What is a Bird?

Birds (class Aves) are a group of vertebrate animals with feathers and toothless beaked jaws (Fig. 5.34). They have wing-shaped forelimbs, four-chambered hearts, and hard bony skeletons. Birds reproduce sexually and lay hard-shelled eggs in nests protected by the parents. They occur on all seven continents including Antarctica and can survive a wide variety of environmental conditions ranging from the tropics to the poles.

Many bird species live in aquatic environments. Familiar examples from freshwater habitats include ducks, geese, and swans. Other bird groups rely heavily on freshwater resources. America’s national bird, the bald eagle (Haliaeetus leucocephalus), feeds primarily on fish.

There are approximately 470 species of marine birds, or seabirds, on Earth (Fig. 5.35). These species live in coastal marine habitats and on islands. All marine birds rely primarily on ocean resources for food and habitat.

Evidence of Common Ancestry and Diversity

Today biologists consider birds to be a type of reptile (Fig. 5.22 A). The most recent fossil and phylogenetic evidence indicates that modern birds are actually a subgroup of upright-walking dinosaurs (Fig. 5.22 B). While other dinosaur groups faced extinction around 66 million years ago, birds survived and diversified dramatically. There are approximately 10,000 known bird species today.

Like other reptiles, all birds are amniotes. Birds lay hard-shelled eggs containing amniotic sacs (Fig. 5.36). Marine bird eggs often have a camouflage pattern, an adaptation that reduces losses to egg-eating predators.

Also like other reptiles, all birds are tetrapods. The typical walking front limbs of other tetrapods have evolved into wings in birds. Birds stand and walk on their rear limbs. Wings allow most bird species to fly. Most marine birds can flap their wings and lift off from the ground. However, some birds are flightless. Penguins and some other marine birds use their streamlined wings to swim underwater. Large flightless birds like the ostrich, emu, and cassowary have small stubby wings. These wings are examples of vestigial structures or structures that have lost their original function during evolution. They are used to regulate body temperatures and to maintain balance while running.

Marine Birds

Many groups of birds are adapted to living in or near water. Ducks, geese, swans, herons, and flamingos are all familiar examples of freshwater birds. Marine birds not only need adaptations to survive in saltwater, but also often far away from land. The most common marine birds fall within four large groups: penguins, tubenoses, pelecaniformes, and charadriiformes.

Penguins

Penguins (order Sphenisciformes) are flightless marine birds found almost exclusively in the southern hemisphere. Figure 5.37 demonstrates some examples of penguins. Major penguin populations are found in the Southern ocean basin in Antarctica, South America, New Zealand, and South Africa. One species occurs in the cold, nutrient-rich waters of the Galápagos Islands near the equator. Contrary to depictions in popular media, penguins do not occur in the northern hemisphere alongside polar bears. Penguins feed primarily on fish and small invertebrates like krill and squid. There are between 17 and 20 species of extant penguins.

Tubenoses

Tubenoses (order Procellariiformes) are a diverse group of marine birds characterized by one or two tubes running along their straight hook-tipped bills. Examples of tubenoses include albatrosses, petrels, and shearwaters (Fig. 5.38). There are approximately 125 extant species around the world. Tubenoses mostly nest in colonies on remote islands free from predators. Their diets consist primarily of fish, squid, and small crustaceans.

Pelecaniformes

Pelecaniformes (order Pelecaniformes) is a group of medium- and large-bodied aquatic birds. Examples include pelicans, frigatebirds, gannets, and cormorants (Fig. 5.39). Although this group encompasses a wide diversity of body forms, most pelecaniform birds have four webbed toes and bare throat patches. There are approximately 60 species within order Pelecaniformes around the world. Most pelecaniform birds eat fish. Pelicans have adapted large throat pouches that allow them to scoop small fish from the water. Cormorants catch fish with their hooked beaks by diving down from the water surface from a floating position.

Charadriiformes

Charadriiformes (order Charadriiformes) is a diverse group of small and medium sized aquatic birds. Examples include plovers, gulls, terns, skimmers, sandpipers, puffins, and auks (Fig. 5.40). There are approximately 350 charadriiform species worldwide, with 305 species considered to be marine birds. Wading shorebirds such as plovers and snipes pick small invertebrates from the sediment on beaches and tidal mudflats. Larger charadriiform birds like gulls and puffins hunt for fish and invertebrates or scavenge on carrion. Gulls­­­, often referred to as “seagulls,” are perhaps the marine bird most familiar to most Americans. However, species in the gull family are rarely found in Hawai‘i and other tropical island communities.

