AUTHOR BLOG: The real story behind murres’ pear-shaped eggs

Tim Birkhead

Linked paper: The pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces by T.R. Birkhead, J.E. Thompson, and R. Montgomerie, The Auk: Ornithological Advances 135:4, October 2018.

murre eggFor the past six years, Jamie Thompson, Bob Montgomerie, and I have tried to understand why murres produce a pear-shaped (pyriform) egg.

It started one evening in 2012 when I watched a well-known TV presenter take a murre’s egg from a tray of birds’ eggs in a museum. “The reason it is this shape,” he said, “is so that if it is knocked, it will spin on its axis rather than rolling off the cliff ledge.” He demonstrated this by spinning the egg.

I was appalled. That idea was nonsense and had been dismissed over a century earlier. Yes, if you take an empty eggshell you can indeed lie it on its side and spin it like a top on its side. But a murre egg full of yolk, albumen, and a developing embryo will not spin like that without undue force.

Having offered to send the presenter the papers pointing out why the spinning-like-a-top idea was wrong, I had a sudden crisis of confidence, and decided I had better re-read those papers myself.

I soon realized that the more widely accepted view — that a pyriform egg rolls in an arc and thereby minimizes the risk that it will fall off the breeding ledge — was not very convincing either. The rolling-in-an-arc idea gained support initially by some experiments in the 1960s using model eggs (made from plaster of Paris). But it was later found that model eggs simply do not roll like real eggs. Subsequent experiments with real murre eggs provided no convincing evidence for the rolling-in-an-arc idea, either.

What’s more, incubating murres invariably orient their egg with its blunt end directed up the slope, in towards the cliff, so that if the egg does roll, it will roll out to the edge. If the purpose of the pyriform egg was to prevent it from rolling off the ledge, then it would more sensible for the parent to orientate the egg the other way.

We decided to re-investigate, thinking explicitly about the selection pressures that might influence the shape of a murre’s egg.

We had two ideas. First, murres are poor flyers that breed at high density. As a result, crash landings onto incubating birds are common, so perhaps a pyriform shape confers greater strength and resilience against impacts. That proved to be a difficult idea to test.

Our second idea rested on the observation that murre ledges are filthy with excrement. Perhaps the pyriform shape enables an egg to keep its blunt end clean such that the pores for air exchange do not become blocked. We found that the density of pores on the blunt end of the egg was relatively high and, if you look at the distribution of dirt on murre eggs, most of it is on the pointed end. These results are consistent with the dirt hypothesis. However, it wasn’t clear whether avoiding dirt or avoiding damage from impacts were sufficiently strong selection pressures to have produced the shape.

Then, while climbing on murre ledges in 2017, I had a sudden thought. Perhaps the pyriform shape allows a murre’s egg to rest stably on the sloping ledges that murres often breed on. I had fresh murre eggs and Razorbill eggs (which are much less pointed and more elliptical in shape) to hand, and I tried placing them on a 30o rock slope. The murre egg rested there immediately, the Razorbill egg rolled off (into my hand, of course), and, indeed, there was no way I could position the Razorbill egg stably on that slope.

My colleague Jamie was climbing with me, so I called him over, said “Watch this!”, and demonstrated again. Same result. Then, together with Bob Montgomerie, we devised a series of tests to establish just how stable murre and Razorbill eggs are across a range of egg shapes on slopes of different steepness. We quantified egg shape using a new approach (Biggins et al. 2018). We then conducted two experiments, one using a moving slope and the other using three static slopes at 20o, 30 o and 40 o. We tested to see at what angle each egg would begin to roll on the moving slope and how successful we were at stably positioning each egg on the static slopes.

The results are clear. The more pyriform the egg, the more stable and less likely to roll out of place it is. Our results are NOT about how an egg will roll when it becomes unstable, but about whether it begins to roll in the first place, either when knocked or during changeovers. Our results indicate that the stability of a pyriform egg also makes it easier and safer for murres to manipulate (with their beak, wings and feet) their eggs during incubation and changeovers.




I started studying Common Murres (common guillemots in the UK) Uria aalge in 1972, on Skomer Island, off the coast of Wales, UK.  I have kept that study — whose main thrust is population monitoring — going ever since:

The video describing our murre egg study is here:

Tim Birkhead academic website:


Other relevant papers:

Biggins, J. D., Thompson, J. E. & Birkhead, T. R. 2018. Accurately quantifying the shape of birds’ eggs. Ecology and Evolution 8: in press.

Birkhead, T. R. 2017. Vulgar errors — the point of a Guillemot’s egg. British Birds 110: 456-467.

Birkhead, T.R., Thompson, & J. E., Biggins, J. D. 2017. The point of a guillemor’s egg. Ibis 159: 255-265.

Birkhead, T. R., Thompson, J. E. & Biggins, J. D. 2017.  Egg shape in the common guillemot Uria aalge and Brunnich’s guillemot U. lomvia: not a rolling matter? Journal of Ornithology 158: 679-685.

Birkhead, T.R., Thompson, J. E., Biggins, J. D. & Montgomerie, R. 2018.  The evolution of egg shape in birds: selection during the incubation period. Ibis, in press.