Quite a lot has been written about the Purple Emperor, including most notably ‘Notes & Views of the Purple Emperor’ by Heslop, Hyde & Stockley (1964). A thorough grounding in the literature may well be helpful; what follows is based on my own experience.
In the East Midlands we are fortunate to have the famous Fermyn Woods site where Purple Emperors are easy to see, yet experience gained there may be slightly misleading. When I have seen them elsewhere, be it abroad in France or at home in Rutland, they have seldom come down to tracks but rather spend most of their time around the canopy. Once you have found the right habitat patience may be required – it is not unusual to wait an hour or more between sightings. Sightings at Fermyn are, however, a useful indication that they are likely to be on the wing in Rutland.
In Rutland the best habitat – a combination of oak and most importantly sallow – is found in the north-east, but other woods (e.g. Burley, Prior’s Coppice) have suitable areas so further spread is possible. 2017 is certainly a good year for Purple Emperor, so the increase in Rutland sites is likely to be genuine as well as a reflection of more skilled searching.
I have found that the most productive period is between mid-morning and early afternoon, on warm days (high teens or low twenties) with at least intermittent sunshine. It is important to concentrate the search around areas of sallow, where males fly quite regularly. They are quick on the wing, and there may be lengthy periods between sightings. It is not uncommon to see them only in flight, or settled high in the canopy of oak or ash close to sallows, so binoculars may be required for a good view. ‘Grounded’ males, and females, are seen much less frequently.
We now know that Purple Emperors are present in Addah, Clipsham Park, Pickworth Great and Stretton Woods. It does seem that they are increasing, but they may have been present at low density for some time without being recorded. The challenge is to track their spread as best we can.