Habitats and Lifestyles
Hawks are a widespread and successful group that occurs on all continents and the larger islands. They are absent only from Antarctica, the ice-covered mountaintops, and the most remote oceanic islands. Hawks are primarily birds of woodland and woodland-edge habitats, and reach their greatest diversity in the rain forests of the tropics. They are components of all temperate woodlands of varying density and diversity, but some species occur in a wide variety of more open habitats, including chaparral, grasslands, desert, and tundra. Many species take readily to conifer plantations and ornamental conifer stands, provided that sufficient food is nearby. Black kites (Milvus migrans), Cooper’s (Accipiter cooperi), redtailed (Buteo jamaicensis), and Swainson’s hawks (Buteo swainsoni) exemplify species that tolerate human-modified habitats, especially farmlands, pastures, and orchards. Given their need for large territories and abundant prey, few species have either the tolerance or ability to exploit urban landscapes, although wooded suburbs and city openspace habitats do attract an occasional furtive nesting or roosting pair of red-tailed hawks or brahminy kites (Hiliastur indus). The daily lives of hawks center around finding food and avoiding enemies. To ensure an adequate food base, most hawks maintain large home ranges for at least part of the year, and some remain on territory throughout the year. Ifprey populations suffice, hawks are sedentary, but if they decline, hawks must move elsewhere. Some species are nomadic, often wandering widely in search of food, but most northern species move southward in spectacular migrations along traditional flyways each fall. Red-tailed hawks migrate only a few hundred miles but others, such as the lesser spotted eagle (Aquila pomarina) and Eurasian buzzard (Buteo buteo), fly thousands of miles to their wintering grounds. Their return migration in spring is timed to ensure arrival on the breeding grounds when weather conditions and prey populations are optimal. Most hawks are entirely carnivorous, or nearly so, and use their remarkable eyesight and intimate knowledge of their territory to locate, pursue, and capture food. Their large eyes are set forward in the head with overlapping fields of vision. Each eye has a long focal length from lens to retina, which produces a telescopic vision that enables hawks to spot prey at great distances. Hawk eyes also have two concentrations of visual cells, called fovea, instead of one, as in other birds and mammals. The central fovea and lateral fovea apparently permit the simultaneous estimate of both distance to and movement of prey, thereby increasing hunting efficiency when pursuing agile, fast-moving animals. Most hawks are larger—some considerably larger—than the prey that they take.Manyspecies exhibit pronounced sexual dimorphism, with the female being larger than the male. Bigger females can better incubate eggs and protect young, but the sexual size difference enables exploitation of a much wider prey range. Sexual dimorphism is best illustrated in the bird-eating accipiter hawks, in which the larger female takes grouse and ducksized prey while the smaller male pursues the smaller and more agile birds. Conversely, insectivorous and scavenger hawks show little or no sexual dimorphism. Almost all hawks that hunt live animals have comparatively light bodies on large wings for a low wing loading, which facilitates extended periods of soaring in search of prey and also for carrying prey. Many species show subtle differences in overall size and wing shape that reveal differences in hunting techniques or diet. For example, the long, narrow wings and light bodies of swallowtailed (Elanoides forficatus) and plumbeous (Ictinia plumbea) kites enhance their aerial maneuverability for catching insects just above the rain forest canopy. In contrast, the long, broad wings of hawks and eagles maximize lift needed to soar for hours or to carry medium-sized mammals and birds, along with an occasional reptile or amphibian. The larger hawks and eagles take correspondingly bigger prey up to the size of rabbits, hares, hyraxes, sloths, small antelope, foxes, and young deer. The long tails and rounded wings of bird hawks such as goshawks, sparrow hawks, hawkeagles (Spizaetus spp.) and bat hawks (Macheirhamphus alcinus) provide just the right combination of speed and agility to pursue birds through the often cluttered microhabitat just beneath the woodland canopy. The long, dihedral wings of harriers (Circus spp.) provide a stable flying platform for slow quartering flight over open grasslands and marshes. Harriers use both eyes and ears to hunt small mammals in the tall grasses and, like owls, have a distinctive facial ruff that helps gather sounds to pinpoint prey in the tall grasses. Hunting and dietary specializations abound in this group. At least twelve species of kites are insectivorous or nearly so, but many other hawks and eagles feed opportunistically on abundant locusts, and grasshoppers, ants, termites, and locust swarms invariably attract a variety of hawks and other birds. Reptile specialists include the snake eagles (Circateus spp) and serpent eagles (Spilornatus spp.), which sit quietly for hours, patiently watching for the slow and stealthy movements of snakes. Possibly the most extreme dietary specialist is the rufous crab-hawk (Buteogallus aequinoctialis), which picks through aerial roots of mangroves in search of crabs, but snail kites (Rostrhamus sociabilis) and slender-billed kites (Rostrhamus hamatus) focus almost exclusively for pulmonate snails, which they extract with their long, hooked bills.
