Big Garden Birdwatch
Every year the RSPB run their Big Garden Birdwatch. A nationwide project in which you spend an hour counting the birds that come in to your garden. The idea is to get a number of the most birds you see of each type at any one time.
This gives the RSPB a good idea of bird populations throughout the UK. How are different birds thriving, falling in numbers, or moving around the country. According to the website it's the worlds biggest wildlife survey.