Stone the Crows!

We are all used to stories about the dire consequences of the Ravens leaving the Tower of London (the resident birds are actually offered inducements to get them to stay!) but a report has been received (http://www.guardian.co.uk/environment/2009/jan/13/crows-pollution-iran-environment-wildlife) that crows are leaving Tehran in Iran. It is claimed that air pollution (e.g. high levels of carbon monoxide) may have caused this exodus but, given that crows are opportunistic birds that benefit from waste and the dead bodies of other animals, it seems somewhat surprising that this species is amongst the first to have (apparently) fled the location. Other factors might well be involved.