Yep, I have this scenario happen daily. Very nerve wracking. Especially during hyperlapses. I seem to be on a hyperlapse kick right now so most flights are send the drone out, setup a hyperlapse scenario then let it run autonomously for 10-15 minutes, return. During these flights birds get somewhat aggressive. on one occasion striking the top of the drone, no marks or crash, just kind of a mid air bounce! super duper pucker factor! My obstacle sensors are always going off... front, back, side, above, etc. I guess I've just been lucky so far to not have a full on strike resulting in a crash...
When I watch them, if its just one bird they tend to fly near and then get bored. If it is a bunch then its basically game on. Time for evasive maneuvers. I've found that going straight up is always best. Yes, it can out ascend them because it can go directly up. whereas the birds have to sore in circles to gain altitude (mainly). And hi speed flight in straight line, I seem to be able to outrun 90% of them.
Good luck.