This is especially true for biological data, because of the huge variation. I do this just to provoke Matthew, because he hates trying to tie down that independant spirit, HIM, with numbers. Nevertheless, I do have a lot of data, gleened from the observations from about 30 Upper Thames enthusiasts each year since 2003..
Only from 2009, however, are the data sets large enough to be able to do anything useful with them. For instance, I have been asked the question, what is the time between the first sightings and the beginning of the peak flight period?
So, for this 8 years, 2009 to 2016, the average time was 10.5 days, with a 90% confidence interval of 8.3 to 12.7 days. For example, for this year, the first sightings at Knepp were on the 14th of June, or thereabouts, suggesting that the peak starts from 22nd until 27th June. In the Upper Thames region, the first sightings were on the 17th June, giving the start of the peak from 25th June until 30th June.
We have seen that the peak is rather extended, lasting between one and two weeks.
Depending upon how reliable this is, it is unlikely that we will be seeing many Emperors after mid-July, although we will see tired Empresses.
I look forward to being struck down in ignominy!
All the raw data is available from the appendices of our annual reports, which are accessible on the Upper Thames website, under 'species champions' and then 'purple emperor'.