Clever crows have learnt how to make a meal of cane toads

Members of the crow family, the corvids, show a capacity to learn and solve problems that may be unrivalled in the bird world. It seems their greatest skill is an uncanny ability to turn new and tricky situations to their advantage.

Take the scourge of the introduced cane toad (Rhinella marina) for example. As this highly poisonous amphibian disperses across northern Australia, the numbers of predators attempting to eat them are crashing. The most notable victims are quolls, goannas and certain snakes, which have been all but wiped out in some regions. Crows, however, have learnt how to eat toads by avoiding the most toxic parts. The large parotid glands on the toad’s neck and shoulders are the greatest risk. When a toad is harassed, milky white poison oozes from these glands. Any contact with this ooze is a likely death sentence. In many cases they die before even swallowing the toad.

Crows avoid contact with the ooze by grasping them by the limbs or even the bony brow above the eye, avoiding the body itself. These clever birds have learnt to roll the toads onto their backs, sometimes doing so repeatedly if the luckless toad tries to hop away. Crows know which bits to eat – fleshy thighs, tongues, intestines – and how to get at these from below without contacting the lethal parts.
The torresian crow (Corvus orru) pictured spent nearly 40 minutes manipulating its victim beside Lake Kurwongbah north of Brisbane. The toxic fluids, clearly visible on the toad’s parotid glands, were avoided. During much of this process other crows stood and watched.Were these observers teachers, students, or dis-interested bystanders? I don’t know, but there has been some debate about whether these techniques are culturally transmitted. Scientists argue that the consistent use of the same safe and efficient methods across Queensland, the Northern Territory and presumably Western Australia has involved the transfer of learning, from crow to crow, as the toads’ distribution expands.

Birds may be more tolerant to toad toxins than reptiles and marsupials, it has been suggested, perhaps due to historic genetic exchange between Australian and Asian birds that have co-evolved with toads. Certainly corvids naturally occur with toads on all continents outside Australia.

Sadly, the range of the toad continues to extend westwards across the Kimberley. While there are encouraging signs of many predator numbers rising again after long term exposure to toads, there is no doubt that predators across the toads’ invasion front will be severely negatively impacted.