Pheasants were introduced to Britain centuries ago and do breed in the wild, but the population visible in late summer is dominated by birds released from rearing pens. Published estimates put annual releases of pheasants in Britain in the tens of millions, with red-legged partridges released in large numbers alongside them.
The scale has ecological consequences that researchers have quantified. Because the released birds are large and arrive all at once, the combined biomass of gamebirds in late summer has been estimated to rival or exceed that of all native breeding birds combined, an unusual situation for a wild bird population. Survival is low: most released birds do not persist to the following breeding season.
The effects are contested rather than settled. Studies point to increased scavenger and generalist predator numbers, altered woodland ground flora near release pens, and disease questions, while the shooting sector points to habitat management benefits.