One of the singers is so intent on his song (pointing towards the book) that he fails to notice that he is being robbed by the lute player.
Modern attribution of the painting (to an as yet anonymous follower of Bosch) was based on an analysis of the music in the open book, which shows notes of the chanson Toutes les nuictz by Thomas Crecquillon from 1549.
[2] The work was bought in 1890 for 400 francs by the Palais des Beaux-Arts de Lille from Morhange, a Parisian art dealer.
[1] It was featured with the Dutch title Zangers en musici in een ei in an exhibition in 2008 at the Noordbrabants Museum, 's-Hertogenbosch.
Possible models: Media related to Concert in the Egg at Wikimedia Commons