How does a duckling identify its mother?

In order to explore how birds can identify their kinship from their songs, ornithologist Hayes and his students studied the hatching process of female wild ducks. They put the miniature microphone at the bottom of the duck's nest and then connected it to the tape recorder. They found that the brooding mother duck made a faint croak in the fourth week of hatching, and each croak lasted only 150 milliseconds. At this time, the inside of the hatched egg makes a "squeaking" sound. At first, these sounds were very small. As time goes on, the calls of wild ducks become more and more frequent, and the "Ji Ji" sound in eggs becomes louder and louder. Then the ducklings hatched. Two hours after the duckling was born, two kinds of songs increased fourfold. After the duckling is born 16 ~ 32 hours, the mother duck leaves the nest and swims into the water. She made a quick phone call, as fast as 40 ~ 60 times a minute. So the ducklings walked out of the nest and ran to their mother. From this point of view, ducklings began to hear their mother's voice on the 27th day after hatching from eggs, and hearing played a major role in this process. After the duckling hatches, vision and hearing work together to make the duckling know her mother better.