booby build more elaborate nests, usually a platform of sticks on one of the upper tree branches. Most species lay single-egg clutches. Some species, such as the Masked Booby and Brown Booby, lay two eggs, but the older chick killing its younger sibling. Incubation lasts 41–45 days in most species, although Abbott’s Booby extends its incubation period to 57 days. Both sexes contribute in long stints and no feeding occurs between the breeding adults. Their eggs have unusually thick shells. The chick is born naked and is continuously guarded for the first month, until it can regulate its own body temperature. It is fed on fish remains directly from the parent’s mouth. Booby chicks take some time to fledge and are fed for a lengthy period after they fledge.
The Abbott’s Booby is classified as 'critically endangered' and its breeding range is confined to Christmas Island in the Indian Ocean. Their population has declined in the past due to habitat destruction through forest clearance. The remaining species are not considered to be globally threatened or in danger of extinction. Effective protection of offshore islands where Sulids place their colonies is an essential part of any conservation program for these species.
Sulids have been exploited for their eggs and chicks, for food in the past. The birds were an important source of protein for some communities in northern Atlantic. Many species have seen their numbers artificially being kept to quite low levels for centuries, and have started to recover only in the last few decades. The Peruvian Booby has suffered severely from intensive exploitation of guano for agricultural purposes, which continued until well into the first part of the twentieth century.