Grounded: the Natural History of Flightless Birds

diatryhed.jpg I have been giving a lecture series through the University of Canterbury’s Continuing and Bridging Education on flightless birds and their evolution. Rather than print a handout on paper, I’m posting the reading list and supplementary information here.


Are Kiwi Eggs Actually Big?


Why are kiwi eggs such a large percentage of the female’s body weight? That can’t be fun. One popular explanation is that kiwi are dwarfed descendants of a moa-sized ancestor, and their egg didn’t shrink as quickly as the rest of them. But kiwi ancestors weren’t giant; they had to fly to New Zealand, after all. When I compared total clutch masses for the ratites, though, I found kiwi eggs weren’t that big—they just put all their eggs in one basket. Or baskets in one egg. Or something.


Did Ratites Fly to New Zealand?

flying_ratite.gif The consensus is that kiwi branched off the ratite evolutionary tree long after New Zealand separated from Australia. That means they had to cross 2000 km of ocean, and there’s no record of a flightless bird ever dispersing across an ocean barrier, no matter how small. That means they flew. A Tertiary dispersal and radiation of ratites, with several flightlessness transitions, is the least improbable explanation of their current distribution, and it has the advantage of being consistent both with the fossil record and with a (sensibly) calibrated molecular tree.


Turkey-sized Moa?


The smallest moa are always being described as "turkey-sized": I've done it myself. But is this really true? Could this be another mindlessly-copied comparison, like the "fox-terrier sized" Eohippus that Gould wrote about? My only turkey experiences have been on Thanksgiving, and I'm not sure that the thing on a platter has much resemblance to a real live galliform, let alone a moa. How big is a turkey anyway?


Are Humans Descended from Giant Penguins?


Actually, they probably aren't, but this graphic from the Guardian (via Junk Charts) would seem to suggest that Icadyptes, described by Clarke et al. in PNAS 104(28) is the missing link. I am interested in estimating the body mass of the largest giant penguins, though; those guys were taller than us and much heavier.

Which Was the Biggest Bird?

The main contenders for largest-ever bird (all extinct) are the South Island giant moa (Dinornis robustus), the elephant bird of Madagascar (Aepyornis maximus), the giant mihirung (Dromornis stirtoni), and the giant phorusrhacid Brontornis burmeisteri. Body masses of between 300 and 500 kg have been suggested for all of these, using a variety of limb bone measurements and sometimes dubious scaling models based on living birds. I found a combination of limb bone measurements (femur and tibiotarsus diameter) improved the accuracy of mass estimates, but the margin of error is still huge when extrapolating much beyond the 100 kg ostrich.


A Pachyornis elephantopus skeleton, sent to Richard Owen at the British Museum by Walter Mantell (son of Gideon Mantell, the Iguanodon discoverer). P. elephantopus weighed over 200 kg. Yes, wingless terrestrial birds do retain a rear toe.

Are Birds Small?

Are birds small? Our intuition tells us birds are smaller than mammals—but wait. We're big mammals. Might we be biased? It turns out that flight is the main thing that determines body size, not what sort of animal you are. I found that flying birds are on average bigger than flying mammals (namely bats). More surprisingly, flightless birds are bigger than flightless mammals. (Remember, most mammals are actually quite small; rodents and the like.) So birds are a little bigger than mammals. This infuriates mammalogists.

About Me

I'm interested in the way in which body size affects the shape and habits of animals (the effect of size on shape is known as allometry). I study this (ORCID) in flightless birds, including those of my native New Zealand.


My dissertation (2.3 MB PDF): pdf.gif

Cite as:
Dickison, M.R. 2007. The Allometry of Giant Flightless Birds. PhD thesis. Duke University.

And the Dissertation Haiku version:

All birds flew once, all
Mighty eggs just the right size.
Why are none bigger?

Further Reading

Scaling: why is animal size so important? • Knut Schmidt-Nielsen • Cambridge University Press, 1984

Size, Function, and Life History • William Calder III • Harvard University Press, 1984

On Growth and Form • D'Arcy Wentworth Thompson • Dover Publications, 1992


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