News
Wednesday, 9 May 2012
Chris Thomas
A regular feature: "Did you know........."
Most birds lay several eggs, but they always stagger their production, laying one egg a day for as many days as it takes to complete their allotted clutch. The reason is simply that if a bird had to carry a set of fully formed eggs, it would be unable to take off - because a fully-formed egg weighs up to a fifth of a bird's own body weight!. So one egg is always in advance of the rest - and having laid this first thing in the morning, she is relatively unencumbered for the feeding part of the day.
The production of eggs puts considerable stress on the female of every species. An egg depletes the resources of the female bird in it's formation - she needs to be well-fed, and also requires calcium for making the shell of her eggs - found in the wild in the shells of tiny snails.
Some birds - for example, Swallows and Starlings - maximise their productivity by donating eggs to other birds of the same species. Having completed the laying of their own clutch, they steal into the nest of a neighbour and - Cuckoo-like - lay another egg. If the chick hatches it will be raised by foster-parents - extra offspring for minimum effort by the busy birth parent!
For most birds, incubation usually begins only when the clutch is complete, not when the first egg is laid. This ensures that all the eggs will hatch within a day or two of each other, making feeding the brood easier and giving each chick an equal chance of survival.
How many eggs are laid by each species varies a great deal. Blackbirds and Robins typically lay four or five eggs: a Woodpecker, seven or so: a Mallard ten - and Tits regularly lay into the teens. The record for a Great Tit is twenty-four!