Pre­dated eggs

Bird Watching (UK) - - Your Birding Month -

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

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