City blackbirds live longer but are less healthy than their country counterparts, a study has found.

For the blackbird, the benefits of urban living include better access to food and less chance of being killed by a predator, scientists believe.

The downside is that city birds age faster and are generally less fit.

What accounts for the trend is unknown, but may involve exposure to city pollution early in life.

Professor Simon Verhulst, a member of the research team from the University of Groningen in the Netherlands, said: "This could be present at birth or develop in the first year, as cities are an unhealthy environment."

To study the health effects of city life on blackbirds, the scientists used a well known genetic ageing marker.

Telomeres, DNA structures that form a protective cap on the ends of chromosomes, shorten with increasing age but at a faster rate in stressed or sick individuals.

The team found that year-old city blackbirds had significantly shorter telomeres than rural yearlings living about 18 miles away.

In older birds, the difference was even greater.

Blood samples were taken from blackbirds in five cities, Granada, Seville and Madrid in Spain, Dijon in France, and Turku in Finland.

The signs of premature ageing revealed by shorter telomeres also indicated that city birds were less healthy than those from woodland populations, said the scientists.

Yet the proportion of older birds caught by the researchers in mist nets was higher in the cities.

Co-author Dr Juan Ibanez-Alamo, also from the University of Groningen, said: "This means that mortality is lower in the cities, so the advantages of city life compensate for the negative health effects."

The findings appear in the Royal Society journal Biology Letters.