an elongated enclosure provided with openings at a front end and a rear end, respectively, and a ceiling, the enclosure defining a passageway for an animal;

a door pivotally attached to the ceiling of the enclosure, the door being movable between an open position that allows passage of a target animal through the passageway and a closed position that prevents passage of the target animal through the passageway;

a torsion spring configured to engage the door, the torsion spring including a coil and first and second arms, respectively, extending radially from the coil; and

a top plate secured to the ceiling of the enclosure;

wherein the coil is held to the top plate such that the torsion spring biases the door toward the closed position,

wherein the top plate is provided with a plurality of flanges, and wherein at least one of the plurality of flanges is crimped to the ceiling of the enclosure to secure the top plate to the enclosure,

wherein the top plate comprises a mandrel formed by at least two opposed tabs extending into an opening provided in the top plate, the at least two tabs being truncated and having an elongated gap there between, and

wherein the at least two opposed tabs extend into opposed open ends of the coil such that the spring is held to the top plate by the mandrel.