Anyone who has ever examined a swift will agree that both the genus name Apus (Greek a-pous, footless) and the former designation of the order "Macrochires" (Greek makros, large and cheir, hand) were well chosen. The wing is conspicuously characterized by its nine or ten long primary feathers (Primaries; 10 in the Common Swift) and the compressed block of eight to eleven very short secondary feathers (Secondaries; 7 in the Common Swift). While in the evolutionarily convergent swallows the longest primary feather is at most twice as long as the shortest secondary feather, in the swift the primary feather can reach three times the length.
Primaries and Secondaries together transmit the highest thrust. Additionally, the primary feathers provide downward forces, while the secondary feathers mainly cause the ascent. This special configuration gives swifts high speed. The uniquely long and curved wing shape is well suited for flapping flight but favors gliding flight. Gliding and soaring flight reduces the need for powerful chest muscles: energetic perfection!
Swifts have ten tail feathers. The shape of the tail is very variable among swift species. Many have forked tails, which in some genera are very long and deeply notched. In swiftlets, only slightly furrowed tails occur. The short, straight-cut tails of spinetail swifts end in spiny "spikes".
The plumage of most swifts is dark, as is the case with the Common Swift: Adult birds appear brownish and show blue-violet or bronze-green metallic sheen on the plumage in sunlight when viewed closely. The throat is dull white. A complete downy plumage does not exist in the hatchling. It is completely naked until the 3rd/4th day, then nestling down appears as black dots. They are only formed in the chest, shoulder and back area and disappear under the body plumage around the 20th - 25th day. The quills of the flight feathers usually pierce the skin on the 7th day and break open on the 9th day (depending on nutrition and weather, also up to 2 days earlier or up to 4 days later). At the age of 4 weeks, the bird is fully feathered, except for the neck, whose feathers only begin to sprout after about 25 days and grow until the 35th day (WEITNAUER, 1980). The juvenile plumage is darker and duller than that of the adult bird; juvenile Common Swifts are anthracite gray to almost black with light-edged feather margins, noticeably white on the shoulders, forehead and throat, which is extensively pure white.
The rare case of an almost entirely white Common Swift could be documented in 2003 at the Swift Station in Frankfurt am Main. The not yet fully grown juvenile bird, originating from the Gießen area, had pure white body and flight plumage with the exception of its brownish-gray mottled neck and facial areas and the light gray colored feather edges. Eyes and beak were darkly pigmented as in typically colored Common Swifts.
As with many other bird species, albinistic color deviations also occur in the Common Swift. Individual white feathers are observed more frequently and are usually scattered irregularly in the plumage. In flight, such color deviations can produce confusing patterns that can lead to confusion of partially albinistic Common Swifts with swift species that have white markings, such as Alpine Swift, White-rumped Swift, House Swift and Pacific Swift (TENOVUO, 2003). The literature contains isolated reports of albinistic Common Swifts. GERBER (1958) reports on an almost fledged Common Swift with pure white plumage and red eyes, thus a total albino, which was found dead on September 4, 1951 in Leipzig. He also mentions "a white Common Swift over the southern suburbs" of Leipzig in spring 1944 (DATHE, oral) and a partial albino swift in Sempach (SCHIFFERLI, oral, 1956), as well as his own observation of a partially albinistic Common Swift that was handed over to the Leipzig Museum on August 4, 1953. In this bird, the tips of the tail feathers and almost all wing feathers were white. Further reports of partially albinistic Common Swifts come from England (BRITTON, 1979; CATLEY and SHARROCK, 1978).
Molt
The literature states that young Common Swifts only molt their flight feathers (primaries and tail feathers) in the 2nd winter of their lives (WEITNAUER 1947 and 1977). With the same feathers they left their nest with, they would thus cover the distance Europe-Southwest Africa three times! During their second wintering, the juvenile flight feathers would be renewed, at the same time the young Common 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 overwintered juvenile Common Swifts in the Swift Clinic Frankfurt. There, 8 young Common Swifts, born in June, began their primary molt almost synchronously already at the end of March of the following year. By early May, the 1st to 3rd primary had already been changed. A primary molt at a time when adult swifts are on their return flight to their breeding areas and beginning their breeding activities could be an indication that juvenile Common Swifts do not return to their birthplace in their first year of life at all, but stay in Africa and only begin the migration north in their second year of life.
The 10 primaries are molted from inside to outside, the 7 secondaries from outside to inside, as are the 10 tail feathers (DE ROO 1966). At intervals of one or two days, the corresponding feather on the left and right wing is symmetrically shed and replaced, along with the overlying coverts. The complete regrowth of a large primary feather takes about 4 weeks. The following feather is already shed before the previously molted one is completely grown.