大象传媒

Some birds have a risky policy and lay only one egg. Because of this they must make a nest that is really secure, perhaps in an underground burrow. The kiwi lays one huge egg - the biggest laid by any living bird and a quarter of her body weight. In fact, it contains so much yolk that the chick will be fully developed when it hatches. Laying this huge egg takes a huge effort. In contrast, the blue tit lays tiny eggs - each weighing no more than 1 gram - but she lays one each day for a fortnight or so. Many of her chicks will die though, so the overall survival rate will be similar to that of the kiwi.