Electric current flow is proportional to voltage difference according to Ohm's law, and both the bird's feet are at the same voltage. Since current flow is necessary for electric shock, the bird is quite safe unless it simultaneously touches another wire with a different voltage.
Want a scary job? Maintenance on high voltage transmission lines is sometimes done with the voltage "live" by working from a platform on a helicopter, sitting on a metal platform! The helicopter must make sure it doesn't touch neighboring wires which are at a different voltage.