Found a baby bird, what to feed it? Urgent help needed!

I Googled for Passer montanus. These birds feed on insects when they are young. Their diet changes to mainly grains when they are mature.

Different birds obviously have different diet. Some birds can live on fruit such as banana while others can't.

Some sources also mentioned it is wrong to feed bread to nestling and fledgling. Some people would soak bread in milk before feeding to them. Milk can be a poison to them.