Spock's legs? Judging by the bloated yet satisfied expression on the duck's face, I imagine possibly it's eaten them. This might fit with the perturbed, slightly pained (meaning deep agony, because this is Spock), yet scientifically curious expression on Spock's face. And the tense, ready-for-action stance of the Andorian (Therin?), who is clearly reaching for his phaser.
"A strictly carnivorous duck. Fascinating..."
Then again, if I recall correctly, this duck was raised in a factory by alligators, yes? I imagine it's picked up some unusual habits.