People can't even fall asleep on the subway without
  slumping over onto the person next to them, but birds stay
  steady even in high winds. The secret of the birds'
  tightrope talent is in the toe tendons.

  As a bird sits on a perch, its own body weight stretches
  the tendons in front of the knee and behind the ankle
  joint. This causes the toes to move forward and clasp
  around the perch. Other tendons under the toe bones have
  interlocking layers that stay secured to one another as
  long as the birds' weight is pressing against them, keeping
  the bird from tipping over.