Crows recognise familiar human voices and the calls of familiar birds from other species, say researchers. The ability could help the intelligent birds to thrive in urban environments; using vocal cues from their human and avian neighbours to find food or be alerted to potential threats. The team used recordings of human voices and jackdaw calls to test the birds\' responses. They published the findings in the journal Animal Cognition. Lead researcher Claudia Wascher from the University of Vienna said that, although it was widely known that crows were \"very intelligent\", most studies had focused on their ability to recognise and communicate with their own species. \"In cities crows live alongside jackdaws, magpies and seagulls, and alongside humans,\" Dr Wascher told BBC Nature. \"Some of those people might be very nice to the crows and feed them and others might be nasty and chase them away. \"You even get some people hunting crows. To find out if they might be able to distinguish between these different birds and humans, the researchers studied eight carrion crows kept in the university\'s aviary. The same people feed and interact with the birds every day. So the team recorded five of these people saying \"hey\" and recorded the same word said by five people who \"had never met the crows\". When they played these recordings to the birds, they responded much more - looking up and turning towards the speaker - when they heard the unfamiliar human voices. \"Since humans can be a serious threat for crows,\" explained Dr Wascher, \"it\'s important that if they hear someone unfamiliar, they are on alert.\" Multi-species teams The researchers repeated the same experiment using calls from jackdaws that shared the crows\' aviaries. They played brief \"contact calls\" - short vocal greetings the birds use - from these familiar jackdaws and from jackdaws the crows had never encountered. In this experiment, the team found the opposite result - the birds responded more to the familiar than the unfamiliar birds. Dr Wascher said that this result suggested that crows might \"team up with preferred individuals outside of their own species\". \"We already know that corvids are very specific in which other crows they choose to co-operate with,\" she said. Previous research has show that when the birds are foraging or solving tasks, \"they avoid certain individuals and choose to work with others\". \"So maybe,\" Dr Washcher suggested, \"there\'s also something [like this] going on outside the species.\"