When I started to write about birds’ eggs I wondered whether anyone had established which of the different shapes was most common. Obviously when we talk about something being egg shaped we are usually thinking of a hen’s egg, which is ‘oval’, but with an obvious blunt and pointed end and whose greatest width lies closer to the blunt end. To my surprise nobody seems to have quantified egg shape across all families of birds. Part of the difficulty, of course, is coming up with a simple index of shape. Researchers have devised several complicated ways of describing egg designs but there is no single number that captures the full range of shapes. For this reason most books dealing with egg shape simply show – as I do here – a set of outlines or silhouettes illustrating the different types that exist.
One thing we do know is that as well as being characteristic for a particular species, shape is also fairly characteristic for particular families of birds, too. Owls, for example, typically lay spherical eggs;
Different shapes of birds’ eggs. From left to right: Ross’s turaco (spherical), ruff (pyriform), hummingbird (oblong oval or elliptical), crowned sandgrouse (oblong oval or elliptical), African thrush (oval), Slavonian grebe (bi-conical or long subelliptical), alpine swift (ellipitical ovate or long oval). Redrawn from Thomson, 1964.
waders (shorebirds) lay pyriform eggs; sandgrouse produce oval or elliptical eggs; and grebes produce biconical eggs. As a biologist, two questions come to mind. How are eggs of different shapes made, and why are they the shape they are? The first question is about the mechanics of making an egg; the second concerns the adaptive significance of different egg shapes.
Thinking about how a female bird produces eggs of a particular shape, my natural inclination was to imagine that the shape was determined by the shell: shape and shell created together. The truth is more bizarre. The contours of a bird’s egg are governed by its membranes, the parchment-like layer inside the shell – as my eggin- vinegar experiment suggested – rather than by the shell itself. Once you know that the membrane determines the shape it isn’t too difficult to imagine the process.
In an ingenious X-ray study of egg formation conducted in the late 1940s, John Bradfield could see that the shape of the hen’s egg was determined before the shell had even started to form, prior to entering the shell gland. Instead, he could see that the egg’s shape was established in the isthmus, the region of the oviduct immediately anterior to the shell gland, where the shell membrane around the egg is created. He noticed, too, that the part of the isthmus adjacent to the shell gland is ‘more contractile and more like a sphincter’ than the other end adjacent to the magnum, suggesting that: ‘Since the egg greatly distends the narrow isthmus [region of the oviduct], it is to be expected that the caudal [tail] end of the egg, situated in the more contractile part of the isthmus, will become more pointed than the cranial [head] end.’ He adds, however, that this suggestion is far from proved ‘and the problem remains without a clear-cut solution.
At the end of the egg-shape spectrum opposite to the guillemot are certain owls, tinamous and bustards that lay almost spherical eggs. How is that done? Does the isthmus in these birds lack the sphincter that Bradfield saw in the hen? Or does the egg turn continuously as the membrane is laid down so that the sphincter applies a uniform pressure all over the egg? We don’t know.
In humans the maximum size of a baby at birth is determined by the size of the ‘birth canal – that is, the internal diameter of the pelvic girdle. Our present ability to perform caesarian operations removes this constraint, but prior to the twentieth century and the routine use of caesarian section, babies who were too big – or whose heads were too big – failed to be delivered successfully, got stuck and died, usually along with the mother. Strong selection indeed. Because the bones that form the human cranium are still not fused at birth there is some flexibility (literally), permitting the skull to adopt a different shape during birth and allowing some relatively big-headed babies to be born.
Read more in Tim Birkehad’s The Most Perfect Thing: Inside (and Outside) a Bird’s Egg