Swallowtail (Papilio machaon britannicus): Striking regional specialist.
If you are out and about amongst the broadland landscape during high-summer and the surrounding habitat resembles this, you could be in prime swallowtail country. The presence of milk parsley is the final piece of the jigsaw puzzle.
I'VE READ IN TEXT BOOKS and heard in conversations that the flight of a swallowtail butterfly is weak and fluttering -- lacking in direction. These can only be the words of observers that merely follow the scribbles and misinformed mutterings of the armchair naturalist rather than enjoying firsthand the spectacle of surveying these enigmatic insects winging across the open expanses of water and reed beds that comprise their broadland stamping ground. Never have I studied them in full flight without being incredulous in the knowledge that I'm viewing a diaphanous insect whizzing back and forth across the fen, redolent of a gaudy pipistrelle bat, such is their direct and purposeful flight: a creature pervading of agendas that need to be met and kept. It would be an improbable faux-pas to confuse this striking invertebrate with any other British insect -- very distant, late flying male brimstones may bring about some brief over-excitement -- such is their aerial prowess and striking charcoal and lemon branding.

The drainage of the once vast fenland landscape over the past 150-years -- in order to expose the richest of all agricultural soils -- has been well documented as the number one factor in bringing about the close to catastrophic decline of the swallowtail, regarded as a Near Threatened species due to its extremely restricted distribution and ever fragile habitat, pocketed within the Ant, Bure, Thurne and Mid-Yare valleys: a landscape widely recognised as the Norfolk Broads. The range of Papilio machaon britannicus, largest of the British butterflies, once covered much of southern Britain as observed by the Rev. F. O. Morris in his 1852 publication 'A History Of British Butterflies'. Locations as far afield as Weston-Super-Mare in Somerset, Wooton Glanville in Dorset, the counties of Huntingdon and Cambridgeshire; even Cottingham in the East Riding of Yorkshire gets a mention, as does St James' Palace in London, where it was referred to as the 'Royal William' by James Petiver in 1717 (presumably in recognition of the reigning monarch at that time). Yet even then fenland retardation was a concern, leading Morris to comment that losing the swallowtail would remove "this most conspicuous ornament of our cabinets," referring to the then Victorian obsession of butterfly collecting. (ref. Ivan West's excellent booklet 'The Swallowtail Butterfly', available from Butterfly Conservation and Jeremy Thomas, 'The Butterflies of Britain and Ireland'.)

The fenland habitat that the swallowtail is so reliant upon is a transitory growth that lacks climax vegetation due to the necessary and continual mowing and cutting back of seasonal advance, without which the fen would eventually revert to alder and oak woodland, suffocating the plant community that fen dwelling species depend upon to survive. It is this mix of fenland vegetation -- in particular the grasses, flowers and horse tails that make up what was once referred to as marsh hay, used as bedding and fodder for livestock and the beds of Great Fen sedge (Cladium  mariscus) and common reed (Phragmites australis), used as roofing material -- that provide ideal growing conditions for the most important plant in the swallowtail's lifecycle: milk parsley (Peucedanum palustre). Found across most of Europe and Central Asia, this umbellifer, a member of the Apiaceae family, gets its common name from the pellucid liquid that seeps from the young plant when damaged, which can quite easily be overlooked when walking through suitable fenland surroundings.
To understand the importance of milk parsley in the swallowtail's existence (it is the only foodplant the butterfly will use), a brief look at the insect's lifecycle is of great assistance. It begins (or climaxes) during the latter half of May, when adult butterflies emerge from their pupae with just two prerogatives in mind: feeding and mating. To seek out suitable stands of energy-rich pink or purple fenland flowers -- ragged robin (Lychnis flos-cuculi), marsh and meadow thistle (Cirsium palustre and C. dissectum) and valerian (Valeriana officianalis) are favourites, as is Yellow Flag Iris (Iris pseudacorus) for the early emerging insects -- is the first manoeuvre of the butterfly, before the imminent thoughts of procreation take hold. Once mutually detected, the male and female will spiral and tumble together above the pre-noon landscape, before dropping without warning into the fenland vegetation where they will remain mating for several hours. There is no time to waste however, as the adults will only live for approximately one month, hastening the female to seek out suitable stands of milk parsley to lay her eggs upon. It is here that a phenomenon know as 'Edge Effect' takes place, where she will be drawn to the prominent food-plants growing alongside linear features such as ditches and cleared pathways: fastidious in selection, she will scrutinise each stem and leaf for suitability, laying only one egg on each plant visited once she is satisfied that her future progeny will have the best start in life.

