...which generally fail to include “sham” marks. A sham mark is applied in the same way as a visible mark, and supposed to feel and smell the same, but cannot be visibly detected. In the magpie study, this was done by placing a black mark onto the magpies' black throat feathers.de Waal also discusses work with other species and the “co-emergence hypothesis,” according to which the capacities for mirror self recognition and perspective-taking appear in tandem during both evolution and development.
Placed on the same black throat feathers, the visible mark—a tiny colored sticker—stood out, but only in a mirror. Put in front of a mirror, the magpies kept scratching with their foot until the mark was gone, whereas they left the sham mark alone. They also didn't do the same amount of frantic scratching if there was no mirror to see themselves in. Evidently, their self-preening was guided by visual feedback from the mirror.
Speaking of crows, Nijhuis writes a brief piece on work showing that crows recognize individual human faces.