A Good Egg, Indeed

A wonderfully diverse and cross-disciplinary team published a paper this month in the mega-prestigious journal Science that really struck a chord with me on several levels. The paper is about diversity in shape of bird eggs. Seems simple enough—measure eggs from many species and report the diversity. That is what would have been done in the 1960s. These authors took a seemingly basic question to a different, and modern, level of analysis. Because we now have a pretty good idea of the evolutionary relationships of the major groups of birds (feel free to call it a “tree of life” or “phylogeny” of birds), and because biomechanically interested friends from the realms of physics and engineering have been studying the mechanics of bird-flight dynamics, these authors had the opportunity to correlate the seemingly non-sensible diversity of egg shape among bird species with their mode and time-allotment of flight. A brilliant jump at an opportunity for knowledge, if I do say so myself! For you to consider diversity in flight in birds, let me just put four names in your head: hummingbird, vulture, ostrich, penguin (i.e., true flight, if underwater). You get the picture? I’m sure that you do.

Starting with a reasonable baseline of a simple sphere, such as is seen in crocodilians, which are the closest living ancestors to modern birds (and they certainly do not fly!), the authors showed that the energy demands and aerodynamics of flight are of huge importance, such that birds that spend most of their time aloft or make massive migrations (e.g., swifts, sandpipers) have elongate, ovoid eggs in association with body-streamlining for flight. Birds that fly only short distances, with small territories (e.g., trogons) have nearly spherical eggs, as do the famously flightless ostriches and emus. Penguins truly do fly underwater, so their egg shape is more elongate than spherical. These definitions of the extremes of egg shape, with respect to flying, then make it clear that the majority of birds fall somewhere in the middle. Hence, we have myriad examples of “tear-drop” shaped eggs in species like our familiar American robin.

This is a truly great piece of descriptive biological inquiry that tested baseline hypotheses of form and function, yet avoided the pitfall of inferring adaption. Realize that, prior to this paper, most textbooks had described the utility of egg-shape in terms of “less likely to roll out of the nest” and other unfounded speculations.

Bird eggs aside (as I consider this paper to be so masterful that the story is done in terms of birds!), it got me thinking about eggs of other reptiles. Hmmm…….crocodilian and sea turtle eggs are spherical, other turtles are spherical or elongate, snakes and lizards are variously elongate. None of these animals are flying (except perhaps sea turtles, underwater?). This is the kind of stuff that gets my brain juices flowing. Next project?

Finally, there is a very ironic side to this study, and one which the authors fully admit and embrace. Bird eggs were once especially popular in the upper-class of many societies as objects to be collected. The mind-set of the fad was quite literally similar to baseball cards. Collectors wanted every species, from every locality, and as many years as possible logged into their private collection. This is where my research-brain and my conservation-brain come into collision. Back in those days, there were no regulations, so the term “black market” does not apply. But basic market economics were such that eggs of the more rare species, or off-season breeders, became more coveted and more expensive for the most fanatical collectors. This phenomenon clearly helped lead to the extinction of the great auk (Pinguinus impennus). The same economical phenomenon occurs today, as animals such as tigers, elephants, rhinos, sun bears, pandas and totuava (a fish), become more rare, the market price for their biomaterials skyrockets and makes conservation efforts become financially severely disadvantaged.

The rampant and indefensible over-collection of bird-eggs for the market of private collectors, despite its obvious harm to the conservation status of some species, ironically, turned into a boon of data for this study. Because these collectors were assiduous in their record keeping, their collections ultimately wound up in research collections such as the Museum of Vertebrate Zoology at the University of California. Of course, I don’t condone the ethics of the original egg collectors of years ago, but I do find myself somehow academically very grateful that their collections were ultimately gifted to proper academic museums, allowing studies such as the one here.
Joe Mendelson, PhD
Director of Research

More information: http://www.sciencemag.org/news/2017/06/surprisingly-simple-explanation-shape-bird-eggs