I can think of no instances where birds do any lasting damage to the environment, they may cause minor damage to a small percentage of grain crops, peas and soft fruits, as well as a few fish but on balance the majority of their diet is wild, natural food. Many birds are scavengers and help to keep the environment clean and healthy, while others spread the seeds of the fruits and berries that they consume helping regeneration of native flora and would we not be overrun by flying and crawling insects if were not for the countless billions that are eaten by insectivorous birds every year. I suggest that birds are essential to our existance and therefore can only be considered useful to the environment. Human intervention has taken it's toll on many birds, "game birds" being one example that has had the consequence of raptors being needlessly trapped and killed, and in some cases eradicated completely from particular places, in order to protect the game birds so that they can be killed and eaten by humans.

Human intervention through pesticides has been instrumental in upsetting the natural pattern of bird life, nature took care to balance the natural, pollination+insect+bird cycle all by itself but was disrupted by artificial interference. Thankfully humanity started to see the error of it's ways and began to address this situation when people started to notice that certain species of wild life were beginning to disappear.

I have not mentioned the word harmful in this exercise post today,+ may be that is because birds are not harmful, on the contrary it has long been recognised "that being in nature" is beneficial to recovery after illness and in maintaining good health, so much so that Doctors are prescribing nature courses on the NHS and I saw on the news recently that birdwatching in particular is recommended for mindfulness for those with mental issues, so it looks like it's official birds are useful.