Common Guillemot eggs vary greatly in size and shape

A cliff-nesting Common Guillemot with its single egg
A cliff-nesting Common Guillemot with its single egg

Jim Hurley - Nature Trail

Birds' eggs are objects of great natural beauty; perfect, seamless casings enclosing unborn chicks that have no idea of the complexity of the wonderful world they are about to be born into.

Ranging in size from that of a Goldcrest or Firecrest, hardly stretching to 14mm long along its longest axis, to that of a Mute Swan measuring an impressive 115mm long, eggs come in all shapes from almost round, to oval, to elliptical, and to pear-shaped and pointy.

Surface texture can be smooth or rough, dull or glossy, dense or chalky. And colours range from a pure background shade to one decorated with bewildering combinations of tiny dots, large spots, irregular blotches, nondescript streaks, intricate scribblings and a host of other markings in an array of shades ranging from white to black and a multitude of colours in between.

Take the Common Guillemot, for example, a very common black and white seabird found of rocky seaside cliffs all around Ireland. It does not build any nest and lays a single egg on a flat surface on a rocky cliff. The egg is large and strongly pear-shaped. And the eggs come in all sorts of colours from pure white to cream to buff to brown to reddish as well as shades of blues and greens.

It is believed that the pear-shape evolved by nature selecting eggs that rolled in a circle thereby staying on their ledges. The popular perception argues that, in the past, more oval eggs obviously rolled off nesting ledges resulting in genes for being oval being selected out of the gene pool.

While that is eminently logical and may very well be the case, it is known that the shapes of eggs that birds lay are influenced by their places on the evolutionary tree of life, their body shape, the arrangement of their organs, how much time they spend flying, the amount of calcium in their diets, the numbers of eggs they lay, the needs of the chicks, etc. Consequently, the full answer is more likely to be a trade-off involving several factors.

Why Common Guillemot eggs show such great variation in colour and markings is unknown. It has been suggested that since these birds nest colonially that it may be a means of different birds recognising their own individual eggs, the scribblings on the eggs being, as it were, like names on place settings on a dinner table at a wedding reception.