Why Birds Eggs Have Speckles

Chris - You have been working with William Clocksin and looking at finding out why there are speckles on bird eggs.

Andrew - People have been wondering for at least 200 years why there are little spots on certain sorts of birds eggs. The patterns on most eggs are not actually explainable. We know that in some ground-nesting wader birds the eggs have speckles for camouflage purposes, but most species actually have white eggs. The birds I have been looking at, the great tit, have white eggs with little brown-red spots on them. People have been looking at these and thought that they must be some kind of visual signal. It's definitely not camouflage because they don't look like their background and the mother covers the eggs anyway.

Chris - So why do they have these spots?

Andrew - The story is that the molecule that makes up these little red - brown spots (protoporphorin) has some very useful properties. Small birds are short of calcium and they find it very hard to find enough to form their egg shells during the breeding season. When they are particularly short of calcium they can put in this protoporphorin instead of some of the calcium. One of the properties it has is that it is a semi-crystalline substance which may act a strengthening agent.

Chris - When you are doing this research, you must have to carefully map out where the spots are on the eggs.

Andrew - I've been using a very simple system where I simply look at the eggs and record the spot patterns on different scales depending on the intensity of pigment and the distribution of the spots. That's been fine for the last 15 years with just me doing it, but we want to extend the work further in this species and more importantly, in other species too. This is where William's work comes in. We are hoping he will be able to write a program that will help map out the patterns.

Chris - Do these spots have any significance for the birds inside the eggs that are actually developing? Does it give then a target to peck at, like a weakness in the egg?

Andrew - That's a very interesting question. It turns out that the protoporphorin spots do mark particularly thin areas of shell. In the pigment spots, the shell is on average 18% thinner than the rest of the shell. The spots tend to form a ring around the sir space of the egg and that is actually where young birds hatch from. It is unlikely that they form a map for the chicks I look at because they are blind while they are in the egg. However, it is worth bearing mind that a lubricant within the shell that strengthens the shell from pressure outside would actually weaken the shell from pressure inside. So it would make sense that they peck where those spots are.