About the Barn Owl
Reasons for the historical decline
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Firstly, we should remember that the first reliable population estimate was not produced until 1999 and that evidence of the Barn Owl's historical decline in Britain is largely anecdotal. The often-quoted figure of a 70% decline between the 1930s and 1980s comes from the comparison of two unreliable population estimates. It's quite likely that Barn Owl decline started in the mid-1800s as a result of persecution by gamekeepers, egg collectors and the like. The fact is we shall never know to what extent these activities might have affected the Barn Owl population. Indeed they may have only caused the temporary suppression of local population levels.
However, there is a strong consensus that Barn Owls must have been a lot more common before the mechanisation of farming. Before the late 1800s, when men and horses worked the land, farming was very much less intensive and there can be little doubt that (overall) wildlife was much more abundant. In particular, small mammals would have been much more common when there were more hedgerows, more marginal grazing land, and pasture was mainly permanent, "unimproved", and much less intensively grazed. Stored cereal crops (in ricks or barns) became so infested with mice and rats that some enlightened farmers encouraged Barn Owls into their buildings via special access holes ("owl windows"). For a species that cannot hunt in rain and suffers high mortality in severe winters, imagine how indoor hunting might have helped survival.
In the late 1800s and early 1900s increasing human population levels and the proliferation of farm machines led to increases in the intensity of land management resulting in the loss of Barn Owl habitat. This accelerated during World War II with a drive for Britain to become more self-sufficient. The idea that British farmers had a duty to produce as much human food as possible (which became deeply ingrained during WWII) and a system of government grants that paid farmers to destroy wildlife-rich fields and hedges, continued right up to the 1980s. Ask almost any elderly farmer and he'll probably remember seeing "fluffy white owls" up on the barn wall when he was a lad in the 1930s. The changes in farming practices stimulated by the legacy of WWII, human population expansion, government policies, and consumer pressure for ever-cheaper food, are the main reason behind the Barn Owl's historical decline in Britain.
The first population estimate for England and Wales in 1932 was 12,000 pairs. There may have been 10 or 20 or maybe 30,000 pairs, nobody will ever know. The first reliable estimate for the UK (in the late 1990s) was only 4,000 pairs.
From the mid 1900s onwards, other factors started to work against the Barn Owl population: The proliferation of mouse and rat baits that kill predators as well as rodents. Barns being converted into houses. The first trunk road was built in the 1950s. The trunk road network now kills thousands of Barn Owls every year. And that's not all...
There is more information about this topic in the Barn Owl Conservation Handbook.