Complex sentence: Interbreeding with feral mallards is also a major problem, as the hybrids seem to be less well-adapted to the local ecosystem but still rather common due to the high numbers of feral mallards.
Split into simpler sentences: Interbreeding with feral Mallards is also a major problem, as the hybrids seem to be less well-adapted to the local ecosystem. This interbreeding is rather common due to the high numbers of feral Mallards.