Swifts of the World

Even though many people confuse members of the two swift families with swallows and martins, once one has understood some of the differences, then observed these birds in action – one never forgets that swifts are not only different but very special.

Some special experiences I have enjoyed with swifts were:

• To see and hear a swiftlet echolocate its contorted route through a complex cave to its nest in total darkness, one kilometre from the cave entrance.
• To witness the “playful” power-dives of a needletail, from spiralling at the base of a high cumulus cloud to below head height in a few seconds.
• To discover the fancy streamlines of a treeswift sitting on its all-too-small nest, perched high in a tropical rainforest.
• To have a flock of 20-30 Common Swifts narrowly miss your head as they noisily sweep past in an evening proliferation of energy known as a screaming party.
• To realise after listening to the radio transmitter attached to a nestling Common Swift for 12 hours - through the night, that this fledgling’s first flight was spent thousands of feet up in the air and in darkness.
• To realise that the swift and swiftlets nest is built from or held together by their saliva: Natures first superglue.
• All of these experiences can only make one wonder at the splendid abilities and finesse of this group of birds.

In 1854 the Common Swift (Apus apus) of Europe was also called the Black Martin, and the Chimney Swift (Chaetura pelagica) was called the American Chimney Swallow, and the Glossy Swiftlet (Collocalia esculenta) of south-east Asia was called the Esculent Swallow (Maunders 1854). At the time swifts and swallows were classified in the same family: Hirundinae. They have since been separated into their own families as we have learnt more about them. We are still learning about them, and invite you to share in the enjoyment of discovering new facts about their design and behaviour. So what type of birds are swifts?

Swifts are characterised by:

1. A short neck & humerus.

Humerus bone.

Photo: Common Swift skeleton copyright Jean-Christophe Theil.

2. A protective ridge of feathers above the eye to protect the eye from damage in collisions with prey.

Uniform Swiftlet, Mt Diamond, PNG. Photo: copyright M. Tarburton.

3. Very small bill, but very wide mouth (gape).

Common Swift, Germany. Photo: copyright E. Kaiser.

4. Flying continuously during daylight except when nesting.

Common Swift, Belgium. Photo: copyright Raymond De Smet. Common Swift, France. Photo copyright J.F. Cornuet.

5. Never folding their wings between wing-beats as do swallows.

6. Small but very strong feet and toes that support their full weight when hanging from tree-trunk, foliage or cave roof, while asleep.

Foot of White-rumped Swiftlet roosting in a Lava tube cave, Samoa. Photo copyright M. Tarburton.

Foot of White-throated Needletail, Qld, Australia. Photo copyright M. Tarburton.

7. Long wings that can be swept back to reduce drag and increase speed, allowing them to glide without wing-beats in any direction on the slightest of breezes.

Fork-tailed Swift

Apus pacificus

Qld 5/2/2005

Photo copyright Ian Montgomery: birdway.com.au

The distinctive scythe shaped curve of the wings leading edge generates leading edge vortices providing lift and thrust beyond that of any other birds measured so far and when supported by changing wing-shape, high levels of oxygen-carrying haemoglobin in both blood cells and plasma (Palomeque et al. 1980), swifts are able to achieve many activities while in the air. This helps because they are in the air all day. They take all their food and nesting materials from the air, and they fly up to heights of three or four kilometres (Gustafson et al 1977, Tarburton 2009). They even drink, bathe, mate and can spend the night on the wing (Lack 1956, Tarburton & Kaiser 2001).

Most species have enlarged salivary glands for production of the glue-like saliva used in nest building and for holding their food boluses together until they return to the nest to feed to the nestlings, which they only do at widely spaced intervals. Some species only feed their nestling(s) two to four times a day. Tests show they can go without food for two or three days. This means that young swifts have a cyclone-proofing that most birds do not enjoy. Sexes are similar and both share in breeding activities, such as incubation and nestling provisioning.

Swift physiology combined with their morphology enables them to fly using little energy compared to other birds. Their flying metabolic rate is only 2-5 times their sleeping metabolic rates, whereas most other birds have an increase of about 12 times (Lyuleeva 1970). Lets look at how the morphology of a swifts wing works.

The traditional method of explaining how a bird or plane wing provides lift is to show that air flowing over the top of the wing has further to go than air passing under the wing. So air particles going over the wing are stretched further apart – reducing air pressure, compared to air going under the wing which remains more dense where the higher pressure pushes up. Together these features provide lift.