Other Marine Birds

Several other bird species use coastal and open ocean resources but are not traditionally classified as marine birds. Examples include herons, sea ducks, and many raptor species (Fig. 5.41). Raptors, or birds-of-prey, are general terms used to describe eagles, hawks, falcons, and vultures. The eagle genus Haliaetus consists of 10 species of sea eagles, including the bald eagle, erne, Stellar’s sea eagle, and African fish eagle. The osprey, or sea hawk, (Pandion haliaetus) is another common marine raptor (Fig. 5.41 B). These raptor species use their excellent vision and sharp claws to locate and capture fish.

Structure and Function

Like other reptiles, birds have skin covered in keratin scales. Keratin is a fibrous protein produced in the outer skin cells of many vertebrate animals including many fishes, amphibians, reptiles, birds, and mammals. Keratin is found in scales, horns, claws, turtle shells, bird feathers, and mammal hair. Bird scales are most readily observed on their feet, ankles, and other featherless skin patches.

All birds have feathers covering most of their body surfaces (Fig. 5.42 A). Feathers are lightweight growths of keratin extending from the skin. The two basic types of feathers are vaned feathers and down feathers. Vaned feathers have a stiff supporting shaft running through its center (Fig. 5.42 B and C). Fine branches or barbs emerge from each shaft, held together by microscopic hooks. This system of interlocking hooks allows vaned feathers to support birds’ flight. Vaned feathers also provide birds with color patterns and waterproofing. The term plumage refers to the feather patterns visible on a bird’s exterior surfaces (Fig. 5.42 A). In contrast to vaned feathers, down feathers are usually much shorter with soft central shafts and fluffy branches (Fig. 5.42 D). They grow layered underneath the vaned feathers. These fluffy branches allow down feathers to insulate birds and protect them from dangerous changes in body temperature. Some bird species use their own down feathers to line the interior of their nests (Fig. 5.42 E).

All tetrapods have hard bony internal skeletons (Fig. 5.43 A). These bones are constructed of calcium mineral. In contrast to other tetrapods, bird bones are mostly hollow and lightweight. Ancient Chinese from the Yellow River Valley used the hollow bones of red-crowned cranes (Grus japonensis) to make flutes (Fig. 5.43 B). Having hollow and lightweight bones allows most bird species to fly.

All birds have wing-shaped front limbs. The combination of feather-covered wings and lightweight skeletons allow most bird species to fly. The fastest and longest ranging animals on Earth are birds. Several bird species are flightless and have evolved smaller wings than their flying relatives. Examples of flightless birds include penguins, kiwis, the emu, and the ostrich.

Adaptations

Marine birds have evolved several adaptations that allow them to survive in ocean conditions. Many species that float atop the water surface have evolved webbed feet (Fig. 5.44 A) and dense oil-covered waterproof feathers (Fig. 5.44 B). Examples of surface-swimming marine birds include gulls, albatrosses, pelicans, and puffins. Penguins have evolved streamlined bodies and powerful flipper-like wings (Fig. 5.44 B). These wings allow penguins to swim not only at the water surface, but also submerged below. Many penguin species also exhibit porpoising behavior, leaping in and out of the water often when swimming long distances (Fig. 5.44 C & D).

Marine birds have evolved several adaptations that allow them to survive in ocean conditions. Many species that float atop the water surface have evolved webbed feet (Fig. 5.44 A) and dense oil-covered waterproof feathers (Fig. 5.44 B). Examples of surface-swimming marine birds include gulls, albatrosses, pelicans, and puffins. Penguins have evolved streamlined bodies and powerful flipper-like wings (Fig. 5.44 B). These wings allow penguins to swim not only at the water surface, but also submerged below. Many penguin species also exhibit porpoising behavior, leaping in and out of the water often when swimming long distances (Fig. 5.44 C & D).

All birds have hard external mouthparts called beaks or bills. Birds have evolved a wide diversity of beak shapes (Fig. 5.45). These forms are adaptations suited for a variety of functions. For example, a long thin bill helps avocets, curlews, and many other wading shorebirds (order Charadriiformes) to pick small invertebrate prey from tidal mudflats (Figs. 5.46 A and 5.47 B). Pelicans have adapted a large pouch-like beak and throat that allows them to scoop several small fish from the water in a single gulp (Fig. 5.46 B). Skimmers (genus Rynchops) are tern-like marine birds in the order Charadriiformes. They are unusual because their lower beaks are longer than their upper beaks (Fig. 5.46 C). Skimmers catch small fish by flying very low with their lower beaks slicing into the water (Fig. 5.46 D)

Energy Acquisition

Like other reptiles, birds acquire energy from a wide variety of sources. Many terrestrial and freshwater bird species feed on plant matter like grasses, aquatic plants, fruits, and seeds.

