Mike’s Research

Why Aren’t Birds Bigger?

2007-06-04T20:52:09Z

Birds are bigger than mammals, but the largest-ever bird weighed less than a cow. Why aren't birds bigger? One theory is that as avian eggs get bigger their shells get disproportionately thicker to support their weight. Eventually the shell would be too thick for a chick to break through, and that’s your maximum egg size (and thus maximum bird size). I don’t buy this.

Status: Initial allometry of ratite shell thickness done.

Are Kiwi Eggs Actually Big?

2007-06-04T20:50:39Z

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.

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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 90% finished chapter of my dissertation (June 2006)

Are Birds Small?

2007-06-04T20:48:14Z

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.

Status: A 90% finished chapter of my dissertation (June 2006)

Mysterious Floating Eggs

2006-06-04T21:29:24Z

There have been two well-supported cases of an elephant bird (Aepyornis) egg floating from Madagascar to Western Australia (reference to come). This seems incredible, doesn’t it? And yet it really seems to have happened. It would be nice to know more about the mechanics of this. How bouyant is a fresh giant egg (remember, it has a much greater percentage of shell than a small egg)? What about a dead, dessicated, but intact egg? How long does an infertile egg take to dessicate, anyway (if it even does)? Do floating eggs absorb water thorugh their pores? How fast? And how fast do they travel by wind or current? If we know the answers to all these questions, we’d be better equipped to evaluate the plausibility of trans-oceanic egg dispersal.

Avian Incubation Times

2006-06-04T21:23:09Z

Has their been a comparative study of what factors determine bird incubaiton times? One factors that probably hasn't be taken into account is phylogeny. Another is body size; how does incubation period scale? A third is metabolic rate, which is by no means independent of size or even phylogeny. Clutch size and thus clutch mass probably has some effect as well. Incubation period increases with body size, but past a certain point all pattern seems to disappear (this means extrapolations which estimate the incubation time of giant extinct birds based only on body mass are probably not very useful),