Most Aerial Land Birds
Sooty tern (Sterna fuscata) — It is the most aerial of all birds. Studies have shown that during the period of immaturity and sub-adulthood, which normally runs between three to ten years, it does not visit land at all. Its only necessity to visit shores is to breed. As far as fatigue and rest is concerned it is not of much importance for which it should visit the land. It can take rest while flying, possibly during intervals of gliding or thermal soaring. The most astonishing part is that it can even moult and doze while still airborne, holding the posture, altitude and course.
Sooty terns that have no special anatomical features feed mainly on fish or squids caught from near the surface, while hovering or by seizing in mid-air as many fish jump out of water to escape marine predators. Flying fish, which usually shoot out of water are caught in mid-air. Unlike most of their cousins, they do not plunge-dive; however, surface-dive is occasionally seen.
These birds neither touch land nor water unless it is very necessary and if there is any halt in the flight, without any emergency, it is casual and incidental rather than necessitated by any other reason.
They are not known to frequent shores or near-shore waters, at any place throughout their vast range. Entirely pelagic they are most visible above the open ocean, thousands of kilometers away from land. In one example, a bird banded at Lord Howe Island was found 5,800 km, northwest in the Philippines.
Breeding in large colonies consisting of millions of pairs, fortunately, they are still one of the most abundant birds in the world despite very high rate of predation. According to an estimate in 1950s, they were yielding over ten lakh (one million) eggs per year, which is far more than any other species.
Sooties, as they are commonly known, nest in a pan-tropical belt between 30 degrees N and 30 degrees S. When they arrive at their breeding place they do not land at once instead keep on circling for several hours and calling persistently. Their call has given them the name of “wideawake” because no individual can sleep in such a ruckus.
These ground-nesting birds are classified under genus Sterna (Family Sternidae) having 32 species of which most are migratory and found worldwide. They are small to large sea birds; most of them have grey upper-parts, white under-parts and black cap. Some have short crests. They have long pointed wings and short tails, while some have deeply forked tails with long tail streamers. Due to their pointed wings and forked tails they are also known as “sea swallows”.
They have long, slender to stout, pointed bills and short legs with webbed feet. ‘Sea swallows’ inhabit coastal and inland waters and feed on fish caught by diving. In short closely related to gulls they are among the most graceful of all seabirds.
Like many other birds that live in crowded colonies, terns too have distinctive calls that help both parents and chicks to recognize each other from a distance. Terns usually nest on small sandy islands in large numbers. Till the eggs hatch parents locate their nests by features in their surroundings but once the chicks are out of eggs, they roam about throughout the island. Searching for young in such a situation is a real difficult job for parents; naturally call is the only way of finding chicks by parents and vice versa. When parents come back to the island from feeding grounds they usually circle above it, calling loudly until the chicks recognize their call and respond. Once they have spotted each other by sound there is no problem.
Terns feed on fish, which they catch by diving. They cannot see clearly underwater, but can spot the prey from above, taking aim from the air they dive and the force takes them beneath the water surface where they catch the fish.
Common swift (Apus apus)
It is also among the most aerial birds in the world. Common swift can remain airborne continuously for up to three years, from the time it leaves the nest till it mates for the first time. During this period, it is estimated that it may have covered up to 5 lakh (half a million) kilometers of non-stop flight.
About 16 cm in size this small bird possesses great endurance with the capability of flying an unbelievable 800 km in a single day. It can cruise at a maximum speed of 31 metres/second (112 km/h; 70 mph). In a single year the bird can cover at least 200,000 km. Since they spend such a long time remaining aloft they sleep, eat, drink and even mate, while on wings. This is possible because of the bird’s torpedo-shaped body and long, narrow wings much of which is taken up by the primaries that give propulsion and the secondary feathers, which provide lift.
For very long time experts were not ready to believe that such a small bird can spend nights after nights on the wing floating around in the upper air, where presumably they occasionally doze. Today it has been established beyond doubt that they do spend their nights on wings and the maximum height they attain can be around 7,000 ft.
Swifts (Family Apodidae) are born fliers. Within hours of leaving their nests, young swifts head for southeast Africa from Europe on a non-stop flight covering about 9,700 km. Even on reaching their destination, these young birds do not land there, they feed and sleep on the wing and return to Europe to breed covering the same distance again. Here they touch the ground for the first time in their lives.
Swifts do not breed rapidly. For them there is no need for it because they have few predators, only some small falcons succeed in catching them. Their survival rate of above 80 to 85 per cent and good life expectancy are some of the reasons why they have long adolescence. To feed their young these birds return to their nests with a food ball, which may contain several hundred-food items glued together with saliva. Because of the habit of aerial feeding they often face food shortage created by the variable and inclement weather, but they have also evolved different strategies to counter such problems.
In the first place the interval between egg-laying which is usually two days can be extended to one or two days. Secondly, during incubation, parents normally relieve each other, but when there is shortage of food both parents set out in search of food leaving their eggs unbrooded, as they are exceptionally resistant to cold. Not only eggs but the nestlings too are very hardy.
Swifts are superficially similar to swallows, but are not closely related to any of the passerine species. Resemblances between swifts and swallows are due to convergent evolution, reflecting similar life styles based on catching insects in flight. Family name Apodidae is derived from the Greek word meaning “footless”, a reference to the bird’s small, weak legs. There are around 100 species of swifts, normally grouped into two subfamilies (Cypseloidinae and Apodinae) and four tribes.
Swifts are among the most aerial of birds. Larger species are amongst the fastest fliers in the animal kingdom, with the white-throated needle tail having been reported flying at up to 169 km/h (105 mph).
Compared with typical birds, swiftlet wings have proportionately large wingtip bones. By changing the angle between the wingtip bones and the forelimb bones, they are able to alter the shape and area of their wings, maximizing their efficiency and maneuverability at various speeds.
Distribution and Habitat
Swifts occur on all the continents, though not in the far north or large deserts, and on many oceanic islands. Many have a characteristic shape, with a short forked tail and very long swept-back wings that resemble a crescent or a boomerang. The flight of some species is characterised by a distinctive “flicking” action quite different from swallows. Swifts range in size from the pygmy swiftlet (Collocalia troglodytes), which weighs 5.4 g and measures 9 cm (3.5 in) long, to the purple needletail (Hirundapus celebensis), which weighs 184 g (6.5 oz) and measures 25 cm (9.8 in) long.
The nest of many species is glued to a vertical surface with saliva, and the genus Aerodramus use only that substance, which is the basis for bird’s nest soup. The eggs hatch after 19 to 23 days, and the young leave the nest after a further six to eight weeks. Both parents assist in raising the young.