Taxonomy and Biology

For the past 150 years, the taxonomic classification of the order Apodiformes has included three living families: Apodidae (Swifts), Hemiprocnidae (treeswifts), and Trochilidae (hummingbirds).^6,8,10,11,26 New studies on molecular evolution have revealed that swifts and hummingbirds diverged in recent times from one another; accordingly, hummingbirds were placed in their own order: Trochiliformes.^8,22 This chapter will discuss Apodidae and Hemiprocnidae families only.

The divisions within the Apodidae family remain uncertain, but two subfamilies are generally recognized: (1) the Cypseloidinae with 13 species (primitive American swifts) and (2) the Apodinae with 79 species in three tribes: Collocaliini, Chaeturini, and Apodini (swiftlets, needletails [or spinetails], and typical swifts, respectively).^8,21 An important distinction between these three tribes is that only the swiftlets and the typical swifts use saliva to glue building materials together to make rudimentary nests. None of the Cypseloidinae species uses saliva for this purpose. Apodidae species are found on all continents but Antarctica and inhabit mostly in tropical and temperate areas and close to water with an abundance of insects.^6,8 The species that breed outside the tropics are forced to migrate over long distances because of the extreme seasonal variability of insect abundance in the temperate zones. Swifts present considerable variations in size. The smallest swiftlet (the pygmy swiftlet) weighs only 5.4 grams (g) and measures 9 centimeters (cm) long, whereas the largest one (the purple needletail) weighs 184 g and measures 25 cm.⁷ The plumage of swiftlets is a dull light brown color, and these birds are considered one of the most aerial of all birds, eating, bathing, drinking, roosting, and possibly even copulating in midair.^8,17 They possess a small beak but a large gape that facilitates the aerial capture of flying insects. These small birds are long lived, with life-span reported up to 26 years.⁸

The Hemiprocnidae family comprises one genus with four species of treeswifts also referred as crested swifts. They are distributed from India and South East Asia through Indonesia to New Guinea, the Philippines, and the Solomon Islands. Treeswifts exhibit a wide range of habitat preferences, from deciduous savannah to evergreen rainforest. They differ from other swifts in that their plumage is softer and glossy and they possess long wingtips and long forked tails. Some of them have crests or other facial plumes.⁷ Slightly larger than most swifts, their total length ranges from 15 to 31 cm, with weights ranging from 21 to 80 g.^21,26

The order Coliiformes consists of one family (Coliidae) and six species of mousebirds; they are alternatively referred as coly and colies. They are found in sub-Saharan Africa, and this order is the sole order of birds restricted to the Afro-tropical region.¹⁰ All mousebirds measure 28 to 40 cm from the tip of the beak to the tip of the tail, weighing between 40 and 70 g.⁹ Their plumage is soft, and their tails are slender and long (up to two thirds of the total length of the bird); the two central tail feathers are further elongated. In some species, the tail is so elongated and stiff that it resembles the tail of a rodent.⁶ This characteristic and their sneaky movements through vegetation, similar to small rodents, give them the name “mousebird.” A unique feature of this group is the way they perch or “hang.” They suspend their bodies vertically with their tails pointing downward with their feet widely splayed at the level of the upper breast or neck area while keeping their heads right side up.^9,10 It is their sleeping position of choice. Mousebirds are one of the few groups of birds that do not possess feather tracts.

Unique Anatomy

Apodiformes possess several unique anatomic features. Like their close relatives from the family Throchilidae (hummingbirds and hermits), they have small feet that are used to perch but are not useful for walking or climbing. Although small, their feet have great strength; this, together with the sharpness of their curved nails, the calluses on their tarsi, and their stiff tail feathers, allows them to grip onto vertical surfaces. The primitive American swifts (Cypseloidinae), the swiftlets (Collocaliini), and the needletails (Chaeturini) have anisodactyl feet, in which the hallux is directed backward, whereas the second, third, and fourth digits are directed forward.⁸ In the typical grasping position, all Apodinae have their hallux and second digit spread medially and oppose the third and fourth digits, which are spread laterally as in the grasping positions of chameleons and koalas.⁸ Half or more of the long wing is composed of a long carpus, metacarpus, and phalanx bones. In contrast, they have remarkably short humerus, radius, and ulna. Also, as in hummingbirds, their coracoid is strong and is attached to the sternum by a unique shallow cup-and-ball joint.^6,17,21 The long carpometacarpus supports 9 or 10 long primary feathers and a group of 8 to 11 shorter secondary feathers.⁸ All of these birds have a claw on the manus. Swifts have no ingluvia (crop) or ceca, but a gall bladder is present.^21,22

