Factors that normally limit magpie populations are lack of nesting territories and high mortality of young birds.
The changes in magpie numbers
Until the mid-19th century, magpies were very common in Britain and were popular with farmers because they eat harmful insects and rodents. But from then until the First World War, heavy persecution by gamekeepers caused their numbers to plummet.
Since the Second World War, magpie numbers have increased. Their numbers trebled from 1970 to 1990, since when they have become more stable. Urban and suburban magpies increased much faster than rural populations. In towns they are not persecuted, there is more food available, magpies will nest close to people, which protects their nests from crows, and they can breed earlier in the year because towns are warmer than the surrounding countryside.
Urban magpies will use artificial nest sites and nest materials, and will take food from bird tables, sometimes storing it in man-made structures such as gutters and eaves.
Factors that normally limit magpie populations are lack of nesting territories and high mortality of young birds. The relatively stable population since 1990 suggests that magpies have reached an ecological equilibrium.