Agree with Andy about the impact of otters. A few years ago a BHG nested in the grasses by a stump in the main boardwalk pond on Cors Caron and to my amazement raised a sizeable chick. Whilst doing so one day an otter came into the pond actively hunting and eating fish; the parent gulls went up in high alarm, so loud were their protestations at the mere presence of the beast that other BHG's from the south end of the bog came over to help.
They repeatedly divebombed the otter to within inches of its head and it soon beat a retreat surprised I think as it hadn't been after the chick on that occasion.
The response indicated extreme alarm and recognition of a master predator and showed that group aggression can drive off predators.
Unfortunately I never saw the chick fly as the family disappeared overnight a short time later. BHG's do move their chicks away from the nest area and this one was quite well plumaged as a juvenile so I always hoped it had survived with such good aggressive parents!
The following year I had the sobering experience of watching a BHG in France eat a small ball of fluff that was a Black necked grebe chick!