I love the way a chance conversation can lead to a whole new subject for investigation. The other day, a colleague asked me whether I’d noticed mistletoe (Viscum album) in Shropshire growing where it never used to, and it got me thinking. I don’t remember seeing it growing when I was a child in Wellington and just over the Cheshire border around Audlem, or as an older teenager around Ellesmere. Last time I walked around Colemere there was a splendid clump of it in a hawthorn tree, and Tom has spotted it popping up all over Shrewsbury. We even have a splendid show of it in the garden at The Cut. So is it a recent thing, or did I just not notice it? 

My curiosity piqued, I did a bit of research. It seems there has been quite a lot of interest in the distribution of mistletoe and whether it has declined with the loss of traditional apple orchards, as apple trees are a common host for the parasitic mistletoe. Survey data is available from the 1970s and 1990s, and a current study is looking at how it stands now. It looks like we were right. 

The heartland of the UK mistletoe population is the south and west midlands, especially Herefordshire, Worcestershire, Gloucestershire, Gwent and Somerset. Mistletoe does occur in scattered locations elsewhere but is only regionally abundant in this core area. The northern boundary of the core area is the Clee Hills, according to the Mistletoe Pages, a website full of information about mistletoe.
The counties listed above are indeed rich in apple orchards, but the decline in traditional orchards seems to have no impact on the distribution of mistletoe. 

The plant will happily grow on other hosts – poplar, lime, hawthorn and occasionally oak. Climate change may be having an effect, as the plant has a traditional preference for the warmer, wetter climate of lowland western areas, conditions which are increasing. However, the balance of opinion seems to point at a different factor. Blackcaps (Sylvia atricapilla) are small migrant warblers, historically breeding in Britain but departing to overwinter in southern Europe. Since the 1990s increasing numbers have been recorded here during the winter months. These have been shown to be migrants from Germany and Austria rather than breeding birds simply staying put. Blackcaps are particularly efficient distributors of mistletoe seed. The eponymous associated bird, the mistle thrush (Turdus viscivorus), eats the entire berry and the majority of dispersal relies on the rather sticky seeds being deposited in droppings. 

Blackcaps, however, with their much smaller beaks, eat the skins and pulp rather daintily. The sticky seeds adhering to the beak are then wiped off on a convenient branch where germination might follow. The presence of blackcaps when ripe mistletoe berries are available (usually around February) might be an important factor in the increase in the plant’s range. Relatively few birds in the UK are tempted by white berries – fieldfares and redwings are also recorded eating them, as are waxwings, but the coincidental timing of the increase of mistletoe distribution and the changing behaviour of overwintering blackcaps seems to indicate a causal link.

So next time you spot a glorious globe of mistletoe – the original Golden Bough, with all its mythological and Christmas associations – high in a Shrewsbury poplar or head-height in a hawthorn, and indulge in an al fresco seasonal kiss, remember the tiny bird that flew 700 miles all the way from Germany to plant it for you! The complexities of ecology are truly amazing.

If you want to help to map the spread of mistletoe you can take part in the MistleGo! Survey – visit the Tree Council website. https://treecouncil.org.uk/science-and-research/mistletoe-research/ and download the app.