Three dead robins in a day

We have some ivy growing by the patio which is a favourite nesting spot for our local robins. They raised a brood earlier this year and have come back for a second one. They do not mind us being around and the parents have been absolute stars feeding the chicks non-stop. Yesterday morning I found one chick dead on the patio. It was about 8 days old judging by the feathers. Later on another chick was on the patio, still alive, but clearly too early for it to have fledged properly. I lifted it up with some tissue paper into the ivy to clamber back into the nest and the parents continued feeding. That chick seemed a little older than the one that had died and twice more I found it on the patio, once even on its back struggling to get up. Each time I moved it back into the ivy and the parents continued feeding their brood. By the evening I heard a commotion and found two more chicks dead on the patio, as if they had just fallen out or had been dropped out of the nest by the parents. Those two chicks seemed to be the same age as the dead chick in the morning. The parents continue feeding any remaining chicks in the nest but I am wondering why three chicks may have died in one day. One thought was that the parents may have picked up poisoned insects from some neighbouring garden but surely then all the chicks would be dead? Any ideas what may have happened?