The skeleton of the Common Swift has developed some special features in adaptation to its purely aerial lifestyle compared to other bird species.
The Common Swift has a relatively large, markedly domed cranium. The size of the eye socket is striking. Compared to other birds of its size and body mass, the Common Swift has very large eyes that dominate the face, guaranteeing a wide field of vision and being necessary not only for fast flight maneuvers but especially for hunting small prey. A swift can turn its eyes noticeably forward, thus reducing the divergence angle of the eye axes, which allows the bird to fix on prey binocularly until the last moment before snapping. In the contour feathers of the head, the eye lies embedded in a niche, additionally protected rostrally by a fine, protruding, brush-like row of feathers. Similar to many birds of prey, a prominently projecting feather edge runs above the eye on a bony base, descends towards the nose and forms the front boundary of the eye niche. The niche construction, covered by feathers like a ridge, undoubtedly serves to protect the swift"s eyes, which are particularly at risk during potential collisions, but could also serve as a windbreak or prevent foreign objects from entering the eye when flying through insect swarms. The appearance of this distinctive brow ridge leads many people who find swifts to mistake their foundling for a small bird of prey.
The Common Swift has a short, triangularly shaped, tapering upper beak that includes relatively large nostrils, and a delicate, arch-shaped curved lower beak that inserts below the ear at the jaw joint. When the beak is closed, only the small, horny, slightly downward-curved tip of the beak is visible. When the swift opens its beak, the mouth opening reaching up to below the eyes appears enormous. The highly elastic throat area ("throat sac") spans between the arch-shaped lower jaw branches; the open mouth is reminiscent of a landing net. Feeding adult swifts collect the caught insects, stick them together with saliva into a hazelnut-sized food ball and transport them to the brood. There is no crop.
In the Common Swift, the relatively broad, densely feathered head appears to sit directly on the shoulder girdle. The compact, streamlined head-body line conceals an arch-shaped curved cervical spine with 12 cervical vertebrae. An elongated neck section would break through the energetically favorable torpedo shape and would thus be a disadvantage for the continuously flying insect hunter. Nevertheless, the head is very mobile. Like owls, the Common Swift can also turn its head 180°. When clinging to house or rock walls, e.g., when searching for nesting sites, the ability to look all around is certainly useful. Even more important may be the extraordinary flexibility in the seemingly short neck area for the vital feather care, which swifts usually have to perform during flight: To clean and smooth even the longest primary feathers by pulling them through the beak requires perfect balance and movements of the head and neck that appear acrobatic.
The Common Swift has a complete shoulder girdle skeleton, consisting of the narrow, curved strap-shaped Clavicula (wishbone), the relatively long, flat, saber-shaped Scapula (shoulder blade) and the compact Coracoid (coracoid bone) as the strongest bone. Compared to other bird species, the very compact, squat arrangement of the individual shoulder girdle bones is noticeable.
The Coracoid, which sits broadly and flatly on the Sternum (breastbone), and the Clavicula, designed like a tension spring that holds the shoulder joints apart, are of crucial importance for flight performance. Presumably, the short, compact form and small distance from the shoulder joint to the attachment of the Coracoid to the Sternum ensures more direct and improved force transmission from the wing to the body. Furthermore, the massive, compact skeletal anatomy in the shoulder girdle possibly reduces the risk of injury to which a bird with high flight speed and an exclusively aerial lifestyle, flying day and night, is particularly exposed. Powerful muscle masses covering the upper arm, shoulder and front chest provide additional protection against fractures and dislocations.
The wing skeleton of the Common Swift shows the characteristic adaptation for a high-performance flier with a short, compact upper arm, relatively short forearm and extremely long hand and finger area.
The upper arm of the Common Swift seems to serve solely as the attachment for the powerfully developed shoulder, chest and upper arm muscles, in which it is completely embedded, effectively protected from fractures. Proportionally one-third longer than the upper arm are the ulna (Ulna), to which the ten secondary feathers attach, and the cranially relatively unprotected radius (Radius), whose exposed position makes it an "Achilles heel" for the Common Swift when the front edge of the wing hits obstacles.
The metacarpus (Carpometacarpus) of the Common Swift is as long as the ulna and radius and thus proportionally considerably longer than in most other bird species. Through the fusion of several carpal bones and a tight articular connection to the 2nd finger, it has considerable stability and serves as the attachment for the 1st - 5th primary feathers. In active flight, the greatest stress falls on the primary feathers and the hand skeleton of the bird. In this area there is hardly any musculature, but almost only tendons; hand and finger bones are covered only by thin skin and covering feathers.
