About these Web-pages

These web-pages provide information about the 36 butterfly species resident in Bedfordshire, plus two regular migrants. Species that are occasionally seen but are not believed to breed in the county have been omitted.

The following explains what is to be found on the species pages...

Record distribution map The Data

378,363 records of 1,611,415 adult butterflies in Bedfordshire covering the period 1990 to 2023 were analysed on 02/02/2024 to produce the distribution maps, flight-time charts and other statistical graphs in these web-pages.

The data were provided by members of the Bedfordshire Natural History Society (BNHS), Butterfly Conservation (BC) and the general public. Thank you to everyone that has submitted records. These web-pages couldn't have been produced without you. Thanks too to those that have assisted with computerising the data, especially Charles Baker the previous County Recorder, who entered and verified the bulk of the data prior to 2003.

To have amassed so much information over the last 34 years is a tremendous achievement and a credit to everyone that has participated. We have quite a good understanding on a broad scale about overall distribution, abundance and trends of our local species, but butterfly populations are constantly changing, so we need to keep a constant watch on what they are up to, and where they may need our help, by continuing the recording effort.

As can be seen from this map showing the distribution of records, coverage is actually uneven with many 1km squares having fewer than 20 records and some areas having none at all. As almost anywhere in the county can have about a dozen species passing through it and others may see over 30, this shows that much of the county is still under-recorded, even after so many records have been submitted.

There is still therefore an opportunity for everyone to contribute knowledge, especially by visiting under-recorded areas and helping to fill in the blanks. There are likely to be unrecorded colonies just waiting to be discovered. See the Recording Butterflies section for how you can get involved.

Species density map

This map shows the species density per 1km square reported over the period 1990-2023.

To a degree this reflects the uneven distribution of recording as well as the actual density of species, i.e. note the large number of areas with 12 or fewer species reported.

Again, this shows the value in visiting areas "off the beaten track" in order to enhance our knowledge of Bedfordshire butterflies.

This chart shows the number of records submitted per year since 1990.

Transects (shown in red) are fixed routes at specific sites that are walked every week for 26 weeks every year, from April to September. These allow scientific comparison of one year with another. The sites at which these have been walked have fluctuated over the years, as has the number of transects in operation in a particular year.

The dark blue bars represent the number of records submitted by all other means, mostly by naturalists' personal observations, but occasionally as the result of targetted surveying. As online recording has become more prevelant the proportion of these records has increased.

This chart shows the number of 1km squares covered by the records each year since 1990.

Bedfordshire has an area of 1,232 square kms, but because there are a lot of partial squares around the boundary Bedfordshire is actually overlapped by 1379 1km squares, so this is the maximum number of squares that could be reported in a year if every square was visited and returned at least one record. That would be an amazing achievement, but even the highest total so far of 632 1km squares visited in 2020 is an impressive 45.8%!

Some of the early years' records were recorded using tetrads (2x2km squares) and 10km squares, so cannot be mapped to 1km squares and therefore do not appear in this graph. Tetrads are too vague for recording and shouldn't be used. When used for mapping records, tetrads hide the detail of the four 1km squares they each contain and may give a false impression of high coverage, so I don't use them.

Species Accounts

Species accounts appear in the taxonomic order listed in Agassiz, D., Beavan, S.D. & Heckford, R.J. 2013 Checklist of the Lepidoptera of the British Isles. Royal Entomological Society, being the most up-to-date checklist available at the time of publishing. This radically changes the order from what we've recently been used to, so don't blame me if you keep going to the wrong place in the list!

Scientific names also follow this checklist, being the latest ruminations of taxonomists who sometimes change their minds about what to call things. For example Purple Hairstreak has been variously known as Thecla quercus, Quercusia quercus, Zephyrus quercus, Neozephyrus quercus and is now Favonius quercus, but for how long? Thank goodness for common names!

1990-2006 2007-2023 1990-2023

Distribution Maps

Distribution maps for each species are presented at a resolution of 1km even though this reveals the locations of our rarer species. Habitat degradation and site destruction due to development are generally considered greater threats to the loss of colonies than collectors, hence it is thought better to indicate at the 1km square level how sparcely distributed some species are in order to highlight the threats they face. (Sites where rare species can be found are generally well known to collectors and can easily be found elsewhere on the internet).

The numbers (9-2) along the bottom of the maps and up the side (2-7) are the Ordnance Survey's 10km gridlines. These allow the 1km squares to be located using OS maps. The Bedfordshire boundary is also shown.

The colours of the squares on the maps represent the number of records for that 1km square. Ranges were selected to roughly equalise the quantity for each range.

