If you listen real close right at the beginning when the aircraft just comes into view, about the 4-second mark, you can actually hear those two herons hitting the compressor fan, about 1/2 second apart. Sounds sort of like 'chip...chip'. The first flame follows almost immediately.
The pulsating flames during the climb-out is the engine auto re-light system hard at work until the engine is shut down by the crew. Those are known as 'explosive light-offs', which you can easily hear in this case, and which happen when the fuel/air mixture is too rich.