I was asked a good question the other day by a visitor – “why is does the guillemot’s egg vary so much in colour when its close relative the razorbill that also breeds on the sea cliffs of the Isle of May have very little variation in its eggs ?” Actually guillemot’s eggs are interesting for other reasons as they have a very distinctive shape being very pointed at one end and rounded at the other. Well there is no point in living with a load of sea bird researchers if you don’t use there knowledge so our tea time conversation was guillemot’s eggs. But the short answer to the question of the colour of the eggs is that they didn’t know. Well firstly making colour actually takes quite a bit of energy so birds would only do it for a reason. There were a few theories, one, backed up by wikipedia, was that the birds made eggs with a unique individual pattern so that they could identify their own amongst others on a busy ledges where there is no actual nest. However guillemots rarely leave their eggs but rather hand over the egg to their partner at each handover during incubation. Some suggested that the colour of the egg showed how fit and able the bird was and therefore what a good partner they would make but it is a bit late to be choosing partners once eggs are laid. As for the shape there are several theories, a) if disturbed, they roll in a circle rather than fall off the ledge but by the end of the season the bottom of the ledges are littered with broken eggs. b) The shape allows efficient heat transfer during incubation, or c)
It is a compromise between large egg size and small cross-section. Large size allows quick development of the chick. Small cross-sectional area allows the adult bird to have a small cross-section and therefore reduce drag when swimming.
So this proved to me to two things 1) guillemots eggs are amazing things, a beautiful colour and a cool shape even if we don’t really know why and 2) don’t ask difficult questions at dinner time.