An injured bird can only be helped if it can be caught. Since a bird with a leg injury or many kinds of minor injury will often be able to fly to get away, it is rarely possible to catch such a bird whether it would benefit from treatment or not.
Catching an injured bird can be difficult and careless handling may cause further injury. Handling must be firm but gentle. Small birds up to blackbird size can be held in one hand. Place your hand over the bird so that its head fits between your forefinger and middle finger. The rest of your fingers will naturally wrap around each wing, holding the bird firmly.
Medium-sized birds are best held with two hands, one over each wing. Handling large birds requires great care because of risk of injury to the handler. Unless you are used to handling large birds, it is best to call an expert rescuer to the bird rather than try to capture it yourself.
Once the bird is caught, examine it quickly and place it in a well ventilated covered box to wait for treatment. Darkness reduces stress and is likely to be the best first aid you can give the bird. It is also the best treatment for shock.
Read more at https://www.rspb.org.uk/birds-and-wildlife/advice/how-you-can-help-birds/injured-and-baby-birds/if-you-find-an-injured-bird/#LjgwFqlwwk6IhYeo.99