Charadriiformes

(Gulls, terns, plovers, and other shorebirds)

Class Aves

Order Charadriiformes

Number of families 13 families

Number of genera, species 91 genera, approximately 343 species

Introduction

The Charadriiformes represent one of the largest avian orders and exhibit an astounding diversity in morphology, behavior, and life histories. Species range in size from 0.06–4.4 lb (25 g–2 kg), with considerable variation in such characteristics as body form, leg length, and bill design. Further, the habits of these birds vary from gregarious to solitary and migratory to sedentary. Reflecting the diversity of this taxonomic group, the Charadriiformes have a global distribution and occur in a variety of environments. As a result of all these factors, generalizations across the group are problematic. However, member species do share a number of common characteristics. Chief among these is a marked preference for inland water and marine habitats. In addition, many species are gregarious and congregate during all or various phases of the annual cycle. The Charadriiformes are generally grouped into three major suborders consisting of shorebirds or waders (Charadrii); gulls, skimmers, jaegers, skuas, and terns (Lari); and auks (Alcae).

Evolution and systematics

The Charadriiformes are an ancient assemblage, with fossil evidence dating as far back as the Eocene epoch, over 36 million years ago. Storrs, Olson, and Feduccia proposed that a group of "transitional" shorebirds represent the basal lineage for all neognaths, the major group of extant birds. A premise of their theory was that ancestral Charadriiformes were one of the few avian groups to survive the massive extinction event of the late Cretaceous period, the same event that marked the disappearance of the dinosaurs. These prehistoric birds are believed to have displayed combined features of modern shorebirds, ducks, and other waterbirds. Since the Cretaceous period, the Charadriiformes have undergone an extensive adaptive radiation that resulted in their diversity of form and function. For centuries, naturalists and taxonomists have struggled with the classification and evolutionary relationships of the different Charadriiformes. Efforts to determine their affinities have relied on an array of morphological, behavioral, biochemical, and molecular evidence and resulted in a myriad of groupings. It is clear that no single trait dictates inclusion or binds this diverse order together. Currently, the Charadriiformes are recognized as a cohesive group based on an assortment of characters. For example, morphological traits common to most members include a schizognathous palate, rump feathers with an aftershaft, a bilobed and tufted uropygial gland, and similarities in syrinx and leg tendons. An alternative taxonomy resulted from the complete avian phylogeny constructed in the 1990s by Sibley and Ahlquist. This effort was based on DNA/DNA hybridization studies and the authors maintained the association of species found in the charadriiform grouping; however, some species were lumped in the collective order Ciconiiformes, along with various other waterbirds. Although considered an important contribution, their methodology has received considerable criticism that has prevented wide acceptance. Undoubtedly, continued research efforts and the advent of increasingly sophisticated analytical methods will result in the further refinement of the Charadriiformes classification. In particular, analyses of mitochondrial and nuclear DNA promise to provide some of the most complete and persuasive information.

General ecology

The Charadriiformes occur in a wide range of water-associated habitats at inland, nearshore, coastal, island, and pelagic areas. These habitats are generally extremely productive and provide a rich source of food for breeding, migrating, and wintering birds. However, some of the most

productive sites also occur in regions where harsh conditions prevail during various times of the year. As a result, the Charadriiformes exhibit various morphological and behavioral adaptations related to survival and the ability to exploit these environments. Fundamental to most Charadriiformes is the issue of osmoregulation, meaning the maintenance of an appropriate balance of body water and salts in a saline environment. This is harder to achieve for those species that inhabit regions with few freshwater sources. However, the Charadriiformes have highly specialized supra-orbital salt glands that provide an effective means of excreting excess salt. Another strategy to exploit resources in regions that are inhospitable at various times is to migrate. Although some species are sedentary, the Charadriiformes generally exhibit a wide range of annual movement patterns. In particular, shorebirds and terns display some of the most remarkable migratory feats. For example, the Arctic tern (Sterna paradisaea) annually migrates over distances of more than 18,000 mi (28,962 km) between Arctic breeding grounds and wintering areas in extreme southern latitudes. The Pacific golden plover (Pluvialis fulva) completes a nonstop journey of more than 2,200 miles (3,540 km) between Hawaii and Alaska in less than two days.

