When visiting a chicken farm, we will find that during the egg-laying period, basically every hen can lay an egg every day, and more than 200 eggs can be laid in a year. However, most wild birds only produce a limited number of eggs per nest. Why is this? During the breeding season, the number of eggs produced by wild birds per nest is relatively fixed, which varies depending on the species and is called the nest egg count. The nest egg count has gradually evolved through long-term natural selection. If the number of eggs laid by the parent bird is too low, it will affect the transmission and dissemination of the individual's genes. If the number of eggs laid by the parent bird is too high, it will exceed the parent bird's ability to raise them. Therefore, under the combined effects of various environmental and food conditions, each bird species has formed its own relatively stable nest egg count.
Although the nest egg count of birds is relatively stable, it can be divided into fixed and variable types. Variable-egg-count birds, such as pheasants and swans, often produce an additional egg if one in the nest is damaged or eaten by other animals during incubation, so that the number of eggs in the nest remains relatively constant. If an egg is removed from a pheasant's nest every day, the female bird will lay an additional egg. However, because egg production consumes a lot of energy and nutrients, there are certain limits to the replenishment behavior of wild birds. Domestic chickens were domesticated from red junglefowls about 8,000 years ago, and their egg-laying habits are also variable. Under artificial feeding conditions, hens can obtain sufficient nutrients, so as long as the eggs laid by them are removed every day after they start laying eggs, it can induce the hens to keep laying eggs. This is the reason why hens have high egg production.
Birds with fixed egg production, such as golden eagles and brown-headed sparrows, only produce a fixed number of eggs per breeding and then begin incubation. Golden eagles usually produce 2 eggs, and brown-headed sparrows 3 to 5 eggs. If an egg in the nest is broken or eaten by predators, these birds will not replace it with another egg, but continue to incubate the remaining eggs.