Help, I have wood pigeons chicks abandoned in a nest. Parent is dead, in an accident. How can I help them

Squabs as they are called are fed by both parents on a liquid known as crop or pigeon milk.
The other parent could still be feeding the remaining chick and unless you are watching every minute of the day you ain't gonna know if it is. My guess is one has already fledged as you say... then the other will soon aswell...Best do nothing and just observe