How to Keep Birds From Eating the Fruit in the Orchard

pauljoseph, I don't know how big your orchard is and this is a possible solution. Hang CDs or DVDs and metallic ribbon in taller trees. Birds will turn away from flashy or bright items. The free movement of the ribbon from the constant change in wind levels will prevent the birds from becoming used to the sight.
String aluminum pans and/or cans in your orchard. Birds will react strongly to loud noises and will tend to stay away if at all possible from them. Take some aluminum pie pans or pop cans and tie them in random places in and around your tree orchard. When the wind blows the pans and cans will bang together, scaring away birds.

Not sure about bats - the bats we have here in the northwest in orchards do a lot of good.

In Australia they would use the plastic netting bags that oranges come in (do you have a similar option in Kerala?) to wrap citrus and grape bunches...individually. Time- consuming and perhaps a bit difficult for taller mango trees!

Rubber snakes in the tree. I doubt the parrots would know about an Owl.

For bats? I don't know...maybe one of those ultrasonic mouse repellent, which don't work on mice BTW, but could probably really annoy bats...or Yoko Ono Albums.