As the climate warms, many British bird species will have to shift their geographical ranges northward to maintain their current habitat conditions. With this change, many bird ranges are predicted to become smaller in the long-term. However, new research by the British Trust for Ornithology (BTO) says that the northern margins of these ranges are moving faster than the southern margins are following. The study shows that the geographical ranges of British bird species are, for now, becoming larger.
Dr James Pearce-Higgins, BTO’s director of science, tells Geographical: ‘What we have tried to do is pull out some general patterns for many different species. However, the nuthatch is individually a good example – 20 years ago nuthatches only got as far as northern England. Now they are residing up to the central belt of Scotland and beyond.’
The ranges of 80 British species were tracked with data from the Breeding Bird Survey. ‘Our results show an average poleward shift in the northern outer point of 3.3km a year,’ say the authors of the study. This was observed over a 15-year period from 1994 to 2009.
The main cause of the northern expansion, the report says, is that the winter temperature has become milder, allowing the species to colonise further up-country. ‘A lot of our species are limited by temperature,’ says Pearce-Higgins. ‘There has been a whole run of mild winters that have probably boosted winter survival rates and allowed the species to colonise areas much further north. That has been quite a rapid process.’
The southern range margin has not moved northward at the same rate. The study says this could indicate that the factors impacting the south are different to those in the north. Instead of temperature, southern margins could be affected by species interactions instead. ‘What is happening in the south is a little bit more complicated,’ says Pearce-Higgins. ‘It’s not like the weather gets too hot for these birds and they disappear. It is probably a completely different and more long-term process such as the arrival of new competitors, predators or changes in the abundance of their food resources.’
Because of different limitations between the north and south margins, the BTO study says the geographical ranges of British birds have experienced a net expansion over 15 years. According to the study, for birds that are capable of adapting their habitats quickly to short-term influences of climate change, Britain has just got ‘bigger’.
‘In the short-term, it looks like climate change is great for all these species because it has expanded their distribution,’ says Pearce-Higgins. ‘However, it doesn’t necessarily mean that climate change is going to be good for them in the long-term. The declines are probably just going to happen further down the line.’