Hieronymus Bosch painted at least 20 owls scattered all over his paintings,often interpreted as a symbol of the occult & the ominous as seen through the eyes of a 15th century christian where the devil embodies temptation, the ever-menacing darkness preying on unsuspected souls. 
Bosch's Owls details:
1-3 "The Garden of Earthly Delights Triptych"-1490-1500-Museo del Prado
4-"The Conjurer" ca. 1502 at Musée Municipal, St.-Germain-en-Laye [1]
1-3 "The Garden of Earthly Delights Triptych"-1490-1500-Museo del Prado
4-"The Conjurer" ca. 1502 at Musée Municipal, St.-Germain-en-Laye [1]
According to the collection of fables Dialogus creaturarum optime moralizatus [1483] or "The Dialoges of Creatures Moralysed", the owl was symbolic of Superbia or the sin of pride and hubris [2]
In "The Conjurer" [1502],Hieronymus Bosch depicts the ever-gullible human mind willing to believe his/her desires when it's both unmoored of reason & reality.The high-rank man leans over transfixed by a pearl in the conjurer's hand while unaware of being picked of his money purse
A Flemish proverb, published ca 1480 in Bosch's hometown of 's-Hertogenbosch proclaims: "No one is so much a fool as a willful fool". Bosch associates the conjurer as a common criminal, luring in the prey [1]
Detail: The frog coming out of the mouth of the central character represent the extent to which the victim has let go of reason and gave in to bestial impulses.
Hieronymus Bosch "The Conjurer" ca. 1502 is at Musée Municipal, St.-Germain-en-Laye, France [2]
Hieronymus Bosch "The Conjurer" ca. 1502 is at Musée Municipal, St.-Germain-en-Laye, France [2]
Portrait of Hieronymus Bosch 1572 engraving with the marvelous verse by Dominicus Lampsonius from his "Pictorum Aliquot Germaniae Inferioris Effigies"
Four fantastic details of flora in Hieronymus Bosch's "The Garden of Earthly Delights Triptych"-1490-1500 @museodelprado
Bosch was arguably the first surrealist painter, 400 yrs before Dali his paintings are populated by the haunting beauty of the fallible: Christian moralistic symbols visualized in the feverish dreams of the fallen man
H. Bosch-Temptation of St. Anthony Triptych [details] c. 1500
H. Bosch-Temptation of St. Anthony Triptych [details] c. 1500
H. Bosch "The Temptation of St. Anthony" Triptych c. 1500 is at Museu Nacional de Arte Antiga, Lisbon [1]
Art historian Erwin Panofsky once wrote on the challenges of decoding Bosch: "We have bored a few holes through the door of the locked room; but somehow we do not seem to have discovered the key”. [2]
