If the current fine weather continues well into June, the first Purple Emperor of the year should appear on Midsummer Day, June 24th, around 1pm, behaving badly.
If the weather wobbles, then June 28th is more likely.
If the weather collapses, then make it July 1st.
The difficulty here is that long-range weather forecasts are still not possible.
The first sightings tend to come from Bookham Commons, Surrey, or Knepp, West Sussex, two well-monitored sites containing sizeable populations. The M25 corridor also harbours a number of 'early' sites. NW Kent (around Gravesend) should be even earlier. As with all our butterflies there are early-appearing, mainstream and late-appearing populations ('early', 'mainstream' and 'late' sites).
The flight season is likely to start much later to the north and west, in early July.
It is likely that numbers will vary greatly from region to region, and even site to site, depending on the impacts of last July and August's heatwaves, in which eggs boiled in the bag, baby larva desiccated and droughted sallows dropped myriad sub-canopy leaves, especially on less moisture-retentive soils.
At sites where the drought impact was relatively low, numbers could be remarkably high, as winter predation rates have been low and the weather was fine during the crucial end of May / early June period (pupation time). Well-known sites in this category include Bookham Commons and Alice Holt Forest, Hants, and possibly also Savernake Forest, Wilts (though the FC has taken a number of ride-side sallows down there).
Also, and more critically, the shorter the period iris spends in the vulnerable pupal stage, the more we see on the wing - the longer, the fewer. And, of course, much depends on flight season weather.
My guess is that numbers could be relatively impressive in the western and northern parts of the Empire, but relatively poor overall in the Southeast and East Anglia regions, where last summer's drought was most severe - but note the local exceptions mentioned above.
Numbers at Knepp should be relatively low, as the drought was severe there.
We can anticipate the butterfly turning up in new districts this year, not because of clandestine introductions of captive-bred stock but because of natural spread during anticyclonic weather last July.
We shall find out... and all shall be revealed and celebrated on this illustrious but rather clunky Blog, as it unfurls; and also on Twitter @MatthewOates76.
Oh good, it's allowing me to post piccies again...
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