The egg, minute and lime green in colour when first laid (shifting to brown with age), hatches after 7 - 10 days, when a tiny larva (caterpillar) emerges, resembling of a bird dropping, its black, spiny form sporting white blotches on either flank. For a solid month the miniscule eating machine will gorge itself on milk parsley from whence it will morph through four instars (moults) and grow to the size of a person's little finger, all the while attaining a lurid mint-green, black and tangerine colouration in a vain bid to deter avid predators (sedge warblers, bearded tits and reed buntings in particular): the larva have a final form of defence should their colouration fail them: a two-pronged growth called the osmaterium is raised from the top of the head when under attack, simultaneously releasing an acrid perfume likened to rotting pineapple: however, the success of this defiance is varied, particularly when used on seasoned predators that have seen it all before. Pupation is the next stage of life, and by this time (July) the parent insects will have perished, leaving their offspring to continue the lineage. Vacating the now tatty looking milk parsley (several larvae can be present on one plant) and moving to the bottom of alternative vegetation, the pupae will form -- adopting the colour of its chosen anchorage in an upright attitude, attached by silken threads -- to effect one of two actions: to emerge as an adult during August, or hibernate over winter and emerge the following year.

The drainage of the fenland landscape... has been well documented as the number one factor in bringing about the close to catastrophic decline of the swallowtail...
Resplendent in its gaudy bright yellow and black colourations, the swallowtail butterfly is hard to miss at close quarters. They are seldom still however, even when feeding, as their weight makes balancing on flower stems difficult. 
A cropped close-up of a swallowtail butterfly's beating wing displays the resplendent colours that have made them the apple of many-a-naturalist's eye. The British race Papilio machaon britannicus is slightly darker than its continental cousin, P. m. gorganus.

The fenland habitat... is a transitory growth that lacks climax vegetation due to the necessary and continual mowing and cutting back of seasonal advance...
A number famous of sites are easily located within the broadland network to view these magnificent creatures, some even holding open days inviting members of the public to attend guided tours, offering-up the possibility of extremely close exchanges with the butterflies. One such location is the Wheatfen, fabled former home of the noted Norfolk naturalist Ted Ellis. Other prime spots are Reedham Marsh at the How Hill Trust, R.S.P.B. Strumpshaw Fen and N.W.T. Hickling Broad. A worthwhile piece of advice: you don't need to get up early; the swallowtails rarely fly before 10.00am on warm, sunny mornings!

Interesting fact...
The Continental race of swallowtail butterfly, Papilio machaon gorganus, varies only slightly in appearance from its British cousin, being paler in colouration and slightly larger. It regularly flies across the English Channel, visiting several southern counties each summer but has one trait not shared with the Anglo Norfolk variety: it will lay its eggs on a wide selection of umbellifers, including wild carrot and fennel, liberating itself to colonise new territories out of reach to the British insect.
Beating its wings frantically to stay balanced, a swallowtail butterfly feeds at a thistle flower growing in the butterfly's Broadland home. These spectacular insects prefer the purple flowers of thistle, valerian and ragged robin, although they'll take what they can when favourite flowers are not in bloom.


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Text and photography © James Williamson | info@norfolknaturesafari.co.uk
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