My Grandad is a keen birdwatcher and I sometimes go out with him, but I can hear the birdsong much better than him. You can hear sounds which range from very low sounds, low frequency, to very high sounds, high frequency. All humans can hear sounds of about the same range of frequencies, but as you get older the ear gets stiffer and it can not hear the higher sounds as well as it used to.
Most bird song is of high frequency.
Below this range we feel rather than hear the sound, it is said to be infrasonic. Above this range we cannot hear the sound at all, it is said to be ultrasound. However this range is for the average person and the ability to hear the high frequencies drops quickly as you get older. Whether or not you can hear a sound also depends on how loud it is. A very loud sound can hurt.