Newts, toads and slow worms have also been given new ponds along the £90m route, which is due to open next month.
The specialist work has been carried out by Rhondda Cynon Taf council and contractors Costain, who were legally required to outline wildlife protection plans before the bypass was approved.
The Welsh Assembly Government has funded the bypass, including the dormice bridges.
A Rhondda Cynon Taf council spokesman said: “The council and Costain, its contractor in delivering the bypass, is proud of the working relationship it enjoys with the Countryside Council for Wales and the Environment Agency, who required the ecological work to be carried out before planning permission could be secured for the road.
“Both statutory bodies fully endorsed the ecological work undertaken by the team which enabled planning permission for the road to be built.
“Such measures are now commonplace across the country and adhere to the current legislation in protecting species of this nature.”
