The additional coverage was completed earlier in 2013 and we are now embarking on the analysis of the data gathered. Just to recap, the main aims were to assess overall population size, compare numbers and distribution since the last survey conducted in 1999, and to gather new information on distribution of the species across different habitats.
A secondary but highly novel aim of the survey was to assess the pairing status of individual Nightingales. No previous survey of a breeding bird species in Britain, and possibly in Europe, has attempted to estimate what proportion of the territorial population might be pairs.
Understandably, most estimates of ‘breeding populations’ are based on singing or territory holding birds, yet this takes no account of how many of these birds may actually be paired and therefore probably breeding.
Taking the 2012 and 2013 counts together, we now have a good basis for estimating population size and habitat use. Overall, a total of about 3300 territories were recorded.
The preliminary map shows the distribution of these territories recorded during main survey visits. It is worth pointing out that in both 2012 and 2013 there was a remarkable record of a territory holding bird in Cleveland, perhaps the most northerly record for many decades.
Statistical techniques have been used to estimate the numbers of birds likely to be present in areas which weren't covered and also to correct for detection – this is necessary because the probability of detecting all the territorial birds that are actually present on the two early morning visits is considerably less than 100%.
Although we have not yet calculated the final population estimate for 2012/13, we now believe that the 1999 survey underestimated the population, mainly because strong emphasis was placed on nocturnal visits which tend to underestimate paired birds which are thought to sing less at night than unpaired birds.
There are plans to write two journal papers based on the Nightingale survey together with an article in BTO News. The first will focus on numbers, distribution and habitats. We hope this will be submitted to a journal early in 2014. The second will address the issue of pairing success and will explore whether this varies with factors such as habitat type, position in the range and size of local population.