Thursday, 23 July 2009

Urban birds are lazy

The divide between town and country is one that you often see mentioned in the news. Be it a story about economics, social attitudes, crime or even quality of life, the differences between town and country are much trumpeted. One difference that you might not expect, however, is the difference that has been found between how particular bird species behave in towns and how they behave in the wider countryside. Research that I have been involved with has just revealed that urban birds are lazy, at least when it comes to getting up in the morning to visit garden feeding stations! Urban-living populations of Robins, Greenfinches, Blackbirds and many other species arrive at garden feeding stations, on average, a few minutes later after first light than their country cousins.

Of course, this is not really a case of lazy birds but the result of different factors operating in each of the two habitats, influencing the birds’ behaviour. On cold winter nights (the time of year when we carried out our study) many small birds struggle to survive the low temperatures and long nights without access to food. In order to keep warm, many of these small birds will burn off much of their body’s fat reserves, reserves that need to be replenished as soon as possible after dawn. It is these energy losses that prompt the early arrival at garden bird tables and hanging feeders, the birds arriving as soon as it is light enough for them to see what they are doing.

Now you might think that with all the street lighting that exists within our cities, urban living birds would be able to start feeding earlier because of the extra light available to them. That this is not the case suggests that another factor is operating on our urban birds, working in the opposite direction. This factor is temperature. Because of all the heat that escapes from offices, shops, factories and houses, our urban areas are typically several degrees warmer than the surrounding countryside. In fact, some of the largest urban areas (think of London, Birmingham and Manchester) may be as much as eight degrees warmer than the surrounding land. This is known as the urban heat island effect and in winter it can counter the effects of lower ambient temperatures. If it is less cold overnight for urban-living birds then they will not have to use up so much of their energy reserves keeping warm, meaning that they are not so pressed to replenish fat reserves come first light. This allows them to make a more leisurely start to the day than hard-pressed relatives who happen to be living in rural areas. Sometimes an urban existence pays dividends.

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