Three sets of maps are provided. One, the default, shows the whole 32-year period of 1990-2023. Beneath the map are links for selecting two 16-year ranges, one for 1990-2006 and one for 2007-2023. Just move your cursor over the link to the range you wish to see and the map will change to show it. (Javascript needs to be enabled for this to work). This allows changes in distribution, abundance, or possibly just recording emphasis, to be observed for each species.

Maps for all species and time-periods can be viewed together on the Distribution Maps page.

Where a species was reported as present on a 1990-2006 map, but is absent on a 2007-2023 map, please try to visit the old 1km square and see if you can confirm it is still there - it may simply have been unrecorded in the last 16 years. It is tempting to visit "honeypot" sites for guaranteed views of certain species, rather than visiting out-of-the way places to make discoveries, but please try to be a little adventurous if you can.

This map shows the distribution for Marbled White - move your cursor over the date ranges to see how this common species has expanded its range and is now beginning to be recorded in the north-east.

Graphs

Eight different graphs are presented on each species page showing different statistics derived from the records. Collectively these can be examined to see what they indicate about the status of the species and how it is changing over time. The graphs are now interactive - move your cursor over them to inspect the data.

As many of the records are the product of casual recording (i.e. untargetted and unsystemmatic) it isn't always possible to draw solid conclusions about how a species is faring. Long-term trends may indicate a change in range or abundance, or possibly just recording habits, so use the information with care. A basic trend line is included in each graph, but as this may not reflect the trend in the most recent years it should also be used with care.

Be aware, for example, that during the early 1990s fewer records were submitted, and a higher proportion of records had 2km or 10km precision grid references, which cannot be assigned to 1km squares. Apparent increase of range, or increase of abundance, later in the decade, may be attributable to this. You should study all the statistical data together before reaching any conclusions.

The example graph on the right showing the number of Wall Browns reported per year however truly does reflect a dramatic decline in abundance of this species in Bedfordshire, and it is probably now extinct here!

Flight Times

The number of adults reported during each standard week of the year are presented as a graph that reflects the flight times of the species, e.g.

The period since 1990 has been divided into two ranges of equal duration, analysed and then displayed separately to allow any shift in flight times to be discerned. The numbers on the week number axis are between the two bars for that week's data. The heights of the bars of the two periods have been normalized to show abundance as if the same number of records had been submitted during each period. This allows crude comparison of the species' abundance in both time periods.

The graph above for Small Tortoiseshell shows lower reported abundances during both summer broods during the later time period, but the reported abundance of overwintering adults emerging in the spring is similar.

Remember that each period contained warm, normal and cool years (and months) and therefore the width of a peak in the graph is wider than is likely to be seen in any single year. This is because warmer weather can advance the flight period by a week or two relative to a normal year, and a cool period can retard it by a similar amount.

The records have not been processed in any way to reduce the effects of casual recording, or dramatic events. Thus if lots of people happened by chance to disproportionately report a species in a particular week then this will result in a bigger spike for that week (and the converse will produce a dip). Also, if a species is particularly abundant in a certain week in one or two years, this can produce a similar spike.

Once the limitations of these graphs is understood they can be a useful aid to know when to look for the species on the wing.

Information

The Information section in each species' page presents basic information about the species.

Wingspan of adult butterflies vary, so for simplicity have been stated simply as:
Small: Less than 40mm
Medium: Between 40mm and 60mm
Large: Over 60mm
based upon an average wingspan over both sexes.

There are many good books about butterflies available that can provide greater depth if required and make fascinating reading.

The status of each species as published in "Richard Fox & Emily B. Dennis, 2021, A revised Red List of British butterflies. Butterfly Conservation" is stated. For Bedfordshire's species these fall into four categories: Least Concern, Near Threatened, Vulnerable and Endangered.

The Natural Environment and Rural Communities (NERC) Act 2006 Section 41 lists Species of Principal Importance in England. Bedfordshire has eight resident butterfly species on this list: Dingy Skipper, Grizzled Skipper, White-letter Hairstreak, Small Blue, Duke of Burgundy, White Admiral, Wall Brown and Small Heath.

Contact Details

Keith Balmer is the current Bedfordshire County Recorder for Butterflies. (Peter Glenister shared the role with him from 2003 to 2019, then he retired). If you would like to know more about Bedfordshire butterflies, or assist with recording, please contact Keith:

Keith Balmer
6 Salcombe Close
Bedford
MK40 3BA

Tel: 01234 304741

Email: butterflies AT bnhs.org.uk

This Website

This website was produced by Keith Balmer and received its usual bi-annual update in February 2024 to include the 2022 and 2023 seasons. I prefer to make this information available online rather than in book form because it can then be regularly updated. This is its ninth edition!

All material on this website is copyright. Please ask if you wish to use anything.

All maps on this website were produced using DMAP.