The Charadriiformes display considerable variation in mating and social habits. Most species are monogamous and in many cases maintain pair bonds in successive breeding seasons. Adherence to a monogamous system, particularly in the Alcae and Lari, is possibly related to the requirements of nest defense and bi-parental care of young. In contrast, polyandrous and polygynous behavior occurs in a number of the shorebirds (Charadrii). Examples include the phalaropes (genus Phalaropus; family Scolopacidae), jacanas (family Jacanidae), and some sandpipers (family Scolopacidae). Another fascinating aspect of Charadriiform social systems is the gregarious nature of many species during various phases of the annual cycle. This is most pronounced in the colonial nesting habits of gulls and terns (family Laridae), and alcids (family Alcidae). Colonies range in size from hundreds of birds to hundreds of thousands of birds. Additionally, a small number of shorebirds, such as the banded stilt (Cladorhynchus leucocephalus) of Australia, are semi-colonial or colonial breeders. Outside the breeding period, many species also occur in large flocks on migration and at wintering sites. For example, it is estimated that over five million shorebirds stop at the Copper River Delta in Alaska each spring.

Nest construction in most Charadriiformes is relatively crude. In fact, many of the cliff nesting seabirds (e.g. Alcae) lay eggs directly on rock ledges with no nesting material. Shorebird nests generally consist of a shallow scrape lined with small pebbles and bits of vegetation. Perhaps the most unusual nesting habits of the Charadriiformes are found in some murrelets (genus Brachyramphus) and some sandpipers (Scolopacidae) that build nests in trees or reuse nests constructed by songbirds. Clutch size ranges from 1–4 eggs with an incubation period of three or more weeks. Shorebird chicks are precocial and leave nests shortly after hatching, while many of the seabird chicks (Alcae and Lari) remain at or near nests for extended periods. The diversity of Charadriiformes is exemplified by their range of diets and foraging strategies. Diet ranges from animal to vegetable matter and in some cases is highly specialized. For instance, many species are piscivorous. Of these, the terns are skilled fliers with relatively long, narrow wings and a slight build. Foraging individuals plunge into the water from above to deftly catch fish near the surface. In comparison, the alcids are stockier marine birds, with short wings well adapted for use when swimming underwater in pursuit of prey. Also, their bills are narrow and sharp-tipped with ridged edges for holding onto fish. The skimmers (genus Rhynchops), are primarily tropical birds with a bill design that is unique among birds. In this group, the lower mandible extends further than the truncated upper mandible. When foraging, birds skim low over the water and use their specialized bill to grab fish from near the surface of the water. Another important food among the Charadriiformes is aquatic and terrestrial invertebrates. The group that has evolved the greatest diversity of foraging strategies associated with an invertebrate diet is the shorebirds, which exhibit a wide array of bill and leg morphology. For instance, the plovers are relatively short, stocky birds of upland and shallow wetland habitats. In most cases, these birds are generalist, visual foragers with short bills that are used to pick small invertebrates from the surface of the substrate. In contrast, the sandpipers and other scolopacids forage almost entirely in wetland habitats of coastal and inland areas and often use tactile cues to capture prey. Their bills vary in length and in some cases are highly sensitive, with the ability to detect prey items that live in mudflats and other soft sediments. A number of the alcids are plankton feeders with short, wide bills and strong, flexible tongues to aid in handling prey. Also, these birds have a modified gular pouch that is used for storing and carrying food to their young. In addition to animal matter, many Charadriiformes are omnivorous with a diet that includes varying amounts of vegetable matter. The sheathbills (family Chionidae) are generalists of extreme southern latitudes. During some parts of the year, particularly when other food sources are scarce, a significant portion of their diet consists of algae. Species following this diet include the enigmatic plains-wanderer (Pedionomus torquatus) and seedsnipes (genera Thinocorus and Attagis). More than 50% of the plainswanderer diet consists of seeds and in spite of their name, most seedsnipes subsist almost entirely on buds, leaf tips, and small green leaves.

Conservation status

Whereas a number of charadriiform species have expanded their range and increased in numbers, the vast majority face an uncertain future and in some cases have experienced severe declines. A total of 34 different species are considered vulnerable, endangered, or critically endangered by the international community. For example, the black stilt (Himantopus novazealandiae), once common in New Zealand, now numbers less than 100 birds and ranks as one of the most endangered species in the world. It is clear that the greatest threats facing the Charadriiformes and other avian groups arise from the effects of expanding human populations. Complicating matters, there are various aspects of charadriiform ecology that increase the vulnerability of the group. In particular, the productive waters of coastal and inland habitats attract not only large numbers of birds, but also the activities of humans. These areas are increasingly the focus of development and exploitation of resources, such as fish and petroleum. In addition, the tendency for many Charadriiformes to congregate during all or various phases of the annual cycle increases the potential impacts of various threats, such as habitat loss and catastrophic events. In some cases, exposure to such forces is not a recent

