Source : Webster's Revised Unabridged Dictionary (1913)

Zygodactyl \Zyg`o*dac"tyl\, Zygodactyle \Zyg`o*dac"tyle\, n.
   [See {Zygodactylic}.] (Zo["o]l.)
   Any zygodactylous bird.

Source : WordNet®

zygodactyl
     adj : (of bird feet) having the first and fourth toes directed
           backward the second and third forward [ant: {heterodactyl}]