You must know...
According to the law of the Republic of Poland – the Act on Nature Conservation, 16.10.1991 – one cannot take a bird out of its natural environment, because:

all animal species living in the wild are protected by the law (in case of birds, it pertains to all bird species except common pigeon) and cannot be caught, kept outside their natural environment. Moreover, one cannot have in possession neither dead not living wild animals - taking wild animals out of their natural environment is forbidden.

From the point of view of nature (first and foremost from the point of view of the evolution of species) an animal which is not able to survive, should not survive but it should become food for others. Natural losses among birds, especially right after they leave their nest, amount to a few dozen percent per year (depending on the species, season and place it can even be 90 %) and it is normal. Such is a breeding strategy of all the species and it has existed for millions of years! A pair of great tits can breed up to 20 chicks within a year, and over a hundred in great tit’s lifetime. Nonetheless there is no significant increase in the number of these birds, which means that the majority of them dies.

From the point of view of nature helping an injured or sick bird has no significance. However, there is one exception form the rule, when a environmental disaster caused by humans (e.g. leakage of oil) kills hundreds of birds at the same time.

Otherwise helping an injured or sick bird or other animal is nothing but a humane deed, which has no significance from the point of view of nature protection

We take in a bird-foundling
One has to remember that to take a bird out of its natural environment is the last resort. However, if you decide to help a sick bird or a chick, that has fallen out of the nest your intervention should be limited to the very minimum. Some birds get used to human presence very quickly and it may be difficult to get them back to the wild. One should never try to tame a wild bird.

In case of a very sick bird you have to try estimate its chances to survive. Be realistic – if a bird is seriously hurt and its chances of surviving are poor do not desperately try to save it, and have the bird humanely destroyed.