In a nutshell, where the Census Team at the BTO identifies trends in bird populations, so the Demography Team provides some of the tools to explain these changes.
Changes in birth rates (productivity), death rates (mortality and survival), immigration and emigration are what drives these changes in populations, so it is these that we are interested in. Even at the small scale, what we see can be fascinating. Just since the Christmas break, one small net in my Norfolk garden has caught 99 Blue Tits and Great Tits.
Now this isn't an exceptional catch, but what was interesting was that three quarters of these birds were adults. So where are all the young birds?
The past two summers have been incredibly poor for tits, with the wet weather causing havoc with adults trying to feed hungry chicks. Both species have had their lowest ever productivity, producing half as many chicks compared to the 'norm'.
So will this run of poor breeding seasons affect the population as a whole? Demography may well have the answer, so watch this space.