Seven pairs of ribs can be identified in the Common Swift. The rib cage encloses the internal organs from front to back in a pear shape. The large, dominant Sternum provides the attachment for the powerful flight muscles, which when viewed from the side has approximately the shape of a backward-pointing acute triangle.
In the course of evolution, the necessity for ground locomotion has largely become obsolete for the Common Swift and has been reduced to a crawling motion in the nesting cavity, equally skilled forwards and backwards. The wings are used alternately for balance support. The upper body is almost parallel to the ground. The hip joints sit high in the rear body area. The center of gravity is low; obviously the entire body construction is designed for flying and only allows a clumsy-looking type of locomotion on the ground, which can nevertheless be surprisingly fast and agile. In confrontations, the adult swift can even stand up tall and run towards an opponent with spread wings; similar behavior is observed in hungry begging juvenile swifts who pounce on the food-bringing adult bird with wildly beating wings.
The short straight thigh, knee joint and long narrow lower leg lie laterally against the body and almost completely merge with the torpedo-shaped body silhouette. Notable is the short, strong tarsus (Tarsometatarsus) and especially the remarkable claws.
Like most bird species, the Common Swift also has four toes. BEZZEL and PRINZINGER (1990) and HUMMEL (2000) state that in swift-like birds all four toes point forward. GLUTZ and BAUER (1980) also list as a characteristic of the genus Apus that all four toes are directed forward postembryonically.
This is contradicted by our own observations of the patients in the Swift Clinic: The 2nd - 4th toe of the Common Swift points forward, while the 1st shows great flexibility and is moved like a "reversible toe" inward and forward. Only in resting position does it happen that all four toes are held directly forward. When climbing and swinging, a sideways arrangement of the 1st and 2nd as well as the 3rd and 4th toe is shown, similar to a grasping hook. Otherwise, all four toes on the foot of the Common Swift are evenly distributed sideways and forward like the fingers of an outstretched hand, and when released from the ground, they immediately reflexively seek hold, continuously opening and closing. Their considerable mobility and flexibility are striking.
The unique pincer foot
In birds, each toe consists of a basal phalanx and a claw bone that carries the horn sheath. Intermediate phalanges are inserted between them: one for the 2nd, two for the 3rd and three for the 4th toe (HUMMEL, 2000). The situation is completely different and possibly unique in the bird world for the Common Swift and its larger relative, the Alpine Swift: All four toes consist only of the basal phalanx and claw bone. There are no intermediate phalanges.
Studies on other members of the order Apodiformes would be of great interest to determine whether Common Swifts and Alpine Swifts occupy a special position regarding the anatomy of their feet, or whether this highly unusual feature for birds also occurs in other swifts.
The scientific name of the Common Swift – apus, without foot – testifies to a long-standing misconception that persists to this day, when colloquially the reduced or even atrophied feet of the swift are mentioned: "Apodes, that is called without feet / not that they have no feet at all / but because they cannot use them" (GESNER, 1669).
A Common Swift has anything but reduced or atrophied feet. Its hind limbs are just another example of a process of evolutionary adaptation and reduction (not simplification!) to what is essential for its living conditions. It has extremities that are equally suitable as tools and weapons.
The functions and advantages that result for the Common Swift from the skeletal anatomical peculiarity of its two-jointed pincer foot are obvious. For a bird that naturally never lands on the ground or in trees (exceptions are some rare individual observations of Common Swifts sleeping hanging in trees in Africa), a "normal" bird foot suitable for grasping and holding branches has no adaptive advantage, but a strong, hook- or pincer-like gripping tool with which the bird can securely cling to vertical walls and ledges when searching for nesting sites certainly does. Toes with only two joints allow optimal force application. Such a hook or pincer foot is also extremely effective as a weapon or for defense. Common Swifts in shock or panic can dig so firmly into a human hand that they perforate the skin and can only be released with the utmost difficulty, because the inward-pointing, sharp claws anchor themselves like barbs.
In juvenile swifts, the gripping strength of the claws is of particular importance and prevents them from falling out of the nest. This early-developed gripping ability reaches extreme and astonishing proportions, for example, in African Palm Swifts, which breed vertically on the back of a palm leaf and whose young must cling to their swaying nesting site almost upside down. It is to be assumed, but would still need to be proven, whether the anatomy of these swifts shows similar adaptations.