Guillemot deaths at Arrochar, Loch Long

Good evening all, 

A few days a go I was scouring the beach at Arrochar with my daughter's and came across 6 Guillemot that had been washed up. All within 100 meter stretch. There was no sign of injury and they all looked healthy, except they were all dead. Then we came across another which was only just alive, again looked healthy asthetically, but again it was almost dead as I was able to stroke it's back whilst having a glance over it to see if there was any injury. Eventually it managed to get to the water and disappeared. Does anyone have any idea what could of caused this? Is it normal? Or do you suspect they've someone been poisoned?