If the angle of attack of the wing is increased, drag is increased, slowing the wing down as well as the attached bird or plane. In fact one leading authority on the evolution of birds (Welty 1975. p.2) referring to bird design said that birds compared to mammals “simply dare not deviate widely from sound aerodynamic design. Nature liquidates deviationists much more drastically and consistently than does any authoritarian dictator.” It is always dangerous to make generalisations, and particularly so for scientists.

Greater knowledge arrived in 2004 when Science journal broke the news that:

“The current understanding of how birds fly must be revised, because birds use their hand-wings in an unconventional way to generate lift and drag.” (Videler et al. 2004). They use the lift for gliding and flapping flight and they generate high drag when they want to brake or land. This finding was based on solid moulds made in the shape of the wing of the Common Swift (Apus apus) being studied in flowing water tunnels at Groningen & Leiden Universities in the Netherlands.

These researchers found that the arm-wing uses the traditional aeronautics system, but that the distal hand wing (which in swifts is most of the wing) is much thinner in cross-section and is designed to set up a series of vortices along the front upper edge of the wing. These are called leading edge vortices (LEVs) and they travel along to the tapered tip of the wing, providing lift all the way and then are released at the tapered tip without the normal amount of drag behind the wing.

Then these modified aerodynamic theories were shown in 2008 to be inadequate by testing, not models but the swifts themselves flying in low turbulence wind tunnels at Lund University, Sweden (Henningsson et al 2008, 2011). The Dutch had already postulated a theory (Lentink et al 2007) to explain how a swift can glide and make sharp turns at both low and high speed and even to sleep on the wing, by changing the shape of the wings. Now the Swedes were demonstrating it to be true.

This new research showed that in swifts the upstroke of the wing produced thrust as well as lift equal to 60% of the lift of the downstroke. Most birds do not produce any lift on their upstroke. They also found that the lift/drag ratio in the swift was the highest (13) of any bird measured so far. The team believes these phenomena result from the stiffness of swift wing feathers, the sweep of the wing, the tapered tip, as well as a changing wing shape. The swift wing generates clockwise leading-edge vortices on the downstroke and anti-clockwise leading-edge vortices during the upstroke. Put all this together and it has the effect of increasing manoureveability in a bird that is otherwise designed for speed. For a bird that feeds on flying insects this would be a big help. This also answers the question insect and swift researchers could not answer till now. How come many species of swifts and swiftlets at times catch disproportionately more flies than other orders of flying insects, when compared to the proportion of flies to other flying insects in the air, (Tarburton 1986) & when entomologists tell us flies are the most manoeuvrable group of flying insects? (Hespenheide 1975).

SYSTEMATICS

Swifts are grouped into two families: the APODIDAE (Typical Swifts) and HEMIPROCNIDAE (Tree Swifts).

Typical Swifts Tree Swifts

Class Aves Aves

Order Apodiformes Apodiformes

Family Apodidae Hemiprocnidae

Sub-Family Cypseloidinae Apodinae

Tribe Cypseloidini Chaeturini Apodini Collocaliini

Genera Cypseloides Mearnsia Aeronautes Collocalia Hemiprocne

Streptoprocne Zoonavena Tachornis Aerodramus

Telacanthura Panyptila Schoutedenapus

Rhaphidura Cypsiurus

Neafrapus Tachymarptis

Hirundapus Apus

Chaetura

Number of genera & species:

19 genera; 103 species

Distribution

Almost world-wide. Main regions avoided are Antarctica and the Arctic, with the Arctic being given a wider berth in central and eastern Asia.. Most species live in the tropics.

Habitat

Swifts forage on aerial plankton taking a wide range of flying insects and hatchling spiders that drift through the air on strands of web, and so may be seen over a wide range of terrestrial habitats, but for roosting and sleeping they are dependent on cliffs, caves, hollow trees or branches or foliage of either tall trees, or smaller trees on high ground. However, it has been shown that some swifts can spend the whole night in the sky (Tarburton & Kaiser 2001).

Size

Length range = 90-250 mm; Wing Length 86-234 mm Mass range = 5-205 g. The smallest species is the Pygmy Swiftlet Aerodramus troglodytes of the Philippines with a length of 90-92 mm (tip of bill to tip of tail), wing length 86-99 mm and weight 4.5-6.8 g. (Hartert 1897, Oberholser 1906, Rand & Rabor 1960, Dickinson 1989, Dunning 1993). The largest swift is the Purple Needletail Hirundapus celebensis and though found in Indonesia and the Philippines, very few birds have been measured. A small sample were length 229-234 mm, Wing 218-220 mm, and weight 170-203 g. (Morse & Laigo 1969).

