Do you know why some birds migrate? The length of daylight affects the nervous system of birds. When the days get shorter, the birds get some sort of a signal from their brains to migrate to warmer places as they realise winter is round the corner. When daylight increases to a certain level, they again get a signal to return to their original homes. Millions of different varieties of birds migrate every year to warmer climates and return to their breeding grounds with extraordinary precision the following spring.