Caterpillars key to urban blue tits’ low breeding

Many animal species suffer reduced reproductive success in urban habitats, despite wide-spread supplementation of breeding and feeding opportunities. In some years, the breeding success of city birds is devastatingly low. Biologists have now shown conclusively that in urban blue tits, reduced breeding success is linked to poor nestling diet and in particular to scarcity of caterpillars, their preferred nestling food. The research adds to growing concerns that urban environments can become ecological traps for urban-dwelling species. The increasingly rapid process of urbanisation has now placed more than 50% of the human population in cities. Birds and other species can be attracted to these habitats by human food and shelter, but these benefits can be offset by major ecological deficits, as now shown for blue tits. Although blue tits are widespread songbirds that appear to do well, their breeding failure in cities can be severe.