Sources : Merlin

Pliny the Elder [1st century CE] (Natural History, Book 10, 95.205): There is a small bird called the aesalon that breaks a raven's eggs, whose chicks are preyed upon by foxes, and it retaliates by pecking the fox cubs and the vixen herself; when the ravens see this they come to their [the foxes] aid against the aesalon as against a common foe. - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum Birds 5.13): It is called asalon, as Pliny says, it is small and breaks the raven's eggs. The asalon chicks are attacked by foxes, so she wanted to hurt the fox's cubs and the fox itself. And when the ravens see this, they help the foxes, as if against a common enemy. This bird lives among thorns; and therefore he hates donkeys that devour flowers and thorn leaves. - [Badke translation/paraphrase]