I can think of no instanties where bird do any lasting damage to the environment, they may damage a smalll percentage of grain crops and  soft fruits, and a few fish but on balance the majority of their diet is wild and natural food. Many birds are scavengers and help to clean the environment while others spread the seeds of the fruit and berries that they consume helping nature regenerate the native flora and would we not be overrun by the flying and crawling insects if it were not the countless billions that are eaten by insectivors. Birds are essential to our existance and therefore can only be considrered useful to the environment. There are places in the world where birds are persecuted, partridge, pheasant, grouse etc, and the raptors the depend on them for food are in turn persecuted by human cruelty.

Birds play an important role the natural world and if left alone deal with natural events eg, drought and disease and recover in due course.Human intervention inthe nateral word has had devastating affects on bird life as well as global warming but this also has a few pluse. In my life have seen accidental and intentional reintroductions  in this country  due to global warning human intervention and that can only be good.