Exercise 2 solution (sample answer)

A common method (same route, same time window, count every bird) makes counts comparable between people, places and years, so a change in numbers reflects the birds and not a change in how people counted. Without it, a rise could just mean more volunteers or longer watching.

Possible biases from the chapter: untrained volunteers may misidentify birds (Thelen and Thiet, 2008); accuracy varies with task difficulty and experience; most participants share similar backgrounds, which may shape where counts are done (the article states the participant profile; the link to place is my inference; check).