event. Historically, many charadriiform species were harvested by humans for meat, feathers, oil, and eggs. Often, this consumption was part of subsistence hunting with limited impacts on the persistence of bird populations. However, expanding human populations and advances in exploration and technologies progressively led to dramatically increased harvests of some species. Large flocks of sandpipers were easy targets for hunters during migrations and at wintering areas, with reports of wagons filled and barrels packed with birds for market. It is thought that such persecution led directly to the demise and probable extinction of the Eskimo curlew (Numenius

borealis). Harvests of colonial nesting sea birds (e.g., Alcids) have been equally dramatic. Explorers, whalers, fishermen, and traders often stopped at seabird colonies to replenish stores by harvesting large numbers of birds and eggs. Over time, the ability to market this resource led to even greater takes. A noted example involves the great auk (Pinguinus impennis) that was driven to extinction in the mid-1800s. A growing body of research and expanded monitoring efforts has resulted in a greater understanding of the impacts of harvests on bird populations. Knowledge of dramatic declines has led to increased protection through legislation and the regulation of hunts. Even so, in some regions harvests continue at unsustainable levels. For example, an estimated 300,000–400,000 thick-billed murres (Uria lomvia) were killed in Greenland in 1988-89. The widespread loss and alteration of interior and coastal wetland habitats has been implicated in the decline of numerous species. For example, in North America, the mountain plover (Charadrius montanus), the piping plover (Charadrius melodus), and the snowy plover (Charadrius alexandrinus) have all experienced significant range contraction and population declines as a result of the degradation of interior upland, wetland, and riverine systems. In various coastal areas, there is also the constant danger of catastrophic oil spills. The progressive increase in the world harvests of fish and other marine resources has also led to increased interactions of fisheries and marine birds. Troublesome negative effects on birds include entrapment in fishing gear, prey depletion, and disturbance. Other major conservation issues include the introduction of non-native species (particularly at seabird colonies), disturbance from human recreational activities, and the potential effects of climate change.

Suborder Charadrii

Shorebirds, also referred to as waders, are the largest and most diverse group of Charadriiformes with 11 families and 216 species. This includes the thick-knees (Burhinidae); plovers (Charadriidae); sheathbills (Chionidae); crab plovers (Dromadidae); coursers and pratincoles (Glareolidae); oyster-catchers (Haematopidae); jacanas (Jacanidae); stilts and avocets (Recurvirostridae); painted snipes (Rostratulidae); phalaropes, snipes, and sandpipers (Scolopacidae); and seedsnipes (Thinocoridae). These birds occur in a wide range of environments including coastal and inland wetlands, grasslands, and even deserts.

Suborder Lari

The single-family Lari suborder consists of the gulls, terns, skimmers, jaegers, and skuas (Laridae). The skimmers and terns are primarily piscivorous and occur in marine and freshwater habitats. Skuas and jaegers are found at nearshore, coastal, and pelagic areas. These aggressive birds are known for their kleptoparasitic and predatory habits, in many cases preying on the young of other seabirds. The gulls are generalist foragers, occurring in a range of inland, marine, and coastal habitats.

Suborder Alcae

Also consisting of a single family, the auks (Alcidae) are restricted to marine environments of the Northern Hemisphere. In these regions, the auks fill the niche of the penguins found in southern latitudes. Auks are short-winged birds that are specialized for diving to capture fish and feed on plankton. A majority are colonial nesters, often nesting on cliffs at rocky coastal areas. In some cases, breeding colonies number in the tens to hundreds of thousands. A notable exception are the murrelets, some of which nest in trees and may occur many miles (kilometers) inland.

Resources

Books

Burger, A.E. Oil spills. New Brunswick: Rutgers University Press, 1997.

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Hoatzin to Auks. Vol 3, Handbook of the Birds of the World. Barcelona: Lynx Edicions, 1996.

Feduccia, A. The Origin and Evolution of Birds. New Haven: Yale University Press, 1996.

Schreiber, E.A., and J. Burger, eds. Biology of Marine Birds. Boca Raton, CRC Press, 2002.

Sibley, C.G., and J.E. Ahlquist. Phylogeny and Classification of Birds: A Study in Molecular Evolution. New Haven: Yale University Press, 1990.

Van Der Liet, J., and J. Grindle, eds. Bird Families of the World. New York: Harry N. Abrams, Inc. Publishers, 1978.

Periodicals

Chu, P.C. "Phylogenetic reanalysis of Strauch's osteological data set for the Charadriiformes." Condor 97 (1995): 174-196.

Friesen, V.L., A.J. Baker, and J.F. Piatt. "Phylogenetic relationships within the Alcidae (Charadriiformes: Aves) inferred from total molecular evidence". Molecular Biology and Evolution 13 (1996): 359-367.

Peter Martin Sanzenbacher, MS