Courtship and Nesting
Most hawks breed each year, but some of the larger tropical eagles and snake eagles may breed once every several years. All phases of the breeding cycle of hawks are dependent on prey abundance: In low prey years pairs often do not nest and may not even establish a territory, while in high prey years nesting success, measured by the number of eggs produced, hatched, and young fledged, is highest. Tropical hawks may nest at any time of year, but usually do so during the dry season. Temperate species are more strictly tied to a spring and early summer nesting cycle when prey abundance is maximal and longer days permit extended hunting periods. The breeding cycle of temperate species begins when males or mated pairs claim and advertise territories by often extravagant displays that may include sky dancing, dives and swoops, perches, calling, and posturing. Unmated males court females with aerial displays such as circling, following flights, sky dancing, and food transfers from males to females. As courtship proceeds, the pair bond is cemented by synchronized flights, following flights, mutual preening, food begging by the female, and courtship feeding. Territory size depends on food needs and prey density. Smaller tropical hawks may limit their breeding activities to a hundred acres or so, while the larger eagles that inhabit open habitats, such as the Scottish golden eagles, sometimes establish home ranges of thousands of acres on the moors and grasslands. Most hawk territories are coneshaped, with little or no overlap permitted near the base of the cone, which is typically centered near the nest site or feeding site. Hawks build stick nests in trees (for example, Cooper’s hawks, sharp-shinned hawks), on cliffs (ferruginous hawks, Buteo regalis), bluffs and outcrops (Swainson’s hawks, Buteo swainsoni), on structures (ospreys, red-tailed hawks), or directly on the ground (harriers). Depending on habitat, some species are flexible in choice of substrate; thus, red-tailed hawks may build their nests in trees, on ledges, less frequently on structures, or directly on the ground. Many hawks (ferruginous hawks, Swainson’s hawks) readily accept artificial platforms such as bridge abutments, piers, telephone poles, and towers. Both members of a pair bring sticks to the nest site, but the female does the actual work of nest building, arranging the materials into a compact nesting platform with a shallow bowl. Many species refurbish one or more nests year after year, sometimes resulting in massive nesting platforms of sticks and branches after many years of use. Nests are generally lined with dried grasses or mosses and many may decorate them with sprigs of greenery. Depending on species, fromone to ten eggs are laid at two- to four-day intervals, and incubation begins immediately. In most species, the female incubates the eggs while the male supplies her with food, which he may deliver to her or to a nearby site. In some species the male may replace her for brief periods of time to allow her to rest, roost, or forage. The female is often highly secretive during this nesting phase as she hunkers down to incubate the eggs. The young hatch in about one month in the smaller accipiters and kites, and up to two months in the larger eagles. Eggs hatch asynchronously, producing a nest of uneven-aged young. The female keeps constant company with the newly hatched young, brooding them and feeding them with food supplied by the male. As the young grow, their food demands increase and both male and female spend most of their time hunting food to feed the young. Adults become increasingly aggressive with young in the nest. Generally, both adults participate in nest defense, although the female is almost invariably the most aggressive. Warning cries (most hawks), power dives (many buteos), and determined slashing attacks (goshawks) can drive all but the most determined intruders away, but unfortunately have little effect on humans intent on destruction of the nest and young. Otherwise, the only natural predators of eggs and young are nest-robbing birds, chiefly corvus species, mammals, other larger diurnal raptors, and the larger owls, especially the Bubo species, which take young during the night. Survival of the young depends entirely on the ability of the adults to find sufficient food. If prey is sufficient, all of the young successfully fledge, but if prey populations decline, one or all of the smallest and weakest young of a nest die of starvation. Nestling fratricide has been observed inmany species and may help ensure survival of the strongest young at the expense of their younger or weaker nestlings. After leaving the nest, the young usually stay with the adults for several weeks or longer—a year or more in some tropical eagles—learning how to perfect their hunting skills and avoid enemies. Many retain a streaked or spotted juvenile plumage for several years that may help conceal them from potential enemies. This plumage is replaced at sexual maturity by adult coloring that probably promotes sexual advertisement and territorial displays.
Conservation and Economic Importance of Hawks
Humans are the most consistent and potent threat to hawks. Historically, huge numbers of hawks were slaughtered by farmers, hunters, gamekeepers, and trappers for sport and as control measures designed to increase game and poultry populations. Many hawks still fall victim to illegal shooting by hunters and sportsman who prefer live target practice, while accipiter hawks are sometimes shot by pigeon fanciers or grouse managers to protect their interests. Generally, the larger buteo hawks and eagles are at greater risk because of their size and habit of selecting open, conspicuous perches for territorial posturing and while hunting. Globally, some hawks are still slaughtered for the taxidermy market, some are taken for falconry, and some are captured as part of the lucrative international trade in zoo specimens, despite the fact that they are all federally protected in most countries of the world. Hawks also fall victim to pesticides and industrial wastes, which accumulate within their tissues and eventually reach harmful levels that result either in death of the bird or reduction of its productivity. Toxic levels of chemicals have been documented in almost every hawk species on every continent and have particularly affected populations and productivity of fish-eating hawks, such as the osprey and bald eagle. While all of these forms of deliberate and inadvertent human persecutions take an annual toll, the most persistent threat to hawk populations results from habitat destruction or fragmentation and deforestation. Habitat losses are especially critical for hawks, which require large areas of undisturbed habitat for home range to provide a food base for themselves and their young. Although no species have become extinct in the last several hundred years, twenty-nine species are on the current list of endangered or threatened species, and it took concerted efforts by state and federal programs on many levels to save several species, such as the bald eagle and osprey. Bans on dichloro-diphenyl-trichloroethane (DDT) and other environmental poisons, along with extensive programs of captive breeding, nesting platforms, and reintroduction programs have helped reestablish bald eagles (Haliaetus leucocephalus) in North America and red kites and white-tailed sea eagles (Haliaetus albicilla) in Scotland. Conservation programs continue to target the protection and recovery of many species throughout the world, including the harpy eagle in Latin America and the Philippine eagle (Pithecophaga jefferyi) of the Philippine Islands.
No comments:
Post a Comment