There are many shapes and sizes of eggs in the natural world but the ones we're most familiar with are, of course, those laid by birds. For the past month or two birds have been pairing up, mating and building nests. Breeding takes its toll on parents and females need to eat the right foods in order to reach optimum breeding condition - a clutch of blue tit eggs in your garden's bird box, for example, contains more calcium than the mother's entire skeleton!
The real eggs of Easter
Timing your breeding season with the availability of food is crucial - both to help the parents build up energy and nutrients as well as for the chicks to have suitable food once they've hatched. When you consider that each blue tit chick requires about 100 caterpillars a day and that each blue tit nest has an average of 10 chicks, you begin to realise just how important this timing is. As our climate changes, there is a risk that this timing becomes mismatched, especially for migrating species such as chiffchaff or cuckoo, and research is ongoing as to whether this is already making a difference.
Shells are essentially calcium carbonate so they’re basically white. White eggs would be easy for a predator to spot, though, so birds use two different pigments – one derives from bile and provides blue and green shading, the other derives from blood and creates spots and patterns. It’s thought that speckled patterning helps to strengthen the eggs – later eggs in a clutch have more patterning than earlier eggs and this is probably because the female bird has lower calcium levels; the patterning offers additional protection.
Just like we like to live in different places, so do birds. Some of us prefer bungalows or flats, having close neighbours or living in splendid isolation. Where a bird nests dictates the colour of the eggs. Birds that nest in trees and hedgerows, like dunnock and song thrush, often have blue or green eggs, birds that nest in dark holes, such as blue tits and nuthatches, need to have pale eggs so that the parents can see them and birds that nest on the ground often have brown or speckled eggs in order to be well camouflaged. Guillemots, a bird found on the UK’s coasts, nest very close to each other in large colonies and their eggs are very differently marked in order to allow each pair to recognise their own egg.
Recent research showed that eggs laid in colder areas tend to be darker in colour because they absorb heat more quickly and release it more slowly – an important benefit if parents need to leave the nest to find food. This means that eggs in the UK may be darker shades than those in the Mediterranean.
We’re all familiar with the shape of a chicken’s egg, or one of our chocolatey Easter eggs come to that, but not all eggs are the same. Those of cliff-dwelling birds like the guillemot, for example, are often conical at one end and it’s thought that as well as making them less likely to roll off a slightly sloping cliff, this also helps parents to move the egg between them to keep it warm. There are other, more practical reasons – the shape of an egg could well be a factor in helping to keep the body streamlined whilst in flight!
It’s important not to disturb breeding birds (and it’s illegal to take their eggs) so please keep dogs on a lead when you’re out for a walk and think about when and how you cut your hedges and tidy your garden. You can visit our wildlife gardening for ideas on how to help wildlife where you live.
If you don't have a space for a nest box to help out your local birds, why not consider renting a nest on one of our nature reserves instead?