Anyone who has ever examined a swift will agree that both the genus name Apus (Greek a-pous, footless) and the former name of the order “Macrochires” (Greek makros, large, and cheir, hand) were well chosen. The wing is strikingly characterised by its nine or ten long primaries (Primaries; 10 in the Common Swift) and the compressed block of eight to eleven very short secondaries (Secondaries; 7 in the Common Swift). Whereas in the swallows and martins, which have evolved along convergent lines, the longest primary is at most twice as long as the shortest secondary, in swifts the primary can reach three times that length.
Primaries and Secondaries together generate maximum thrust. In addition, the primaries provide downward-directed forces, while the secondaries mainly produce lift. This particular configuration gives swifts their high speed. The uniquely long, curved wing shape is well suited to flapping flight, but favours soaring flight. Gliding and soaring reduce the need for powerful pectoral muscles: energetic perfection!
Swifts have ten tail feathers. Tail shape varies greatly within the swift family. Many species have forked tails, which in some genera are very long and deeply notched. In the swiftlets, only slightly notched tails occur. The short, square-cut tails of the needletails end in spiny “spines”.
The plumage of most swifts is dark, and this is true of the Common Swift as well: adults look brownish and, seen close up in sunlight, show a blue-violet or bronze-green metallic sheen on the plumage. The throat is dull white. Hatchlings have no complete covering of down. They are entirely naked until the 3rd/4th day, after which nestling down appears as black dots. It develops only on the breast, shoulders and back and disappears beneath the body plumage around the 20th to 25th day. The quills of the flight feathers usually pierce the skin on the 7th day and burst open on the 9th day (depending on nutrition and weather, up to 2 days earlier or up to 4 days later). At the age of 4 weeks the bird is fully feathered, with the exception of the neck, whose feathers only begin to sprout after about 25 days and continue growing until the 35th day (WEITNAUER, 1980). The juvenile plumage is darker and duller than that of the adult; juvenile Common Swifts are anthracite grey to almost black with pale-fringed feather edges, conspicuously white on the shoulders, the forehead and the throat, which is pure white over its whole area.
The rare case of an almost entirely white Common Swift was documented in 2003 at the Swift Rescue Center in Frankfurt am Main. The not yet fully grown juvenile, which came from the Gießen area, had pure white body and flight feathers, apart from its brownish-grey mottled neck and face and its light grey feather fringes. Eyes and bill were darkly pigmented, as in normally coloured Common Swifts.
As in many other bird species, albinistic colour aberrations also occur in the Common Swift. Individual white feathers are seen fairly often and are usually scattered irregularly through the plumage. In flight, such aberrations can create confusing patterns, so that partially albinistic Common Swifts may be mistaken for swift species that have white markings, such as the Alpine Swift, White-rumped Swift, Little Swift and Pacific Swift (TENOVUO, 2003). The literature contains isolated reports of albinistic Common Swifts. GERBER (1958) reports an almost fledged Common Swift with pure white plumage and red eyes – a complete albino – which was recovered dead in Leipzig on 4 September 1951. He also mentions “a white Common Swift over the Südvorstadt” district of Leipzig in spring 1944 (DATHE, pers. comm.) and a partially albinistic swift at Sempach (SCHIFFERLI, pers. comm., 1956), as well as his own observation of a partially albinistic Common Swift handed in to the Leipzig Museum on 4 August 1953. In this bird the tips of the tail feathers and almost all the wing feathers were white. Further reports of partially albinistic Common Swifts come from England (BRITTON, 1979; CATLEY and SHARROCK, 1978).
Moult
The literature states that young Common Swifts do not moult their flight feathers (primaries and tail feathers) until the 2nd winter of their lives (WEITNAUER 1947 and 1977). This would mean that they cover the route from Europe to south-western Africa three times with the very feathers they left the nest with! During their second winter in Africa the juvenile flight feathers would be replaced; at the same time the young swift would become sexually mature and return to Europe in adult plumage in the 2nd spring of its life.
This is contradicted by recent observations of juvenile Common Swifts overwintered at the Swift Rescue Center in Frankfurt. There, 8 young Common Swifts hatched in June began their primary moult almost synchronously as early as the end of March of the following year. By the beginning of May, primaries 1 to 3 had already been replaced. A primary moult at a time when the adult swifts are on their return flight to the breeding areas and are starting to breed could be an indication that juvenile Common Swifts do not return to their birthplace at all in their first year of life, but remain in Africa and only set out on the northward migration in their second year.
The 10 primaries are moulted from the inside outwards, the 7 secondaries from the outside inwards, as are the 10 tail feathers (DE ROO 1966). At intervals of one or two days the corresponding feather is shed symmetrically on the left and the right wing and replaced by a new one, together with the coverts lying above it. Full regrowth of a large primary takes about 4 weeks. The next feather is shed before the previously moulted one has grown to its full length.