Survey analysis. Question for RSPB

Am fairly new to this, and have just submitted my 2012 survey. My question is this ; how can meaningful conclusions about species decline or increase be made on the basis of an hours survey? For example my tally was 8 species, but if it had been last Sunday it would have been 15 when there was snow on the ground and it wasn't as windy. I know I have jays, wrens, coal tits, song thrush, starlings, etc In and around my garden, but they just happen to not be here today! So if the RSPB say "there has been a reduction in Jay numbers" what do they base this on?