Between five and 10 million red-legged partridges, a non-native species, are released on UK shooting estates every year.

All of these birds come from intensive breeding farms where their parents are confined year-round in tiny cages.

In November 2015, investigators from the League Against Cruel Sports visited three of Britain's largest red-legged partridge breeding farms which supply British shooting estates:

Heart of England Farms, Claverdon, Warwickshire

Hy-Fly, Pilling Lane, Preesall, Lancashire

Southern Partridges, Court Barton, Yarnscombe, Devon

Video footage (see below) reveals that the breeding partridges on all three farms were confined in small, barren cages without perches, enrichment or litter. They were surrounded on four sides by solid walls, their only view of the world being the sky above and piles of excrement below.

Two or three years egg-laying in cruel confinement. Then shot

Breeding partridges spend two to three years confined in these battery cages, released only when their egg production declines. Then they'll be shipped out to shooting estates just like their chicks.

Caged birds on all of the farms we visited exhibited repeated jump-escape behaviour - futile attempts to fly away - a clear sign of stress. This can lead to 'scalping', open wounds on the top of the bird's head from repeated impact on the mesh roof.

Our investigators also spotted one bird with its head sticking out under the cage wall in what appears to be a futile attempt to escape. Sadly, when freedom does come for these birds it will be short - a few weeks loose on an estate before shooting season begins. Of course many birds will not survive that long and will die in the cage.

At one farm, Southern Partridges, the cages were predominantly wooden and in an extremely poor state of repair with exposed nails and holes in the walls between cages large enough for birds to fit through. This could lead to fights and injuries as partridges are extremely choosy about their cage mate.

The wooden structure also makes disinfection impossible, increasing the risk of illness and disease among the birds.

At the other two farms, Heart of England and Hy-Fly, the cages were essentially metal boxes with wire mesh for the floor and roof. Neither farm provided adequate cover for the partridges during adverse weather. At both sites the food and water were automated, just like most industrial poultry farms.

Free range and natural? If only ...

One of the partridges we filmed at Heart of England Farms had been debeaked - the tip of the upper mandible had been sliced off. This is a routine practice in chicken battery farms and is done to prevent injuries when aggressive pecking inevitably occurs due to the stressful conditions.

The same bird also had a metal 'bit' forced in its nostrils which is designed to limit damage done by pecking as it impairs the bird's ability to close its beak fully.

This industrial production of birds is little different to the battery farming of hens for eggs, yet the shooting industry continues to promote shot game as free range and natural. In fact laying hens in the EU now have greater protection than partridges on game farms and could not legally be kept in these small, barren cages. The semi-wild nature of farmed partridges makes their extreme confinement even more inhumane.

The conditions that partridges and pheasants endure on breeding farms are so bad that even the shooting industry itself via the British Association of Shooting & Conservation once pushed for a ban on breeding cages.

The League continues to call for a ban on these cages as well as a full and independent inquiry into the whole commercial shooting industry.

Dr Toni Shephard is Head of Policy & Research at the League Against Cruel Sports.

To find out more, including what you can do to help stop this cruelty, visit league.org.uk/stopshooting.