The abundance of mistletoe, Easter and two garden birds

Bob Jarman writes about a plant that you might typically think of in connection with Christmas, and discusses two species that help spread it.

Mistletoe is part of our Christmas traditions but in the Middle East it is also part of the Easter story.

The crown of thorns forced onto the head of Christ by Roman soldiers as he carried his cross through the streets of Jerusalem was originally cut from a bramble bush. It is said the original bush grows in the garden of the Bishop of Jerusalem and a single cutting from it grows in St Catherine’s Monastery in the Sinai.

The Easter story is that God was so angry with the bramble bush he changed it into mistletoe so the thorn bush became a fleshy, spineless, parasitic plant.

Visit Jerusalem at Easter and you can buy an “authentic” crown of roses souvenir mounted in a glass topped case. The small label on the back of the case says “Made in China”!

Mistletoe grows as separate male and female plants and only the female plants have the white berries. In the last 20 years mistletoe has become an abundant plant, growing on trees throughout the city. Perhaps this increase is due to global warming, perhaps it is also due to an increase in birds eating the berries and spreading the sticky seeds.

Mistletoe is a parasite and grows into the bark of trees and taps the water and nutrients the host tree absorbs through its roots. The flesh of the berry is edible but the seeds are not and birds wipe them off onto the tree’s bark where they germinate.

There are two species of birds in particular that spread mistletoe, the mistle thrush and a small warbler – the blackcap.

I have seen territorial mistle thrushes aggressively defending mistletoe, even defending whole trees that have dense berry-bearing clumps. Mistle thrushes sing from treetops over winter from November to April and a recent survey counted 23 singing males across the city. They nest early, usually in March.

The blackcap is a species of warbler. They are greyish in colour; the males have a black cap and the females and young males a brown cap. They are summer visitors, but many have recently over-wintered in the UK in increasing numbers especially in the last 20 years, which has coincided with the increase in mistletoe.

I have also seen blackcaps defending berry-bearing mistletoe clumps. A recent survey in the book The Nature of Cambridge located 39 over-winter blackcap sites in the city. They are birds of shrubs, hedges and especially gardens in winter often eating from bird feeders and they are probably responsible for the increase in mistletoe on garden apple trees.

I have seen some garden apple trees so densely parasitised with mistletoe that they are dying, probably due to their severe infestation. This is not good news for the host tree or the parasite!

The late Roger Istead used coloured leg rings on blackcaps he caught in his Harston garden expecting to find a single bird but ringed 12 different blackcaps one winter and 13 different blackcaps the following winter. All were different individuals from those of the previous winter.

Other ringing results show that wintering blackcaps in the UK come from central Europe – Slovakia, Austria, and southern Germany. These birds migrate northwest to the UK in October/November instead of migrating south west to Morocco to spend the winter.

This change in migration behaviour may be having another remarkable effect. Some observers claim that overwintering UK blackcaps develop a greyer plumage and thicker bills. This change in bill shape is a modification to feed at garden bird feeders rather than their usual insect diet.

Our over-wintering blackcaps return to central Europe and breed earlier than blackcaps returning from north Africa. Perhaps a new species is evolving before our very eyes just like “Darwin’s finches” in the Galapagos. They seem to be establishing a separate and different population, and mistletoe is part of this story.