According to Larry Solomon, an owl, in Bosch's work, symbolizes "Satan looking for victims."

http://solomonsmusic.net/BoschSymbols.htm

According to Kyla Ward, "Further evidence of Bosch's medievalism is the symbolism he uses, and the amount of it. A well-established language of direct allegory existed, that allows interpretation of pictures such as the Garden, and his intricate Hells, in the triptych of which the Garden is the centrepiece and other works such as The Last Judgement, and a similar panel in the Haywain triptych. A dead give-away is the owl. An amazing number of Bosch pictures, when you look for them, contain the tiny shapes of owls peeping from nooks and crannies. They are evil lurking, symbols of witchcraft and demonology, and the Garden is full of them being cuddled and nursed."

http://www.tabula-rasa.info/DarkAges/Bosch.html