Pelecaniformes (Pelicans and Cormorants)
Family: Cormorants and Anhingas
Family: Boobies and Gannets
(Pelicans and cormorants)
Number of families 5 families
Number of genera, species 9 genera; 62 species
Evolution and systematics
Twentieth-century ornithologists typically recognized six families in the order Pelecaniformes. Included were: the tropic birds (Phaethontidae, genus Phaethon) with three species, the pelicans (Pelecanidae, genus Pelecanus) with seven species, the cormorants and shags (Phalacrocoracidae, genera Phalacrocorax and Leucocarbo) with 34 species between them, the anhinga and darter (Anhingidae, genus Anhinga) with two species, the gannets and boobies (Sulidae, genera Sula, Papasula, and Morus) with nine species among them, and the frigatebirds (Fregatidae, genus Fregata) with five species. In this treatment, however, Anhingidae is classified as Anhinga, a genus of the family Phalacrocoracidae. At least six other families, showing characteristics similar to those living, were believed to have disappeared since the Cretaceous. Among the most spectacular of these were the pseudothorns. Bony-toothed bills and wingspans in excess of 18 ft (5.4 m) characterized these primordial predators of the Eocene (60–40 million years ago).
The tropicbirds and frigatebirds may well be the most primitive families in this diverse assemblage. Indeed, Limnofregata azygosternon, dating from the lower Eocene, is among the oldest known aquatic birds in the fossil record. Early additions to the cormorant (Phalacrocoracidae) and pelican (Pelecanidae) lines first appear in the Eocene-Oligocene boundary (40 mya) and early Miocene (22.5–5 mya), respectively.
Owing to the widespread geographic range of some of these birds, often occurring in discrete subpopulations, the reader should not be surprised that there is a great diversity of opinion regarding the number of extant species and subspecies. Of greater significance, however, is the accumulating body of evidence, published in the 1990s and early part of the 21st century, that casts doubt on the legitimacy of the order itself. Early avian taxonomists had qualified the group's membership largely on the basis of superficial internal or external morphology, most notable the characteristic totipalmate (webbed between all four toes) foot structure. Unlike all other water birds, families assigned to this taxon had webbing that connected all four toes. DNA-DNA hybridization data and nuclear and mitochondrial DNA sequences paint a very different picture. Results of experiments published by Charles Sibley and John Ahlquist in 1990 suggested that, while the cormorants/shags, anhingas/darters, and boobies/gannets enjoy a close genetic relationship, the other taxa are phylogenetically remote. Pelicans showed the greatest proximity to Africa's shoebill (Balaeniceps rex) and hammerhead (Scopus umbretta). Similarly, frigatebirds appeared most closely related to the petrels (Procellariiformes), penguins (Sphenisciformes), and loons (Gaviiformes). Meanwhile, tropicbirds, perennial outliers even in traditional classification systems, showed little relationship to any family. Independent phylogenetic investigations conducted later by Van Tuinen et al. supported much of the findings and caused some to wonder whether the order was actually polyphyletic and a classic example of convergent evolution.
Some of the most readily recognizable birds in the world belong in this group. Most show obvious adaptations to an aquatic lifestyle and the diagnostic characteristic shared by every one is the presence of webbing connecting all four toes, although this is much reduced in the frigatebirds.
It is their bills that have accrued the assemblage its greatest claim to fame. The enormous bill and, when distended, gular pouch of a pelican makes it difficult to confuse with any other animal. At lengths of up to 20 in (50 cm), the bill of the Australian pelican (Pelecanus conspicillatus) is the largest of any living bird. Much more representative of the group, however, are the more modest, often serrated, hooked-tip bills found in almost every other Pelecaniform family. The exception is the genus Anhinga, which is characterized by a straight, rapier-like bill of less than 4 in (10 cm).
An unfeathered gular pouch, although present in every species except the tropicbirds, is not always visible from a
distance. Invisible from any distance, of course, are the subcutaneous air sacs that serve to absorb the impact shock of those species that plunge dive. These air sacs account for the surprisingly light weight of birds in a group that includes some of the largest birds capable of flight.
The pelicans are the largest members of the group and may weigh up to 33 lb (15 kg). Two species have been recorded with wingspans exceeding 9 ft (3 m) and lengths of over 80 in (180 cm). By contrast, the smallest tropicbirds may weigh as little as 10.5 oz (300 g). If not for its characteristic tail feather, one species would measure as little as 15 in (38 cm) as an adult.
