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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
Birds have five main strategies for incubating eggs, and all five have been known for more than 150 years. It had been felt that every conceivable pattern for accomplishing the essential task of incubating the embryo in birds’ eggs had been recorded. The basic categories are incubation by (1) both parents; (2) one parent only; (3) other adults of the same species; (4) other species (i.e., cuckoos); and (5) non-animal heat (e.g., megapodes using mounds of compost). However, in 1985 I discovered that Australian Swiftlets, which produce a clutch of one and cannot find enough food to feed two nestlings at the same time, are able to equal the annual productivity of other swiftlets such as the Fijian White-rumped Swiftlet, which produce clutches of two.[1] This is achieved despite the fact that both species can only find enough insect prey to raise their clutches for 125 and 150 days, respectively.
Australian Swiftlets employ an unexpected strategy: a second single-egg clutch is produced once the first nestling becomes homeothermic and is incubated by the first nestling. This strategy is so finely tuned that in most cases the second egg hatches the day after the first nestling fledges, allowing the parents to continue feeding the second nestling until it fledges just before the flush of insect prey reduces at the end of the wet season in the savanna environment.[2]
Another design feature is that most swiftlets have the ability to echolocate and find their way into the caves every night to sleep and to breed. I have found nests up to one kilometer from cave entrances in Samoa and Australia.[3] Nesting in caves reduces the number of predators that can end the lives of the adults or the nestlings, and swiftlets are designed with the ability to adapt their nesting behavior to their predators. In Fiji and Australia, they mostly nest high in the cave above a smooth area of roof or wall over which pythons cannot climb. In the Cook Islands, they spread their nests out inside the caves to avoid the two species of crabs that climb all over the cave walls, looking for nestlings to eat. In the highlands of Papua New Guinea, they nest on the floor of the cave as there are no ground-based predators.[4]
Swiftlets are able to build nests under the roof of caves because they are designed to make their nests out of their saliva. One species makes nests of pure saliva (nature’s superglue), another places its own feathers in the nest, and most mix the saliva with Kangaroo Grass (in Australia), moss (Samoa), filmy ferns, and/or coconut fiber (Fiji). The only other birds that can echolocate are the oilbirds of South America, and in this God’s creative ability and sense of humor are glimpsed, for oilbirds are vegetarians, feeding on oil palm fruit during the night and sleeping in caves during the day—the reverse timing of cave usage by swiftlets.
A third design feature is that swiftlets, like swifts, can fly all day without resting. Even during breeding, they hold the insects they catch in a pouch below their mouth that is designed to hold over 750 insects.[5] Their nestlings are designed to grow slowly, which means they can survive not only being fed just two or three times a day, but even during cyclones that prevent parents from catching food for several days.[6]
Why is it that swifts and swiftlets can fly all day? Work at Groningen University showed that their wings work quite differently from other birds. They use leading-edge vortices to provide lift.[7] Subsequent studies at the Lund University wind tunnel have shown some of the design features that enable low energy flight even while pursuing insects all day.[8] In swifts and swiftlets, the upstroke of the wing produces thrust as well as lift equal to 60% of the lift of the downstroke. Most birds do not produce any lift on their upstroke. Additionally, the lift/drag ratio was found to be the highest of any bird measured so far.
These advantages result from the stiffness of the feathers of the wing, the sweep of the wing, the tapered tip, as well as a changing wing shape. This wing design generates clockwise leading-edge vortices on the downstroke and anti-clockwise leading-edge vortices during the upstroke. All these elements together have the effect of increasing maneuverability in a bird that is otherwise designed for speed. This is a big help for a bird that feeds on flying insects.
Such a range of design features, all essential to enable swiftlets to do what they do, with none of them able to achieve their end results by themselves, speaks of an intelligent origin and a Creator who planned nature not only to achieve a multitude of diverse ecological relationships but also to occupy our minds for eternity.
NOTES
[1] MK Tarburton. The breeding biology of two populations of the Whiterumped Swiftlet in Fiji and Queensland, with special reference to factors that regulate clutch size in birds [PhD thesis]. [Palmerston North (NZ)]: Massey University; 1987.
[2] MK Tarburton. Breeding biology of the White-rumped Swiftlet at Chillagoe. Emu 1988; 88:202–209.
[3] MK Tarburton. White-rumped Swiftlet colony size and locations in Samoa. Helictite 2011; 40(2):35–49.
[4] MK Tarburton. An unusual nest site for swiftlets. Muruk 1990; 4:66–69.
[5] MK Tarburton. The food of the White-rumped Swiftlet Aerodramus
[6] spodiopygius in Fiji. Notornis 1986; 33:1–16.
[7] MK Tarburton, SR Tarburton. Colony stability of cave-nesting Australian Swiftlets in Queensland: what are the impacts of severe weather events? Australian Field Ornithology 2013; 30:131–151.
[8] JJ Videler, EJ Stamhuis, GDE Povel. Leading-edge vortex lifts swifts. Science 2004; 306(5703):1960–1962.
Michael Tarburton is retired dean of the School of Science and Technology at Pacific Adventist University. He holds a professorship from that university and a PhD in Zoology from Massey University. He has published one book and 64 research papers as well as coauthored seven papers, mostly on swifts, but also on other birds and one mammal. Currently he is involved in research on swifts, swiftlets, and Tawny Frogmouths as well as maintaining three web-pages: www.swiftsoftheworld.info, www.evidenceforevolution.info, and www.birdsofmelanesia.net.