Chickens derive from jungle foul meaning they express their natural behaviour when provided with the shade and shelter offered by the trees.

Birds will venture further into their range and live a stress-free life, which is good for the farmer and good for their hens.

Over the past 10 years, the British Free Range Egg Producers Association’s 400 members have planted in excess of one million trees.

At West Newbiggin Farm near Darlington Philip Twizell has planted around 4,000 trees and hawthorn hedges to provide shelter for his 16,000 birds.

"It certainly works in terms of encouraging the birds to come out of the house, the trees create a relaxed environment for them. Anything you can do to keep the birds stress-free is great," he said.

The vast majority of farmers meet the cost of planting the trees themselves, recognising the associated welfare benefits.

Broadleaved trees have predominantly been planted because of the cover they offer but a variety of species found in English woodlands can also be seen growing on free range units.

And the trees often have wider benefits. Farming on the Dorset-Somerset border, Jason Thompson has planted more than 300 trees over the past nine years at Silverthorne Farm near Sherborne.

This includes an avenue of a vigorous hybrid willow which grows three metres a year. Orchards each side of the hen house offer shade and encourage the birds out of the shed.

"We usually go for apples, plums and pears – anything that provides cover for the birds but also gives us some fruit in season," he said.

Many farmers grow fruit trees to provide another crop on their farm. Four years ago Tony and Gwen Burgess planted an orchard at Birchgrove Farm near Aberystwyth which has increased food production for their acreage and reduced their carbon footprint.

"We’ve had a bumper crop of fruit this year and we’ll definitely plant more fruit trees as it has been a great success," Mrs Burgess said.

Trevor Sellers established Belton Poultry Farm near Uppingham in Leicestershire 11 years ago and has planted more than 3,500 trees.

He said: "When I bought the farm I planted trees to enhance the situation and I was one of the first into the Sainsbury’s woodland scheme.

"I’m a big advocate of birds being outside – they have the choice to go in and out but I prefer my birds to be out of the shed and the trees encourage that."

Biodiversity is important on Ben Wharfe’s aptly-named Sapling Home Farm near Knutsford in Cheshire. He has planted oak, ash, silver birch and field maples and a wildflower meadow alongside 600 metres of new hedgerows.

"We chose different species of hedgerow plants to provide flowers, berries and cover year round which benefits all the wildlife on the farm," Mr Wharf said.

Tree planting is only one of the ways many free-range egg farmers are investing to improve both hen welfare and the bottom line of their businesses. Husbandry, housing and other wildlife initiatives all contribute to happy hens and top quality eggs.