Mill Hill and its Butterflies
by Andy Horton

Mill Hill Local Nature Reserve is situated on the chalk downs north of Shoreham with  splendid views over the Adur valley. It is visited by dog walkers, ramblers and people enjoying the view, but there are lesser known aspects of the hill, its natural wildlife and an unusual story about the Chalkhill Blue Butterfly.

Horseshoe Vetch
Hippocrepis comosa

Horseshoe Vetch (Photograph by Andy Horton)
Horseshoe Vetch with seed pods

Historically, Mill Hill was the second most important site for the Chalkhill Blue Butterfly in the whole of England. Alas, with modern agricultural practices most of the original chalkhill downland with special herbs has now disappeared. The most important herb is a small yellow legume called the Horseshoe Vetch which is toxic to most living organisms but its leaves are eaten as the only food plant of the Chalkhill Blue and Adonis Blue Butterflies.

Early in the last century this herb could be found in large swathes of the downs, but it is now restricted to small pockets, and one of the largest of these are the five acres of the lower slopes of Mill Hill. This small herb only grows on low fertility chalk downs, chalk pits, wasteland and road embankments.  It flowers in profusion on Mill Hill during the month of May where it can be seen from miles away.

The Chalkhill Blue

The Chalkhill Butterfly has one brood every year and the blue male emerges in mid-July, followed by the brown female about a week later. At the end of the July and beginning of August, in a good year (the last was 2003) the whole downs can become alive with blue butterflies so it appears impossible to walk on the springy turf without stepping on them. The actual numbers are about 750 to an acre on Mill Hill in 2003, but much less in subsequent years.

Chalkhill Blue (Photograph by Andy Horton)
Chalkhill Blue Butterfly female

Shoreham Bank (Mill Hill) was nationally the most famous of all of the Sussex butterfly localities, yet its reputation was gained from just one phenomenon - the numbers and aberrations of the Chalkhill Blue. The site's 20th century history has been mainly recorded by two giants of the net - R. M. Craske and A. E. Stafford.
The Chalkhill Blue. that archetypal downland butterfly. has been nationally celebrated on the Bank since at least the 1820's - but. after a century and a half of tradition, I have seen more than one aged lepidopterist's eyes fill with tears when discussing the insect's modern history on those hills …
A Revised History of the Butterflies and Moths of Sussex
Colin R Pratt
Publisher:  Booth Museum of Natural History
CD-ROM  1999

The most unusual part of the story is that from the early 19th century to the 1950s there was a craze for butterfly collecting including a search for unusual forms of the Chalkhill Blue which were killed and pinned and are now in the large Cockayne Collection now at the Natural History Museum in London. The story is documented on a CD-ROM by Colin Pratt.

"Our family lived at The Mill House, Mill Hill, from around 1933 until about 1967, and every July we saw the “Butterfly Men’ walking past onto the Downs. My father used to tell us that they were interested in the blue butterflies."
Heather Clark (née Eager), Ryde, Isle of Wight

Butterflies of Shoreham

Mill Hill hosts 33 species of butterfly, some like the Common Blue, Brown Argus, Small Heath and Dingy Skipper are downland specialists. Others like the Wall Brown, Grizzled Skipper, Small Skipper, the Small Blue, Small Copper, the common Meadow Brown and Marbled White favour the downs but are found on wasteland elsewhere, whilst the most colourful butterflies seen are locals or immigrant butterflies like the Painted Lady that will visit the downland flowers for nectar. Woodland butterflies like the widespread Speckled Wood inhabit the Hawthorn scrub on Mill Hill.

I've watched you now a full half-hour;
Self-poised upon that yellow flower
And, little Butterfly! indeed
I know not if you sleep or feed

Adonis Blue

William Wordsworth

What makes Mill Hill so important for the blue butterflies? The answer lies in the very special nature of the low fertility chalk soil, undisturbed by the ravages of ploughing and heavy grazing. The small downland herbs lose out to larger common opportunist weeds in rich soil. This fragile environment at Mill Hill is unique because historically it was managed as an unimproved rabbit warren and taken in public ownership before World War II. Many of our Nature Reserves are like that, presented to the nation because the land was infertile uneconomic and unsuitable for agriculture.

A large part (724 acres) of the downs including Mill Hill were presented to the people of Shoreham in 1937.Just over 30 acres still remain as public open land and a Local Nature Reserve.  This is divided into about 11 acres of grassland and meadows above the ridge, about 9 acres of scrub, the copse and glades at the northern end, and about half of the prime Chalkhill Blue area of 6.4 acres of herbland remaining. 6 acres has been lost to a Sycamore woodland on the southern slopes.

The Future

What does the future hold for Mill Hill and its butterflies? The answer is that it is up to man. The downs are a man managed environment and has been for two millennia. Left to nature the land would quickly revert to woodland by a process called ecological succession. Woody hedgerow shrubs like Privet, Brambles and Hawthorn would rapidly invade the downland and shade out the small herbs and then without their food plants the butterflies would be eliminated.  This has already happened in my lifetime with six acres completely lost to a Sycamore wood.

In the past the land owners or commoners would use the wood for fuel, but in modern times it is a cost to maintain the Nature Reserve. On Mill Hill, it is Privet that is the main invader and this needs to be removed by professionals. If it is not removed the butterfly site will be destroyed in just a few years.

Web Site
Mill Hill Wildlife Reports 2009

A list of the Flora of Shoreham-by-Sea including Mill Hill has been compiled by Betty Bishop.


In the last good year in 2003 the numbers of Chalkhill Blue Butterflies recorded were:

Chalkhill Blue
11 July 2003
 First visit
11 July 2003
20 July 2003
21 July 2003
30 July 2003
2 August 2003
7 August 2003:
20 August 2003

 UK Butterflies Chalkhill Blues 2009

Butterfly & Large Moth List 2009
Butterfly Habitat Notes

Observations of the habitats of the Small Heath Butterfly

Observations of the habitats of the Small Copper Butterfly

Some Notes on the local Meadow Brown Butterfly populations

Notes on the Wall Brown Butterfly

Bird's Foot Trefoil & the Common Blue Butterfly

Some other notes on resident butterflies and moths in the Adur area (Part One)

Some other notes on resident butterflies and moths in the Adur area (Part Two)

Adonis Blues notes from the downs near Shoreham

Chalkhill Blues notes from the downs near Shoreham

Skippers of the downs near Shoreham

Observations of the other Butterflies of the Adur district area and a few absentees

Observations of some of the smaller Moths in the Adur district area

Etymology of the word "butterfly"

Lead Agencies for designated Local Nature Reserves

Clouded Yellow Butterfly  (Link to a recommended photograph by Dave Appleton)


Adur Butterfly Flight Times