Glancing at the photograph we might well exclaim "ouch", or "boasting again", but fortunately for the feral pigeon things are not what they seem!

In fact, the eggs have been laid by a swan that is standing nearby and just after the photo was taken she leaned forward and hissed at the pigeon to warn it off.

For most species, including the swan, birds lay a single egg early each morning but the female will not begin to sit on them and incubate until her full clutch is complete.

This ensures that eggs will all hatch at approximately the same time and fledglings leave the nest within a few hours of one another. However, there are exceptions to this rule.

For example, the barn owl which lays from four to seven eggs begins to incubate directly the first egg is laid so the young hatch at intervals of from two to four days.

It may seem cruel to us but when there is a shortage of voles and mice, the owls main diet, the last fledgling to hatch being the 'runt' of the batch, may be killed and eaten by its siblings, thus ensuring that some survive when prey is scarce.

The cuckoo lays up to twenty eggs, each one deposited in a different nest of dunnock, reed warbler or similar. The cuckoo removes one egg from her host's nest, probably eats it, and in its place lays one of her own which is slightly larger but similarly patterned.