In the late 1980s I remember driving through Normandy and being impressed by the amount of Mistletoe (Viscum album) growing at the tops of roadside Poplars; there was nothing like it just across the Channel in England. The only Mistletoe I knew of then grew on back garden apple trees in Huntingdon Road and Coldham’s Lane – I suspect these Mistletoes had been deliberately established by gardeners. The clump in Coldham’s Lane was half the size of the small apple tree on which in grew.

A paper in Volume 51 of Nature in Cambridgeshire (Cadbury and Oswald, 2009) describes the explosive increase of Mistletoe in Cambridge. Most of their records come from west Cambridge – the University parklands, gardens and sports fields; I am sure the increase was just the same in the residential north and east of the City but less obvious. The authors speculate that two decades of milder winters enabled Mistletoe to establish itself so successfully.

Every time I travel along Madingley Road I am impressed by the number and size of Mistletoe clumps; lower down in the apple trees and hawthorn there is just as much Mistletoe. An apple tree in a Huntingdon Road garden is so infected with Mistletoe that the host has died. In Chesterton and Arbury it is well established in garden apple trees and boundary trees to recreation grounds.

A principal vector of Mistletoe is the Mistle Thrush (Turdus viscivorus). It’s an early nester and establishes territories from mid-winter, singing from the tops of trees. Its song is like a Blackbirds but less varied, loud and mournful! It feeds on winter berries and vigorously defends shrubs and bushes with berries within its territory, including Mistletoe, from other thrushes. I don’t know if the generic name for thrush means what it sounds like but Mistle Thrush droppings are very “viscivorous”! Mistletoe seeds are excreted by the thrushes with their sticky seed coating intact and stick to branches and stems. The seeds do not need any pre-germination treatment by passing through the thrushes’ gut. Mistle Thrushes probably account for most of the Mistletoe established in the tops of trees.

There is another bird which may be a more efficient Mistletoe vector. Blackcaps (Sylvia atricapilla) are “scrub” warblers nesting in low thickets and feeding in the woodland mid-storey. They are mostly summer visitors. Between 1981 and 2011 the number of Blackcaps overwintering in England, Scotland and Wales has increased by 57% (Balmer et al., 2013). Most of these overwintering birds are from central Europe and have migrated NW to the UK rather than SW to Iberia. (See Blackcaps blog). Milder winters and the easy availability of food from garden bird feeders are thought to be the reasons for this change in migration behaviour. Blackcaps do not excrete Mistletoe seeds but wipe them off their bills directly onto a tree branch with the seeds sticky seed coat, viscin, attaching the seed to the host plant. Blackcaps are therefore probably the reason for the explosive increase in Mistletoe in garden trees and shrubs.

Viscin was (and probably still is) used to make bird lime – the sticky paste that illegal trappers spread on stems and branches in southern Europe to catch migrating songbirds. The cruel irony is that their major target species is the Blackcap.

Bob Jarman

January 2017

Please let us know where you see Mistletoe growing in Cambridge. See our Mistletoe survey for more information.