Without a dryer vent cover, your dryer vent is vulnerable to all kinds of debris that can enter it. This debris can include rain, snow, blowing dust and even bird nests.
Making a Dryer Vent Cover
Remove the top and bottom lids from a small empty tin can. Use a pair of tin snips to cut lengthwise through the can. Make a 2-inch flange on one edge of the tin sheet you've made. Bend the flange down, and place this sheet above the vent cover with the flange against the wall. Screw the flange to the wall. This will be your vent cover.