Most marine birds are carnivorous predators. Wading shorebirds (within the order Charadriiformes) like plovers, sandpipers, and snipes mostly feed on small invertebrates plucked with their pointed beaks from coastal beaches and mudflats (Figs. 5.46 A, 5.47 A, and 5.47 B).

Many marine birds are active predators of swimming prey such as fish, squid, and jellyfish. Gulls, terns, and albatrosses generally feed by scooping prey from the water surface or shallow diving. In contrast, pelicans, penguins, puffins, and cormorants are capable of diving several meters below the water surface by flapping their powerful streamlined wings underwater.

Gannets and boobies (family Sulidae within order Pelecaniformes) feed mainly by plunge-diving into the water after their swimming prey (Fig. 5.48 A). This is accomplished by locating schools of fish and squid from heights of up to 45 meters (m) and then diving steeply with their wings folded back (Fig. 5.48 B). Plunge-diving gannets strike the water surface at speeds up to 100 kilometers per hour (62 miles per hour) with their heads and necks outstretched and beaks tightly shut. They can plunge-dive to 5 m deep and swim down further if necessary.

Some marine birds also acquire their necessary energy from other birds. Gulls, frigatebirds, and other species are known to steal food caught by other birds (Fig. 5.49 A). This type of biological interaction is called kleptoparasitism. It can occur between different species (“interspecific”; Figs. 5.49 A and B) as well as between members of the same species (“intraspecific”; Figs. 5.49 C and D). Large marine birds like gulls, frigatebirds, giant petrels, and pelicans have also been observed preying on other birds.

Growth, Development & Reproduction

All bird offspring hatch from hard-shelled eggs laid in nests. In many species of marine birds, the parents provide some degree of parental care for their young offspring (Fig. 5.50). Parental care can include incubating eggs and providing food, shelter, and protection from predators.

The plumage or feather patterns on most bird species changes as they grow from juveniles into adults. Newly hatched chicks are often covered in thick, fluffy down feathers (Figs. 5.51 A and 5.51 B). This insulating layer of feathers eventually molts or falls off as the young bird grows into adulthood (Fig. 5.51 B).

Courtship is the ritual behavior displayed between potential mates of the same species. Examples of courtship behavior can include dance-like movements, vocalizations or calls, and displays of physical beauty, strength, fighting prowess, or nest building skill. Several species of marine birds display complex courtship and mating behaviors. Mating is the pairing of two individuals for sexual reproduction. All birds produce offspring through sexual reproduction.

To review sexual reproduction, go to the Growth, Development, and Reproduction topic in Algae and Aquatic Plants.

Albatrosses (family Diomedeidae within the “tubenoses”) are well known for their complex courtship behavior. This includes vocalizations and display behavior where courting birds will touch beaks (Fig. 5.52 A), spread their wings, and then point their beaks skyward (Fig. 5.52 B). Ornithologists, or bird biologists, call this courtship behavior “sky calling.”

In many bird species, the females select male mating partners based on these courtship displays. Frigatebirds (Fregata spp.) are well known for the males’ bright red throat pouches. Male frigatebirds inflate their throat pouches in a courtship display for females (Fig. 5.52 C). Male boobies (Sula spp.) exhibit “parading” behavior for females by walking upright, swaying from side to side, and alternatingly lifting their brightly colored feet (Fig. 5.52 D).

Most bird species are socially monogamous. Monogamy is the condition where an organism has only one mating partner, often for their entire lifespan.

Behavior

Many bird species are highly social and occur in large groups (Fig. 5.53). Almost 95 percent of all marine birds breed in large colonies. A rookery is a colony of nesting and breeding birds. Colonial nesting birds typically breed and hatch their eggs all at once. This strategy is an adaptation that can overwhelm predators with more newly hatched chicks than they can consume.

Unlike most other animals, most birds are capable of efficient travel over long distances. This ability allows these birds to migrate long distances in search for food, habitat, and other seasonally limited resources or conditions. Migration is the regular seasonal movement of species, typically north-south in direction between wintering and breeding grounds. The bar-tailed godwit (Limosa lapponica; order Charadriiformes) is a large wading shorebird (Fig. 5.54 A) that takes the longest non-stop flight of any bird species. Scientists made this discovery by satellite-tracking tagged birds migrating from New Zealand to the Yellow Sea over 9 days without stopping (Fig. 5.54 B).