All pigeon houses, dovecotes, or structures or enclosures shall be enclosed in such manner as to prevent the escape of any pigeon or dove therefrom; provided, however, that the owners or keepers of carrier or homing pigeons shall be permitted to liberate not more than twenty-four carrier or homing pigeons for exercise and flying at any one time, and said pigeons shall be at such times under the supervision and control of such owner or keeper. (Prior code § 9.04.030)