At first glance, these birds do not strike one as being particularly colorful. Most are primarily black or dark brown, and several are predominantly white. Upon closer inspection, however, a few colorful surprises emerge. Some of the most striking blues and greens in the animal kingdom may be seen in the eyes of some pelicans and cormorants. Colors of the bare skin of a few, most notably the boobies, is particularly striking. With the onset of the breeding season, still others develop unique pastel hues of pink or red. The plumage of the Phalacrocoracidae is unusual in that it is permeable to water.
The greatest numbers of these birds are found in tropical and temperate regions. Overall, the group is not as tolerant to cold as some water birds. None is found at the North or South Poles. Nevertheless, the imperial shag (Leucocarbo atriceps) frequents the coasts and waters surrounding the Antarctic Peninsula and four species, representing the Phalacrocoracidae and Sulidae, are present north of the Arctic Circle.
Owing to the group's heavy dependence on aquatic ecosystems, it is not surprising that it is largely absent from most arid regions. However, during migrations, even small or ephemeral water bodies may prove satisfactory for short visits.
Open ocean, seacoasts, rivers, lakes, and ponds comprise the habitats favored by Pelecaniformes. Gannets, boobies, tropicbirds, and frigatebirds are dependent entirely on marine ecosystems, while anhingas and darters are most often found in freshwater environments. Some pelicans and cormorants are equally at home in saltwater, brackish estuaries, or freshwater.
In areas where they occur in abundance, such as the west coast of South America, these birds play a major role in the ecological processes upon which they ultimately depend. The nitrogen-rich excrement, or guano, of those species is legendary, and the microorganisms that feed upon it form the foundation upon which a spectacular web of life is spun.
One might speculate that the role these animals play in stabilizing the populations on their prey has diminished somewhat with the rise of large-scale commercial and sport fishing. Nonetheless, like so many predators, they are more apt to take organisms whose fitness has been compromised by disease, injury, age, or other factors and, in this respect, serve to enhance the overall health of those populations. By the same token, it is those birds least able to defend themselves, in particular hatchlings and eggs, that are most likely to succumb to still greater predators. Even healthy adult birds are no match for sharks, crocodilians, large birds of prey, and a host of carnivorous mammals. No species preys exclusively on these birds.
As a general rule, these birds are colonial. They are frequently found in association with other relatively large colonial birds including, but not limited to, other families in this group. The proclivity to assemble in colonies in some families, in particular the tropicbirds, appears to be influenced more by a lack of suitable nesting sites than by any economies of scale, such as the detection of predators. Indeed, among the entire group, there are remarkably few examples of developed predator alarm calls. Croaks, grunts, and other rather uninspiring vocalizations are typical of these taxa. The exception is the shrill scream of tropicbirds. The ear-piercing utterances reminded sailors of a bosun's whistle, and they aptly named them "bosun birds."
Territoriality is most marked during the breeding season, when nest sites are at a premium. Displays of aggression may be used as an effective weapon. Ritualized threats preclude these and may involve open-bill gaping, hissing, or size-enhancing postures. There is more than ample time for such activities because, unlike many animals, these birds spend relatively little time feeding. They secure daily sustenance, often, in as little as 30 minutes although significantly more time and energy may be expended traveling to and from foraging sites. The group is a diurnal one, and several species exhibit less activity at midday than they do in the morning and late afternoon. Many species spend their lives in restricted ranges, but those dependent on temperate freshwater habitats migrate before ice makes fishing impossible.
Feeding ecology and diet
Birds representing these families feed exclusively on aquatic animals, and most eat only fish. Squid, crustaceans, and amphibian larvae serve to supplement the diet of a few species. The range of methods used to obtain prey is remarkable and includes dipping, aerial piracy, surface plunging, deep plunging, pursuit plunging, and pursuit diving. The Phalacrocoracidae, whose membership exceeds that of all the other species combined, has perfected this last technique. Unlike penguins, which propel themselves using their wings, birds in these taxa explore beneath the water's surface by foot propulsion and snatch prey with their bills. Meanwhile, anhingas and darters often impale smaller fish with their bills.
A smaller number of species plunge dive, often from considerable heights, frequently stunning prey on impact. Tropicbirds, sulids, and the brown pelican (Pelecanus occidentalis) alike employ this spectacular adaptation. All other pelicans search out their quarry while cruising the water's surface. In one of the most developed forms of cooperative feeding in the avian world, the larger species assemble in a U-shaped flotilla, sometimes in excess of six individuals, that effectively drive schools of fish into shallower water and to their ultimate doom.
