Migrating geese tend to fly at night, at high altitude above cloud or at least in clear air. They will happily climb, india mike, to reach clear air. The climb can take an hour or more.
On avoidance, at 300 kts the aircraft is flying at approximately 1500 feet in 3 seconds or just over a quarter mile. If it takes one second to appreciate the potential for collision you are just 1000 feet away.
In this instance the aircraft was in a climb at possibly 30 feet per second rate of climb. The perception problem is in 3 dimensions and the PF believed they would pass over the birds.
You have 2 seconds left . . .