On a number of occasions flying objects have been photographed by the Rovers. Some believe them to be UFOs, and in some instances that may well be the case. On other occasions, they bear the hallmarks of being biological in nature.
If birds do exist on Mars, how would they subsist? We know that there is water, in the form of ice at the very least. what would they feed on ? Well we also have indications that there may be rodents as evidenced by the "Martian Rat" and possibly reptilian life as evidenced by several images that could be Lizards.
The potential birds in the NASA images do not fly in flocks, those that have been photographed are flying solo - in much the same fashion that birds of prey on Earth do - hawks, falcons, eagles.