The low-flying Fregatidae has perfected a fourth strategy. Unable to swim with effectiveness, birds from this family take organisms occurring on or, as is often the case with flying fish, above the water's surface in the course of their aerial surveillance tours. It was another strategy, however, that earned these birds their popular name. Like the pirates of old, these airborne buccaneers will pester other seabirds to the point of disgorging their gullets. The frigatebirds then swoop down to claim their ill-gotten spoils.
Courtship and mating among Pelecaniformes is often dramatic. Courtship displays may be performed entirely in the air, as is the case with tropicbirds, or entirely on the ground. Frigatebirds use both venues; the grounded males display their brilliant inflated gular pouches for the benefit of airborne females. There is a bias toward monogamy although this trait seems, once again, largely absent among tropicbirds.
Eggs are usually chalky and may number as many as six in a clutch although several species lay only one. Those that lay more than one egg do so asynchronously, and often only the oldest chick survives. Some will breed biannually in the face of reduced food resources. Many pelagic species, among them the Guanay shag (Leucocarbo bouganvillii), have a reputation as being populations greatly affected by the availability of food resources. Nevertheless, there is evidence to indicate that this has frequently been overstated in the literature. Those birds occurring in temperate zones breed in the spring while tropical forms may breed at any time.
As a whole, this group is more arboreal than other aquatic birds, and most construct their nests in trees. However, a few species construct ground nests, while others use cliffs. Incubation varies from 23 to 57 days, and both parents participate in this activity. Brood patches are lacking in all but the tropicbirds; hatchlings are born naked and helpless. Nestlings take regurgitated food matter from the open bills of their parents. Fledging periods vary significantly and may take up to four months in some pelican species.
In 2001, the International Union for Conservation of Nature and Natural Resources (World Conservation Union) listed 22 Pelecaniform birds as being under some threat. Of these, four species were Endangered or Critically Endangered. Nevertheless, the group has fared better than many vertebrate taxa, and only one species, the spectacled cormorant (Phalacrocorax perspicillatus), has disappeared in modern times. Some species, such as the Guanay shag (L. bouganvillii) occurring in the coastal environs of western South America, may number in the several millions.
Several species have recovered from years of persecution after the establishment of adequate protection, including the elimination of toxins such as the insecticide DDT. In North America, the most notable conservation success story is that of the brown pelican (Pelecanus occidentalis). Once an endangered species, this animal is no longer listed by the IUCN. The range and numbers of the American white pelican (Pelecanus erythrorynchos) and double-crested cormorant (Phalacrocorax auritus) respectively, have also increased. Elsewhere in the world, meaningful action has been taken to restore such charismatic species as the Dalmatian pelican (Pelecanus crispus).
Loss of habitat, pollution, overfishing, and purposeful eradication, largely by those who believe the birds compete with humans for food, continue to erode some species' populations. Particularly vulnerable, of course, are those species with historically small ranges. Many seagoing species are endemic to small island clusters where breeding colonies seem especially vulnerable to predation by introduced species, such as rats. The elimination of vegetation by feral rabbits, and the accompanying reduction in the shade it produces, has adversely affected reproduction at the breeding grounds of some tropicbirds.
Significance to humans
Perhaps it is not surprising that such a diverse assemblage of bird families, as those described in the following chapters, should have had such a diverse impact on human cultures through the ages. Reverence for some species was common, and this respect has not been entirely lost in many cases. Even today, for example, the delightful tail feathers of tropicbirds play an important role in the ornaments of some South Pacific cultures. The guano produced en masse by some species is of enormous value to many agricultural economies. It has been claimed that the Guanay shag is the most economically important bird in the world. Unfortunately, with the rise of industrial and sport fishing, these birds are regarded as pests. At the turn of the 20th century, more than 1,000 double-crested cormorants were slaughtered in the northeastern United States. Those responsible, it is assumed, mistakenly believed the birds competed for game fish. In the Far East, the exceptional fishing skills of Pelecaniformes led to one of the most unusual relationships in the animal kingdom when it was discovered that cormorants and darters could be trained to fetch aquatic prey for their human masters. Many species, among them the boobies and tropicbirds, seem to enjoy the company of humans and their conveniences. Often, these birds will follow ships and even alight on them.
Van Tuinen, M., D.B. Butvill, J.A.W. Kirsch, and S.B. Hedges. "Convergence and Divergence in the Evolution of Aquatic Birds." Proceedings of the Royal Society 268, no. 1474 (2001): 1345–1350.
Jay Robert Christie, MBA