Ornithological trends: This bird has flown WHEN people talk about a "north-south divide" in Britain they are usually referring to house prices, employment and the ratio of private-sector to public-sector jobs. The south scores higher on all such measures. But new data from the British Trust for Ornithology (BTO), a research charity, hints at the growth of another north-south divide - this time to the north's benefit. Every 20 years the BTO produces a superbly detailed atlas of bird life in Britain and Ireland. The 2007 to 2011 edition is cheery: more species are tallied than in previous atlases, and many birds are increasing in number. Compared with two decades ago, 45% of regular native species are ranging more widely while 32% are living in smaller areas; the rest have stayed put. But the most striking news comes from the north. The overall populations of woodland, farmland and migrant perching birds are up in northern England and Scotland but down in the south. The same is true of individual species such as the garden warbler, bullfinch and swallow. The number of cuckoos, a closely-watched species, declined by 63% in England between 1995 and 2010 but by only 5% in Scotland. Raptors are faring especially well in the south, but their numbers are rising in most parts of Britain. Partly this reflects climate change, suggests Simon Gillings of the BTO. Some birds are drawn to warmer winters in Scotland and northern England; visiting migrants may stick around for longer. Hard though it may be to believe during a week of torrential rain, the south is becoming drier, pushing snipe northward. More efficient farming has squeezed some farmland species. Some birds find it harder to make homes in the south, too. Pressure on housing means dilapidated buildings and barns, handy for nesting, have been converted into human dwellings. Between 2006 and 2012 the number of vacant dwellings fell by 17% in London and by 12% in Kent. Over the same period the number of empty houses increased by 16% in Derbyshire and by 10% in Lancashire. Northern mining villages once full of workers are now sparsely populated, points out Ian Bartlett, a birdwatcher in Hartlepool, in north-east England. They have become hot spots for birds and the people who watch them. Cultural difference also plays a part, thinks Mark Cocker, an expert on birds. The "obsession with tidiness" is stronger in the south, he says. Fewer people cultivate gardens; they prefer to cover them in decking and remove weeds from between concrete slabs. Village greens are mowed short. In contrast, Scotland and northern England have more trees, grassland and wind-swept moors. Less popular with humans, rugged parts of the countryside are filling up with a winged population instead.