Common Myths about Swifts

1. Swifts cannot take off from the ground. Even books about swifts say “a downed swift is doomed”, or “With long narrow wings shaped for speed, and short weak legs, a swift that is brought down has a poor chance of survival” (Bromhall, D. 1980. Pp. 48 & 47). This is despite David Lack stating as far back as 1956 (p. 119), that “Contrary to popular belief, it is not impossible for a swift to take off from a flat surface”. Yet Campbell (1964) in The Oxford Book of Birds says “If a swift lands by accident it cannot take off again” I have placed White-throated Needletails on the ground and watched them take off. Paul Jones has also seen one of these needletails crash into his fire lookout-tower, fall to the ground, in a stunned condition, then recover and fly away (personal communication). In 1985 Leidgren published his observation of seeing a Common Swift tumble to the ground in battle with a Starling. When the Starling released it’s grip the swift very easily flew away. Leidgren has also seen one crash after hitting pine branches, but it took off from the ground and entered the nest box as if nothing unusual had taken place. Erich Kaiser & I have watched Common Swifts take off from the floor of his house in Kronberg Germany, and Erich believes this myth started from the observation that injured and starving swifts cannot take off from a flat surface.

2. The legs of Swifts have become almost useless.

Bromhall (1980 p.8) will also provide an example for those who publish the idea that swifts legs are small and almost useless. It is true that they have short legs but they are not useless. They are very strong, with larger species easily drawing blood from a human hand. I have found this to be particularly true with White-throated Needletails. But even at the other extreme, swiftlets feet are very strong. In fact to successfully remove nestling swiftlets from their nests one has to learn how to get them to release their grip on the nest or cave wall/roof, or their claws (toe-nails) (which do not grow again) will be pulled off, so strong is their grip. It is important that swiftlets keep their toenails in order to roost on the cave roof. In most locations that I have studied swiftlets; cats, pythons and rats eat those that that roost near or on the cave floor. Another indicator of the importance of having strong feet & legs is that the legs of White-rumped Swiftlets reach adult size in just 13.5 days whereas the wing takes over 50 days to reach adult size (Tarburton 1986, 1987).

3. Australian Swiftlets emigrate north of the equator for the Austral winter.Australian Swiftlets migrate in the winter time.

Pecotic (1974) visited three swiftlet nesting caves at Chillagoe in the winters of 1965-1967 and not finding any birds in the caves during daylight hours, published that “the birds were away, presumably north of the equator”. If he had gone back in the evening he would have found the swiftlets roosting in the cave. Alternatively he could had looked at the guano pile to determine if there were any white spots of guano on the black areas. The white component disappears in one to two weeks so is a good indicator as to whether the swiftlets are still resident in the cave or not. I have met several people who still thought this bird migrates north for the winter.

It had also been claimed that White-rumped Swiftlets of New Caledonia “must be migratory, as during the summer months we did not observe them” Layard & Layard (1879). The previous year Layard & Layard (1878) had claimed the opposite “common in the cold weather, up to the end of September” and not seen since early October [Summer] so “we think partially migratory”. The birds are known to be resident on New Caledonia all year, and the historical statements are just a further example of observers jumping to conclusions before having adequate data.

4. No nestling swifts have down feathers.

While all swifts studied to date are born naked (i.e. have no natal down), it is now known that the Chestnut-collared Swift (Streptoprocne rutila) and probably also the American Black Swift (C. niger) start to grow a covering of downy feathers when eight to nine days old (Collins 1963). Both these swifts breed in cool damp locations and these downy branches of the early body feathers are assumed to help keep them warm in such locations. A third swift the African Palm Swift Cypsiurus parvus has also been shown to grow these downy semiplumes to help them thermoregulate in cool nest sites during the long absences of their parents (Collins 1965). It is true that most swifts do not grow downy feathers.

5. Swifts always roost before dark.

Breeding Common Swifts are almost always at their nests before dark (Lack 1956,). Erich Kaiser (pers comm) has even had Common Swifts return ahead of bad weather to sleep in their nests, three days after they left on migration and apparently had run into bad weather. However, Lack (1956, p. 126) has documented a series of observations where Common Swifts have been found trying to roost after dark in unusual places – particularly in misty weather. Accumulated evidence suggests these are mostly young migrating birds not familiar with good roost sites that they can find in cold and misty weather, and so they get caught in the open after dark. In Southern Sweden, Holmgren (1993) has shown that migrating Common Swifts coming in to roost in the foliage of trees 10 to 40 minutes after sunset, on both foggy and clear nights are almost always pre-breeding birds, likely from Lapland and on their way to Africa.

