Hatching

Birds have differing approaches to nesting. Most songbirds lay an egg each day, but don't start to incubate until the clutch is completed (Blue tit is a good example). Forming eggs is demanding. A bird has to obtain enough food to produce the eggs and, to do so effectively, she can't afford to spend her time incubating. Once the last egg has been laid, she sits. The eggs warm up and being to develop. All hatch at about the same time and so this approach is called 'synchronous' breeding.

Some birds adopt a different approach. An egg is laid and incubated immediately. A second egg follows a few days later, then a third and so on. Egg-laying tends to be slower because the mother can't gather sufficient food to form an egg every day, with the limited time available to her. The first egg hatches a few days ahead of the second, which is ahead of the third and so nesting is termed 'asynchronous'. Birds of prey generally adopt this approach.

Which of the two methods is superior? Neither. Both systems have survived and so both are effective. Which one is used depends on the nature of the food supply. If food for the baby birds is abundant, as with tits feeding on caterpillars, the synchronous system works well. If it becomes scarce, however, the family may be in trouble. All of the babies will be undernourished and may die.

In an asynchronous nest, the first chick to hatch is largest and gets the lion's share of the food. Only if it has enough to eat, will the next baby be fed and so on. In times of shortage, the youngest chick dies, its body being eaten by the older siblings. If things are very bad, the next youngest dies and so on. It's a cruel system but a ruthlessly efficient one. It means that nothing is wasted and, even in a lean year, the pair stands a chance of rearing at least some young.

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