Only 30 or 40 years ago, the fact that there was an observable and distinct southerly migration by migrant butterflies, was little known and largely dismissed by those 'knowledgeable folk' as nothing more than mere fanciful speculation, with little evidence to confirm it as an actual event.

However, butterfly (and moth) migration is very much an annual phenomenon, although the nature of any southerly movement in the late Summer and Autumn months, is dependent on the number of migrants arriving here the previous Spring. Spring butterfly migration is usually much less distinct, although there have been two exceptionally large Painted Lady invasions into the UK during early June 1996 and more recently in ate May 2009.

An average year typically provides very small numbers of both migrant Nymphalids, with the Red Admiral being the more regular of the two. Autumn migration is a drawn out (but ultimately more reliable) event, although numbers vary from year to year and a good Autumn migration is entirely dependant on numbers reaching us in the Spring. Red Admirals make up the greater percentage of all migrants recorded and peak in September. Painted Ladies generally peak a month earlier in August and Clouded Yellows in September.

Choosing a suitable watchpoint

The key to observing and recording the visible migration of migrant butterflies is to seek an open vantage point. Suitable sites can be found pretty much anywhere, and it often helps if there are hedges that run north to south as butterflies typically track along these. Open fields and large bodies of water are well worth watching, where the observer should position themselves at the southern or south-western end and scan low over the ground or water. Migrants generally fly very low (within 60cm above ground/vegetation level).

Scanning with binoculars will reveal distant butterflies and a good day will involve small, loose groups flying through, but single butterflies make up the majority of any count.

To make the data more useful, the counting period can be divided up into hours. This method will show when peak numbers moved through, something that can help when planning future watches. Purposely recording butterflies actively migrating, is not a pastime that comes with instant success and high numbers.

As already mentioned, a fine September day will usually produce a steady trickle of Red Admirals, Painted Ladies or even the occasional Clouded Yellow moving south, low over the ground. It is not totally usual to have a wider variety of species moving through on the same day and the trigger for this when it does happen, seems to be very hot (over 25°C) or dry weather conditions over the previous few weeks.

Getting a decent total is often a matter of luck. The weather has to be right on the day and of course, the butterflies have to be there. A fine, sunny day in September should reap rewards and such a day after a period of poor weather, will often produce the best results.

Red Admirals tend to dominate any counts made in September, with Painted Lady generally peaking several weeks earlier than might be expected in August, when return movements can pass unnoticed. September has proved to be the best month for Clouded Yellow, based on our records.