Researchers at Rancho Grande, in Venezuela commonly observe eight species of swifts, but only one of them the Spot-fronted Swift Cypseloides cherriei is frequently found being attracted to lights well after sunset in dense fog (Beebe 1949, 1950). Collins (1980) also found the same situation continued there and concluded that this swift might be feeding further away than the other species and on foggy nights are thus more likely to have difficulty finding their way back to their nests. It is suggested that this species feeds further away to partition the resource with the other swift species in the area. This practice might get more food, but on foggy nights they may get caught out.

The Australian Swiftlet is almost always back into its roosting cave before dark (Tarburton pers obs) but the White-rumped Swiftlet (Aerodramus spodiopygius) in Fiji rarely returns before dark and some are still returning 4 hours after dark. The Fijian birds roost in larger colonies than the Australian birds, and so resource partitioning may be occurring within the species, just because the populations are so large. The Fijian species has no competing swifts. More recently I found that Three-toed Swiftlets (A. papuensis) are also feeding a long way from their roost, with most birds coming into the cave 1-3 hours after sunset and some still entering after midnight (Hamilton et al 2001, & Tarburton unpublished). Both these swiftlets are habitually late returning, and because they echolocate, getting lost in fog is not so likely.

6. All swifts have four forward facing toes (pamprodactyly).

Back when it took minutes to take photographs and prior to that, it was natural to describe birds from museum specimens, and one of the problems arising from this methodology was that the toes of many dead birds, shrink into the pamprodactyl position. So it was that many authors have described swift feet as being pamprodactyl. For example see Wildbird 1993, 1994, and Sinclair et al. 1997, 19. Problem is, this is only true in real life for some swifts. Hartert way back in 1892 noted that some swifts (Reinarda, Tachornis & Cypsiurus) use their toes in opposing pairs. Collins (1983) published a photo of the foot of a live House Swift (Apus nipalensis) clearly portraying the lateral grasping format. You can also see that format above, in the photo of the foot of the White-throated Needletail. Next to that photo above you can see the foot of the White-rumped Swiftlet roosting in a lava cave in Samoa. It has three toes forward and the hallux backwards – the anisodactyl arrangement. The Three-toed Swiftlet (Aerodramus papuensis) does not have a hallux or hind toe, to point in any direction. So swifts have at least three types of arrangement for their toes.

References

Beebe, W. 1949. The swifts of Rancho Grande, north central Venezuela, with special reference to migration. Zoologica 34: 53-62.

Beebe, W. 1950. Home life of the Bat Falcon, Falco albigularis albigularis Daudin. Zoologica 35: 69-86.

Bromhall, D. 1980. Devil Birds. The life of the Swift. Hutchinson, London. 96pp.

Campbell, B. 1964. The Oxford book of birds. London. Oxford University Press.

Collins, C.T. 1963. The "downy" nestling plumage of swifts of the genus Cypseloides. Condor 65: 324-328.

Collins, C.T. 1965. The down-like nestling plumage of the Palm Swift Cypsiurus parvus (Lichenstein) Ostrich 36: 201-202.

Collins, C.T. 1980. The biology of the Spot-fronted Swift in Venezuela. American Birds 34: 852-855.

Collins, C.T. 1983. A reinterpretation of Pamprodactyly in Swifts: a convergent Grasping mechanism in Vertebrates. Auk 100: 735-737.

Dickinson, E.C. 1989. A review of smaller Philippine swiftlets of the genus Collocalia. Forktail 5: 23-34.

Dunning, J.B. 1993. CRC handbook of avian body masses. Boca Raton, Fl. CRC Press.

Gustafson, T., Lindkvist, B., Gotborn, L. & Gillen, R. 1977. Altitudes and flight times for Swifts, Apus apus, L. Ornis Scand. 8: 87-95.

Hamilton, S., Erico, J. & Tarburton, M. 2001. Notes on the sixth specimen record of the Three-toed Swiftlet Aerodramus papuensis in Papua New Guinea. Corella 25(1): 12-14.

Hartert, E. 1892. Catalogue of the Birds of the British Museum, vol 16. London. Trustees of the Brit. Mus.

