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.
Kiwi eggs are roughly 20% of the female’s body mass, which is extraordinary for a bird their size. How do they compare with the other ratite birds?
In most birds, egg mass scales at about two-thirds the rate of body mass. One can estimate the body and egg mass of extinct ratites, using bone and eggshell dimensions. Ratites don’t seem to follow the rule for other bird groups, whether you include or exclude kiwi.
But some ratites lay a dozen eggs, while kiwi lay only one or two. If we calculate the total mass of an average clutch and plot that against body mass, it scales at a rate of almost two thirds, and kiwi fall right on the line.
So my conclusion is that kiwi eggs are exactly the size you would expect, given they lay just one.
Status: A dissertation chapter, given as a talk at Evolution 2007, and being prepared for submission.