Thanks to the information that you contribute, BirdLife International can build up a picture of bird numbers returning to Europe, their arrival dates, and which regions the different species are coming back to. We are not attempting to count all of the birds coming back to Europe (an impossible task) but to count a proportion of their population in the same way every year.
The results can be broken down and analysed at different levels (European, country or region), providing a huge sample from which we can investigate trends. In addition, mapping the results for a particular species in any one year gives us a detailed picture of their pattern of arrival.