You may want to used fixed processing, it has an exact time step.
Also you don't want to use counters, because they will run at different speeds based on the frame rate. Use the delta time given by the loop. Accumulate it up to some timer value.
var bird_spawn_delay = 5.0/60.0
var timer = 0.0
if(timer >= bird_spawn_delay):
timer -= bird_spawn_delay
timer += delta