The following information and methodology derives from Butterflies for the New Millenium-Instructions for Recorders published on the Butterfly Conservation website.
Butterflies for the New Millenium (BNM), which began in 1995, is a major project to assess changes in butterfly distribution across Britain and Ireland. The first five years of survey data has been published (see references), but recording is still ongoing, and this data will inform future conservation work.
Butterflies for the New Millennium (BNM), is the name of Butterfly Conservation’s general recording scheme. It covers all species of butterfly across all of the UK, Isle of Man, Channel Islands and the Republic of Ireland. It is run in conjunction with the national Biological Records Centre (part of the Centre for Ecology and Hydrology) in the UK and the Dublin Naturalists’ Field Club in the Republic of Ireland.
Please read this safety information before carrying out a survey.
Butterflies are found in the summer, mainly between April and September. Certain species have a much more limited flight time within this period and single species surveys need to take account of this. If in any doubt consult literature about the species or an acknowledged expert before deciding when to visit. More information about timing of visits is given in the methods section for each of the two methods described.
Details of monitoring frequency for each of the two methods described are given in the methods section below.
Ordnance Survey Maps of the area (Landranger, 1:50,000 series). Site Recording Form (for repeated visits to a particular site). Casual Record Form (for scattered locations).
Two methods are described: 1. transects based on Butterfly transect recording; and, 2. distribution recording based on Butterflies for the New Millenium (distribution recording). The first is the recommended method for ongoing population monitoring, the second aims to identify the distribution of species within a particular area.
Method 1 – Butterfly Transect Recording
This method is recommended for ongoing population monitoring and is based on the Butterfly Monitoring Scheme methodology . In brief, a fixed–route walk (transect) is established at a site and butterflies are recorded along the route on a regular (weekly) basis under reasonable weather conditions for a number of years.
Selection of the transect route
The transect route should be chosen to sample evenly the habitat types and management activity on sites. Care should be taken in choosing a transect route as it must remain fixed to enable butterfly sightings to be compared from year to year. Transects are typically about 2-4 km long, taking between 45 minutes and 2 hours to walk, and are divided into sections corresponding to different habitats or management units.
Butterflies are recorded in a fixed width band (typically 5m wide) along the transect route each week from the beginning of April until the end of September yielding, ideally, 26 counts per year.
Transect walks are undertaken between 10:45 and 15:45 and only when the weather conditions are suitable for butterfly activity: dry conditions, wind speed less than Beaufort scale 5, and temperatures 13 degrees centigrade or greater if there is at least 60% sunshine, or more than 17 degrees centigrade if overcast.
Due to the vagaries of the British and Irish weather, it is rare in practice to achieve a full set of 26 weekly counts. However, a small number of missing values can be estimated using other counts during the season.
Accurate species identification is important. There are several good field guides available, but if in any doubt about an identification, it should be checked by a local expert. Single species counts Rare and threatened species can be monitored using the transect method but just counting the target species rather than all species encountered. Although weekly counts are still required this reduces the time required to walk each transect and therefore the demands on the staff and or volunteer counters. It also reduces the data entry requirements.
Method 2 – Butterfly Distribution Monitoring
This method aims to map the distribution of species within a particular area. Accurate grid references are particularly important, as is accurate species identification. There are several good field guides available, but if in any doubt about an identification, it should be checked by a local expert.
This method requires full coverage of sites and would normally be repeated at intervals of a number of years, rather than on an annual basis.
Defining the target area for monitoring
It is convenient to use a grid of squares (10 km, tetrad or 1 km) to define areas to be searched for colonies. For populous areas, such as much of southern England, a grid of 1 km squares or tetrads (2 km squares) is a convenient way to target recording; in upland/highland areas a 5 km grid is more practical.
Aim to visit sites in as many squares as possible, always identifying records from individual sites with at least a 1 km square accuracy and preferably 100 m (6-figure), so that the data can be used to support conservation work. It may well be that in many tetrads, three out of four 1 km squares may have no useful habitat and records may come from only one 1 km square.
Record the grid reference of the site or at least the 1km square and not the tetrad reference. Large sites should be divided into 1 km squares or sub-sites for recording. See the guidance on recording grid references.
Try to visit different types of habitat in the target area, such as woodland, grassland, hedgerows, farmland and where appropriate, heathland. These will each support a different range of butterfly species. In intensively agricultural areas, seek unploughed headlands, hedgerows, relics of woodland, tracks and footpaths. Set-aside areas and some roadside verges often provide suitable habitat for many species. Urban parks, gardens, churchyards and waste ground may also provide suitable habitats, as may former industrial sites, old quarries and disused railways.
Timing of visits
Try to visit each site at least four times in the season, preferably in early May, mid-June, mid-July and the second half of August, to catch the flight periods of all possible species. Warm sunny weather between mid-morning and mid-afternoon is ideal, although some species may fly earlier and later in hot conditions. Butterflies are unlikely to fly in cool, overcast conditions, but they may be found resting. Re-visits to sites are also valuable to record species whose populations vary widely from year to year.
For repeated visits to a particular site use the Site Recording Form. The Casual Record Form is used to make a note of records from scattered locations.
Enter the recorder’s name and address, the date of the visit, the site name (or the name of a local geographical feature) and the grid reference of the site or locality. Indicate the type of habitat using the codes listed on the form. For each species seen, record the numbers seen at that locality, either using the A-E abundance codes on the form or the actual number of individuals. In the latter case it is useful to give the time (in minutes) spent searching. Full instructions are given on the forms.
Where unusual or threatened species are recorded, give as much detail as possible of the locality so that follow-up visits can be made to assess conservation needs. Confidential sightings can be marked as such on the form.
Return completed forms promptly at the end of each recording season to the local co-ordinator or to Butterfly Conservation.
Butterfly Conservation website
The Millenium Atlas of Butterflies in Britain and Ireland. 2001, Oxford University Press