The swallowtail occupies a precarious niche in the UK. Confined nowadays to the fens of the Norfolk Broads, they depend on the management activity of various conservation bodies and their dedicated staff to maintain the habitat that supports their larval foodplant the milk parsley. This fragile plant can easily be crowded out by more vigorous growth, so needs to be given space to thrive. This happens best when fenland is regularly cut, restraining the more robust plants and giving milk parsley a fighting chance.
The whole Broadland ecosystem is under threat from a variety of factors: climate change resulting is rising sea levels, increasing salinity in the wetlands through salt water inundation from tidal rivers, degradation of some habitats not under strict management, general pollution from farmland runoff leading to eutrophication, proliferation of invasive non-natives, and so on. The usual picture. In some areas it seems increasing salinity is having serious adverse effects on milk parsley, making it grow weak and even disappear from some sites. That could prove disastrous, or maybe not……
As I write this (July 2022), at RSPB Strumpshaw Fen there is a well developed swallowtail larva happily munching its way through a hemlock plant that grows just outside the Reception Hide. I’ve seen almost full grown caterpillars feeding on angelica at Ranworth and a recent study, published in the Transactions of the Norfolk & Norwich Naturalists’ Society (Kevin Radley & Hannah Breach), indicates successful breeding on hemlock to be much more common than previously thought. This may be a recent phenomenon, or possible something that has been overlooked in the past. It does seem though that the butterfly may be evolving its life cycle in response to changing conditions.
The swallowtail we have here in Norfolk forms a distinct race britannicus which is slightly smaller and has subtle variations in wing pattern when compared against the continental race gorganus. Numbers of the continental form do reach the UK every year, and have been proven to breed in the south of England. If these cousins from across the English Channel are successful in colonising England, they may soon move north to hybridise with our natives and therefore eradicate the form. I’ve always had mixed feelings about this. On the one hand there is no doubt that having something special living only in our region is a real boon, enhancing the reputation of the Norfolk Broads as being a unique haven populated by rare creatures. Further, the need to conserve swallowtails has been a major factor in the drive to clean up the Broads, create and maintain pristine fen habitat and develop new reserves, all of which benefits not only the butterflies but a vast wealth of other wildlife. In addition the swallowtail are a major tourist attraction and pull in visitors from all over the country who seek to catch a glimpse of this strong flying jewel in its only UK stronghold. But, on the other hand, the continental race which will breed anywhere and feed on all sorts of umbellifers may well thrive, leading to them becoming more widespread and enabling us to see them in our gardens on a regular basis.
In any event, swallowtails are a delight to see, whatever their origin.
And finally a few pics.
