Why do birds sing so loudly at dawn?

At dawn, the air is calm and other noises, such as traffic, are low. Birdsong travels further and has more impact. Studies suggest a song sung at dawn is around 20 times more effective than one at midday.  Females tend to lay eggs in the morning so it makes sense for males to attempt to attract mates just before.  The chorus of song also reveals which birds are perching where and whether any territories have become vacant overnight. 
The best time to hear a dawn chorus is the first hour before sunrise – which means getting up early or staying up really, really late!  The Wildlife Trusts usually have special events happening up and down the UK on the first Sunday in May to celebrate International Dawn Chorus Day.  You can contact your local Wildlife Trust to find out more.