The main difference between treeswifts and the typical apodid swifts is that treeswifts have a nonreversible hallux that allows them to perch on branches and twigs. Treeswifts lack the claw on the manus. Their tails have a deep fork, accounting for 45% to 70% of the tail length; this is considerably larger than that of the typical swift.²⁶

Mousebirds are classified in their own group (Coliiformes) because they have some unique anatomic features not found on any other birds. Thanks to a special arrangement of muscles and tendons, including two small inner muscles unique to the group and an extension to the hallux of the extensor digitorum longus, they have an incredibly flexible foot structure, which allows them to oppose one or two toes or to turn all four forward.^6,10,22 The position of the toes may change continually and may be different in either foot at the same time. With all four digits pointing forward, a mousebird may hang from a twig, or with the toes facing in opposite directions, it may grasp and perch; the position of the toes change very rapidly, accounting for some of the sudden movements only mousebirds can make. This is the equivalent of having anisodactyl, zygodactyl, or pamprodactyl feet all at once.¹⁰ Another unique anatomic feature is the presence of an “anatomic device,” similar to the ones bats have, which allows them to perch without any additional energy expenditure. The thick flexor tendons of the toes are covered with striated epithelium and pass through a grooved sheath that restrains slippage. These tendons do not insert at the bases of the outer phalanges but do so more distally; so when the leg is flexed, the claws move downward and automatically “engage” in grasping position.¹⁰ This mechanism is so effective that dead birds have been found still perched. Their wings have 10 primaries and 10 ancillary feathers; they do not possess down feathers. Their intestinal tract is short and wide, lacking ceca as might be expected of frugivores birds.

Special Physiology

Members of the Apodidae family show several physiologic adaptations to high-altitude flying. Their erythrocytes are larger than those of other bird species, facilitating oxygen exchange. Their hemoglobin is sensitized for optimal delivery of oxygen in conditions of low oxygen pressure, and their oxygen affinities are higher than in other small species of birds such as passerines.^8,19 Similarly, the erythrocytes of mousebirds are noticeably larger (in length, width, and volume) than those of other birds. The hemoglobin content per erythrocyte is relatively low in mousebirds.²

Torpor has been reported in a number of Apodiformes and Coliiformes species.^{9,14,17,20,21} Like hummingbirds, they use torpor as a way to save energy during cold nights, when food is scarce, or prior to migration. It is a hibernation-like state, in which their metabolic rate is reduced down to one fifteenth of its normal rate, thus saving up to 60% of energy expenditure. During torpor, their heart rate is reduced by about 20%, and cardiac output decreases by around 50%; body temperature may decrease down to 18° C.^2,10,21 Apodiformes and Coliiformes are capable of spontaneous arousal from this torpid state.

In addition to torpor, Coliiformes present behavioral traits associated with evolutionary thermal physiology. Clustering and sun bathing, or sunning, along with torpor, ensure the survival of the mousebird in harsh environments, particularly because their food is of very low caloric value and sometimes is scarce. During sun bathing, they fluff their feathers, exposing their heavily pigmented skin to the sun, spread their wings in an arc, and stretch and tilt their backs exposing the ventral surface of their bodies to solar radiation.^9,10 Irrespective of weather conditions, it is common to see mousebirds hanging from branches in clusters of half a dozen to a dozen birds or more. Sometimes, in harsh conditions, several clusters form one ball. Mousebirds sleep, belly to belly against each other, with the head hunched between the shoulders, minimizing the body surface area exposure to climate elements. Clustering behavior allows mousebirds to save up to 50% of energy expenditure.^10,21