These are the eggs of a song thrush (sometimes called the throstle or the mavis), common across Europe and Asia. It breeds in forests, gardens and parks, laying up to five of its distinctive blue eggs in a cup nest, often choosing snapped-off trees to lodge them, as in this story.
Egg collecting is a peculiarly British impulse, a residue of the colonial era when Victorian explorers brought back booty from around the world. Oology, the study of eggs, was a respectable branch of ornithology. Taking wild eggs was made illegal in 1954, but collecting continued, though there was an informal code of practice among schoolboys who did it that only one egg should be taken from any nest. These days, nesting is severely frowned upon and prosecuted. No longer a hobby of schoolchildren, it is still pursued, often quite lucratively, by adult poachers and obsessives.
Watch and listen to the sound of a British song thrush.