Flaming Herons
I should stay away from herons if you have any matches. Health and safety and all that - watch your eyebrows. That's after the Airport Spokesman stating that it was the herons burning.
The video to me looks as though the engine was surging as the core airflow broke down and then restored itself. Hence the pulse effect. That would have made some interesting sensations in the cabin, between noise and loss of thrust (and its momentary restoration. In surge conditions, flames may also come out of the front of the engine, due to the pressure in the combustor being "unopposed" by the compressor. The cause of this was likely to be the birds' bodies damaging the compressor airfoils and making them less-than-ideal for their purpose. A surge is a major compressor event where the airflow breaks down throughout the compressor, as opposed to a stall, which is either part of a compressor stage stalling, or a stage having passing instability.
I imagine that the fan was also seriously damaged, causing imbalances which in turn caused rubs between blades and the engine case. This can also make a fair number of sparks and flames, depending on the materials involved. Videos of blade-off and bird ingestion tests can be found on some of the engine manufacturers' websites. The after-effects are "interesting".
When the RB211-535 E4 was being quailified/certified, I believe that the birdstrike tests involved, separately, being able to take a large number of sparrows at take-off thrust, 8x 1.5 lb ducklings, and 1 bird at something like 5-8 lbs (I can't remember exactly). At that time (mid-80s), the most difficult test was the duckling ingestion. I believe, too, that the requirement was then for the engine to be able to run at 75% thrust for 45 mins which is supposed to be enough to burn off enough fuel to land again safely. Running at 75% thrust could still be with an engine with fairly high levels of damage-induced vibration.
In this case, 2 birds of this size are a major event. Thank heavens for 2 engines! Great job by all concerned, and well done!