Be like the bird, who
Halting in his flight
On limb too slight
Feels it give way beneath him,
Yet sings
Knowing he hath wings.

[Soyez comme l’oiseau, posé pour un instant
Sur des rameaux trop frêles,
Qui sent ployer la branche et qui chante pourtant,
Sachant qu’il a des ailes!]

Victor Hugo (1802-1885) French writer
“In the Church of *** [Dans l’eglise de ***],” Songs of Dusk [Les chants du crepuscule], #33 sec. 6 (1836)
    (Source)

Full French poem. Alternative translations:

• Be like the bird that, on a bough too frail
  To bear him, gaily sings!
  He carols -- thought he slender branches fail:
  He knows that he has wings. [Source]
• Be like the bird that seeks its short repose
  And dauntless sings
  Upon that bending twig, because it knows
  That it has wings. [Source]
• Be like that bird, that halting in her flight
  A while on boughs too slight;
  Feels them give way beneath her,
  And yet sings, yet sings,
  Knowing that she hath wings.
  [Laura Sedgwick Collins 1890s song, "Be Like That Bird"]
• Thou art like the bird
  That alights and sings
  Though the frail spray bends --
  For he knows he has wings.[tr. Kemble (Butler)]