A population of birds is simply the total birds in a certain area. You can do a population survey of the type of birds that live in a state park, or by a certain lake, or so on. It´s very difficult to do population counts, since you have to physically wander through that area and try to count each bird once and only once.

Population studies can help evaluate how birds live, how they age, how the size of their group changes over the years, and so on.

Birding Glossary