It is impossible to count all birds present at a site during a visit, as not all birds are observed. How does this affect the results?
This does not affect the results. PECBMS focuses on changes in bird populations, not on absolute numbers. The only requirement is that numbers can be compared between years. Standardized field methods have been adopted in each country to assure that no systematic changes in observation effort are occurring.
See more in Methods, chapter 1.1 Counting birds.