As the winter begins to thaw and new life breaks through, life for our wildlife becomes easier. Birds will be off the meagre diets of seeds and nuts that has lasted them through the winter and back on to juicy insects, fruit and, later in summer, berries.

Eating a mixed and varied diet might seem normal for us, but for wildlife it can often require some pretty major changes. Did you know that some birds even undergo physiological changes to their guts to adapt to their changing diets? Our Nature’s Home columnist Dominic Couzens explains...

Capercaillie feed on pine seeds in winter

In Scotland, capercaillie can be seen in trees, feeding on seeds, more regularly in winter. Photo: Ben Andrew (rspb-images.com)

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Why do birds change their diets?

Many species are compelled to switch food sources because (for example) insect abundance rapidly diminishes as the days grow shorter. Because of this, many birds must switch to berries, nuts or seeds during the super-productive days of autumn.

Northern-breeding waders, feeding on insects in the tundra, switch to foraging on worms, molluscs or crustaceans in the estuarine mud.

Members of the grouse family may switch from nutritious berries in summer and autumn to eat pine needles (capercaillie) or shoots (ptarmigan) – mean diets for cold days.

Redwing fatten up on berries to fuel migration

Birds such as redwings fatten up on berries to fuel migration. Photo: Ben Andrew (rspb-images.com)

Which species change their diets?

Everything from waders to tits, grouse to garden birds. The changes don’t only affect resident birds. In the autumn, many migrants will have not only shifted towards eating berries that will generate fatty fuel to power them southward, but will also have eaten far more often than they did before.

And the changes can differ across sexes too. Male and female birds often have different diets. In autumn and winter, it is mainly male goldfinches that forage from teasel-heads.

Lapwing flock together in winter

Usually solitary, lapwing flock together in winter to feed on organisms in estuary mud. Photo: Andy Hay (rspb-images.com)

How do they do this?

Behaviour changes

A change of diet often requires, or allows, a change in behaviour. Birds feeding on worms in the soil – such as lapwings – or touch-feeding for estuarine organisms often pack together in tight-knit flocks, becoming more sociable.

Birds that fed up high in the insect-rich canopy – including tits and nuthatches – can now be watched foraging on the ground.

Blackbirds go the other way; in summer they are ground-foragers, seeking invertebrates such as worms in the soil and leaf-litter, while in winter they move up into trees and shrubs to feast on berries.

During winter, a few species will have devoted many hours to food-hoarding: jays, nuthatches and coal and marsh tits hide long-lasting foodstuffs away in hundreds of secret caches, for use on a tough winter day. Some mistle thrushes buck the social trend and become highly territorial, protecting the berry crop of certain trees – such as hollies – from others, trying to monopolise the fruit for themselves.

Bearded tit adaptations

Beareded tits develop hard stomach plates to break down winter bulrush. Illustration: Mike Langman

Physical changes

Amazingly, alongside these behavioural shifts are internal, physiological ones. For some species, a change of diet requires a change in the physiology of the gut. This can include everything from the bill to the intestine.

Nuthatches eat seeds in the winter and insects in summer; their bills are longer in the summer, not subject to the same heavy wear and tear of hammering that a seed diet requires.

Several birds are known to grow longer guts as they shift towards a less nutritious diet, including several grouse and starlings. In fact, the starling’s gut shifts in length repeatedly as its diet changes, longest in winter so that it can store undigested food.

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Want to know more about amazing changing diets? Take a look at your Spring 2019 issue of Nature's Home magazine. If you don't yet receive Nature's Home, you can sign up here. And don't forget to keep feeding your garden birds as the seasons change. Visit the RSPB shop to find the perfect food.

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