Kiwi

16-04-2019 09:04

Snuffling along the ground at night, the kiwi (Apteryx) with its fur-like feathers, whiskers and muscular legs seems an unlikely bird. Prior to human settlement, New Zealand lacked land mammals (the exception being bats), and kiwi thrived by taking up a almost-mammalian evolutionary niche. Like other ratites (Paleognathae) and several species in New Zealand, the ancestors of kiwi forfeited the central tenant of birdhood — the ability to fly — for it was an unnecessary drain on their energy.

Remarkably, among the ratites the kiwi’s closest relative is not the moa it once shared New Zealand with, nor the emu and cassowary next door in Australia; it is the extinct and enormous elephant bird, which lived in Madagascar (Mitchell et al, 2014). Scientists long thought that the ratites derived from a common flightless ancestor in Gondwana, and dispersed across the Southern Hemisphere while the land mass gradually divided and drifted apart. Inheritance from a common ancestor — it was a neat explanation for why ratites share the same types of skeletal and muscular structures involved in being flightless. But now the tale of ratite evolution appears to have begun in the Northern Hemisphere, with the ancestral birds flying to the Southern Hemisphere via South America (or via Eurasia, in the case of the ostrich), and continuing onward through Gondwana where natural selection opted for flightlessness independently among the different lineages (Phillips et al. 2010, Yonezawa et al. 2017). No longer constrained by flight, most ratites became huge; the kiwi retained humbler proportions. However it’s certainly not a small bird, with the Great Spotted Kiwi (Apteryx haastii) and Tokoeka (A. australis, or Southern Brown Kiwi) getting up to 45cm tall and 3.3kg heavy. The other three species are the Little Spotted Kiwi (A. owenii), North Island Brown Kiwi (A. mantelli), and Rowi (A. rowi, or Okarito Kiwi).

The only bird to have nostrils at the end of its beak, kiwi seek out worms, grubs, other invertebrates, berries, leaves and seeds, largely at night, though the Stewart Island Kiwi (a subspecies of the Tokoeka) often forages during the day to the delight of tourists. They are also known to eat amphibians and freshwater critters; kiwi have even been observed in captivity to fish out eels, give them a few whacks and gobble them up (source). With their strong legs and claws, adult kiwi are usually tough enough to defend themselves against stoats and weasels. But they have still suffered under the impact of mammals introduced by humans.

Kiwi are not often seen by New Zealanders (who, to confuse matters, also call themselves “kiwis”), despite being a treasured national icon. Mustelids (particularly stoats), cats and possums prey on eggs and/or chicks; without management, only 10% of chicks tend to survive to 6 months. Populations are left in decline, compounded by the fact that adult kiwi are also lost to dogs and ferrets. Once numbering in the millions, kiwi have dwindled to an estimated 68,000 today.

Predator control, habitat protection and captive management (collecting eggs and chicks, raising them to sub-adulthood in captivity then releasing them back to the wild) has proven effective to help populations become self-sustaining. The future of kiwi is looking brighter, but this unique bird will need long-term active management to return to its former glory.

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Photo credit

Top photo: Little Spotted Kiwi (Apteryx owenii) by Jim, the Photographer, CC BY 2.0
Second photo: Little spotted kiwi at Zealandia by Kimberley Collins, CC BY 2.0

References

K.J. Mitchell, B. Llamas, J. Soubrier, N.J.Rawlence, T.H. Worthy, J. Wood, M.S. Lee, A. Cooper. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science, 344 (2014), pp. 898-900.
M.J., Phillips, G.C. Gibb, E.A. Crimp, D. Penny. Tinamous and moa flock together: mitochondrial genome sequence analysis reveals independent losses of flight among ratites. Systematic Biology (2010). 59 (1), pp. 90–107.
T. Yonezawa, T. Segawa, H. Mori, P.F.Campos, Y. Hongoh, H. Endo, A. Akiyoshi, N. Kohno, S. Nishida, J. Wu, et al. Phylogenomics and morphology of extinct paleognaths reveal the origin and evolution of the ratites. Curr. Biol., 27 (2017), pp. 68-77.
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