Wood Pigeon Squabs

I’ve been watching the nesting Wood Pigeons in our garden for the last couple of weeks and was delighted yesterday to notice 2 squabs being fed by the Mum and Dad. They looked at least a few days old and were quite big.

Early this morning neither of the parents were on the nest, but later I noticed them hanging around for a short while. Very sad to say the squabs are dead and they appear to have been attacked, but left in the nest. Any idea what could have done this?