As the breeding season progresses, we birdwatchers can’t help stumbling upon the sorry sight of damaged, ‘empty’ eggs strewn on the garden lawn or out in the wider ‘field’. Birds’ eggs are predated by mammals including mustelids and Foxes as well as rodents, but most broken eggs found have been predated by other birds. Generally, it is possible to distinguish between mammal and avian predation. Mustelids such as Stoats may leave distinct teeth marks when they bite the shell. Rats and Grey Squirrels nibble the shell into small fragments, and Foxes will readily bite off the ends of the shell without crushing the rest. Invariably mammals completely clear out the contents. Bird predators (gulls, skuas, corvids, woodpeckers, owls etc) peck the eggs open, causing the shell to dent inwards, leaving a jagged hole, usually in the centre. There are often traces of the egg membrane left inside. Predated shells are usually found well away from the nest, having been carried by a bird to a better place to eat it. Naturally hatched eggs may still be around the nest site, and can sometimes be distinguished by a neat line being pecked round the tip, and the ‘cap’ of the egg being pushed out separately. Other species of baby bird, force their way out after chipping irregular holes which will always look like they are pushed outwards. Identifying predated eggs is pretty similar to identifying fully intact eggs. The best way for you to do it is compare with pictures in a book of eggs!
Razorbill and Guillemot eggs, eaten by skua