The first thing with any injured or young creature is to access the need: does this animal/bird need to be helped in the first place? If there is an injury or if the animal has been attacked it needs help! A bird that has been attacked by a cat for example might not have visible injuries but it most probably still needs help. All it takes is one scratch or small puncture wound and the infection will most likely kill it.
If the baby bird is sitting out in the open, not moving, not being tended by parents, laying flat or appear lethargic, or if it's being pestered by corvids and/or flies: Secure it and phone a rescue!
Is it fully feathered sitting in a fairly hidden place? If so, leave it and watch from afar.
Do the parents tend to it? If so it is probably ok. It is normal for fledglings to spend some time outside of the nest even though they are not quite ready to do their own thing yet. If you are worried, again, observe from afar!
If it's not feathered or is only partially feathered: Are the eyes still shut? Is it on the ground?
This is not right. Secure the bird and call a rescue.
As a predator, we are the slowest that any animal will meet, so if this creature can not escape our efforts to catch it then how is it going to be able to escape a hungry and fast predators like a fox, dog, or cat?
Does this animal/ bird move like a healthy happy animal or bird, does it look old enough to be out on its own? Does it have a full coat of feather?
Treat it like you wound anything newborn, get in a safe place, place soft and dry bedding in the container.
Pick it up carefully, is it cold to the touch? If it is cold to the touch get it warmed up, give it a hot water bottle, heat mat, or just an empty jam jar with hot water in it. Put the source of heat NEXT to the bird, not under, and give it enough space to move away from it if it gets too hot. Make sure it's not so hot that it will burn the bird.
Bring the creature indoors in a quiet area. If you wouldn’t do it to a newborn human baby, don’t do it to a tiny bird or injured hedgehog! Don't force it to drink water - baby birds get their water through the food it's given.
The only difference between a wild animal baby and a human one is you NEVER cuddle wildlife!
Wild animals stress if you stroke fuss or even talk to them! Keep the handling to an absolute minimum and keep it somewhere quiet and warm till you get it to a rescue.
Last but not least: Leaving it outside to fend for itself or "let nature take its course", well, it wouldn’t cross your mind if it was human so don't do it to them?
Does this creature look injured, are there any open wounds? Yes, it needs help! Flies lay eggs that hatch into maggots, these maggots cause pain and suffering! A fly will lay eggs on any animal/bird which is unwell laying or sitting out in the open! There does not need to be a wound. Being eaten alive by maggots is nothing we believe that any creature deserves; wild or not.