Hartert, E. 1897. Podargidae, Caprimulgidae und Macropterygidae. Tierreich 1, i-viii, 1-98.

Henningsson, P., G.R. Spedding, & A. Hedenström. 2008. Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel. the Journal of experimental Biology 211, 717-730.

Henningsson, P., Muijres, F.T., & A. Hedenström. 2011. Time-resolved vortex wake of a common swift flying over a range of flight speeds. Journal of the Royal Society: Interface 8, 807-816.

Hespenheide, H.A. 1975. Selective predation by two swifts and a swallow in Central America. Ibis 117: 82-99.

Holmgren, J. 1993. Young Common Swifts roosting in foliage of trees. British Birds 86(8): 368-369.

Lack, D. 1956. Swifts in a tower. London. Methuen. 239 pp.

Layard, E.L. & Layard, E.L.C. 1878. Notes on the avifauna of New Caledonia. Ibis (4) 2: 250-267.

Layard, E.L. & Layard, E.L.C. 1879. Letters, announcements, &c. Ibis (4) 3: 107-108.

Leidgren, A. 1985. Något om de lappländska tornsvalorna. Fåglar i Norrbotten 2: 10-15. English Translation by Jan Holmgren: Notes on the swifts of Lapland.

Lentink, D., Müller, U.K., Stamhuis, E.J., de Kat, R., van Gestel, W., Veldhuis, L.L.M., Henningsson, P., Hedenström, A., Videler, J.J. & van Leeuwen, J.L. 2007. How Swifts control their glide performance with morphing wings. Nature 446, (26), 1082-1085.

Lyuleeva, D.S. 1970. Enyergiya polyeta u lastochyok i strizhyei. [Flight energy in swallows & swifts] Trans. Dokl. Akad. Sci. USSR. Nauk USSR 190: 1467-1469. Transl. Transactions of the Doklady Akadamii of

Sciences, Nauk USSR

Maunders, S. 1854. The Treasury of Natural History. Longman, Brown, Green & Longman. London.

Morse, R.A. & Laigo, F.M. 1969. The Philippine Spine-tail Swift, Chaetura dubia McGregor,as a honey bee predator. Philippine Entom. 1: 138-143.

Oberholser, H.C. 1906. The status of the generic name Hemiprocne Nitzsch. Proc. Biol. Soc. Wash. 19: 67-70.

Palomeque, J., Rodriquez, J.D., Palacios, L. & Planas, J. 1980. Blood respiratory properties of swifts. Comp. Biochem. Physiol. 67a: 91-95.

Rand, A.L. & Rabor, D.S. 1960. Birds of the Philippine Islands: Siquijor, Mount Malindang, Bohol, and Samar. Fieldiana Zool. 35(7): 221-441.

Sinclair, I. Hockey, P. & Tarboton, W. 1997. SASOL birds of southern Africa. Struik Publishers, Cape Town.

Tarburton, M.K. 1986. Breeding of the White-rumped Swiftlet in Fiji. Emu 86: 214-227.

Tarburton, M.K. 1987. An experimental manipulation of clutch and brood size of White-rumped Swiftlets in Fiji. Ibis 129: 107-114.

Tarburton, M.K. & Kaiser E. 2001. Do fledgling and pre-breeding Common Swifts Apus apus take part in aerial roosting? An answer from a radio-tracking Experiment. Ibis 143: 255-263.

Tarburton, M. 2009. Why are White-throated Needletails and Fork-tailed Swifts often last observed in Southern Australia when migrating northwards? Australian Field Ornithology 26, 19-24.

Videler, J.J., E.J. Stamhuis, & G.D.E. Povel. 2004. Leading-edge Vortex lifts swifts. Science 306, 1960-1962.

Welty, J.C. 1975. The Life of Birds. 2^nd Ed. Philadelphia. W.B. Saunders.

Wildbird Jan 1993, Birders Quiz. Wildbird Dec 1994, Wildbird Q & A. then Collins response in Wildbird May 1995 (Vol 9(5), 2-3.)

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If you are interested in seeing more about individual species of swifts then click this LINK to Species details. It will take me some time to finish them all, but I think it is worth a look to see if the one(s) you are interested in are finished. Thanks for your interest,

Prof. Mike Tarburton.

“retired” from Dean of School of Science & Technology,

Pacific Adventist University,

Papua New Guinea.

If you are interested in communicating with me then retype my e-mail address into your e-mail program: . I am interested in new information, photos, corrections, discussion, or even questions you might have.