About how big is the bird population? Has it gone down because of global warming or pollution?

About 10 years ago, two scientists decided to estimate the total number of birds on the planet. The number they came up with was 200 to 400 billion individual birds. Compared to 5 billion people, this amounts to about 40 to 60 birds per person.
Since the 1500s, birds have been declining both in terms of species numbers (by about 500 species) and numbers of individuals (by about 20-25%). This decline is due to human activities, such as the cutting down of forests, contamination, invasive species, and human-induced climate change.
The world's tropical forest bird population may be permanently reduced by 144 million individuals per year.
Of all the known bird species, 129 are extinct  and 9,787 are living today.
Birds are a vital part of ecosystems and they perform important tasks. For example, birds pollinate and disperse the seeds of many plants, including many crops important to humans. They are also helpful in controlling crop pests. And scavenger birds like vultures help with the decomposition of organic material.
All of these benefits to humans and other organisms will likely decline along with bird numbers and species. Because birds can have an important role in controlling other organisms, their decline may even encourage the spread of disease.