OSPF best route selection with same cost

Hello, I have a topology with multiple parallel path to same destination. It also use same next-hop router and each of them use default cost (not specified in config). In "show route" I can see there is only one active route so it probably means that BIRD don't use balancing between these routes. I cannot see in documentation exact algorithm used for route selection. Can you please explain me how is the best route selected when multiple paths with same cost are available? Is name of interface or IP address next criterion for selection? My goal is not balance traffic but I only want to understand how path selection works on equal links. Thank you for your explanation and help. With best regards, Patrik Rehus