There is some evidence that climate change causes shifts in the phenology of species. Is it possible that a detected decline in numbers of a species is caused by the fact that the observers missed birds because these birds nowadays are singing or nesting a few weeks earlier than in former years?
No, this is highly improbable.
Breeding bird surveys are designed to span the full breeding season of birds from spring through to summer. They typically involve between two and 12 visits to a sample site per year through the breeding season to count birds. In this way, bird surveys span the full period when birds might be singing and nesting, and any shifts to early nesting would lie well within this period. Typically, the observed shifts to early nesting have been around 8 days in the UK so they are still relatively small in relation to the length of the breeding season and survey activity.
Note also that when a survey has just two site visits per year, one ´early´ in the breeding season and one ´late´, the higher count from the two visits is taken as the best measure of that species´ bird abundance on that square, and is used in subsequent analysis. This is the case for the Breeding Bird Survey used in the UK and several other European countries. The use of a maximum figure per species thus avoids the potential problem that earlier nesting might drag the mean count per visit downwards through time. We will however keep this methodological issue under review and check for bias.