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Birds and berries are a remarkable example of interaction in nature.
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Berries - they grow
on trees, bushes, climbing plants and on shrubs at ground level. In the
British Isles alone, more than sixty species of native plants produce
them. Animals love them, especially birds. Thrushes and berries go together
like ice cream and August but these wild fruits are an equally important
ingredient in the diet of black grouse and capercaillies (bilberries).
In the Arctic, even large mammals such as polar bears gorge for days on
blueberries. For birds, the range of berries and the extended length of
the season during which various types become available - from cherries
and elderberries in July to holly and ivy long after Christmas - means
that many songbirds switch from a summer invertebrate diet to a winter
one based on berries. When cold weather arrives, frozen ground prevents
foraging for worms, grubs and fallen seeds, leaving berries as a nourishing
lifeline. What's in this for the berries?
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Birds and berries
are a remarkable example of interaction in nature. The fleshy pulp of
a berry contains starchy carbohydrate which conceals and protects the
seed within. Most berries are also full of vitamins. Rose hips contain
concentrated vitamin C, still used as a medicinal syrup. All this is packaged
in bright, glossy colours designed to attract diners. Birds eat the produce
and spread the precious seeds contained in the berry over a wide area.
They drop or excrete the undigested seeds enabling the plant to spread.
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However, seed-spreading
doesn't work when tough-billed species crack open berries and eat the
seeds. Not
to be outsmarted, hawthorns and other shrubs manufacture rock-hard seeds
that pass through avian digestive systems unscathed. Yew berries are delicious
but have a poisonous coating on the seed. This is designed not so much
as a deterrent, but as a means of ensuring that customers eat the fruit
and then spit out the bad-tasting seed.
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Champion berry-eaters
are blackbirds, song thrushes, mistle thrushes and - arriving for the
winter - redwings and fieldfares. Roving bands scour the countryside.
If the weather is mild they alternate between feeding on earthworms with
time spent stripping berries from hedgerows. Feeding is en masse but not
necessarily cooperative. Sticking together in a flock confers safety and
a shared desire to locate feeding areas. However, when a crop of berries
is pinpointed it is everyone for themself. Mistle thrushes are an exception.
They prefer to defend a berried tree against all-comers.
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Their motives for
this behaviour are two fold. First, they want the berry supply as a larder
that will see them through the winter. This
means that while they protect it fiercely, they may not actually begin
eating the food for some time. Secondly, if hard times do not materialise
then the birds gain a great advantage in having food available at the
end of the winter, enabling them to nest early. Although 'prudence' seems
to be their middle name, protectionist plans can go awry if a visiting
horde arrives. Mistle thrushes may be pugnacious, but defence against
dozens of ravenous redwings and fieldfares is hopeless.
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Starlings, as befits
one of the most successful birds on Earth, are not averse to exploiting
a berry crop - particularly elderberries in late summer and autumn. Starlings
are systematic in their approach and methodically strip bushes from the
top down. They also drop far fewer berries than other birds and leave
bushes picked completely clean. However, for once, they are left in the
shade by waxwings, a close relative from the vast coniferous belt extending
across northern Europe and into Russia
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There is no doubt
about it - waxwings look stunning. The soft brown plumage has a texture
like velvet; they have insuppressible crests and a face pattern to inspire
a make-up artist. In Dutch they are named 'pestvogel' meaning invasion
bird. This is apt. Waxwings are famous for two attributes: sporadically
irrupting across western Europe during winter and, when they get here,
feasting on berries. Although they feed by flycatching for mosquitoes
during the summer, the species is exclusively a berry-eater throughout
the rest of the year. Flocks fly west in a nomadic quest for grub, which
means their movements are unpredictable.
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 Waxwings
pluck and swallow berries whole and then retire to digest lunch before
flying back to plunder more haws, hips or any red berries growing on ornamental
shrubs. They are not fussy, so flocks are as likely to alight in a cotoneaster
bush in a city park as on a rural hawthorn hedge. Hailing from northern
forests, they know little of people and have the endearing habit of being
almost wholly indifferent to human approach.
Just how many berries
do they eat? In early 2001 an unprecedented flock of 500 waxwings spent
four winter months devouring cotoneaster berries in Belfast's city parks
and housing estates. Based on counts made over several days, individuals
consumed a daily average of 500 apiece (twice the body weight). This is
in line with similar estimates elsewhere. Doing maths was never such fun.
How many berries did the flock go through in the 100 days between Christmas
and mid-April? The astonishing answer is 25 million!
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