There are a variety of techniques that bird researchers use to catch birds. These techniques vary depending on the habitat, size and other variables concerning the species in question. For a seagull, a simple funnel trap is an ideal structure to capture one or more birds. Since most seagulls rely, to one degree or another, on human food, and are thus fairly habituated to humans, capturing them should prove easier than capturing other, less habituated species.
Place the bait, which could be food such as French fries, on the ground. With the mallet, drive four posts in to the ground in a square around the bait. The size of the square will be determined by the size of your net. There should be enough net to drape down to the ground and leave little or no space between the ground and the bottom edge of the net. You will also need several feet of slack on one side of the square to form the door.
Place the remaining two posts roughly 2 feet into the square from one of the sides, roughly 1 foot apart, or just far enough apart for a seagull to squeeze between them.
Place the net over the tops of the posts. Secure the net to the tops of the posts with the duct tape if the holes in the net are too large and the posts slip through the holes. The idea is to form a netted enclosure for the seagulls to go in to.
Drape the edge of the net over the two entrance posts in such a way that the two posts form an entrance, without any gaps along the edges. Use duct tape, if necessary, to secure the net to the top of the two entrance posts. Use six zip ties to secure the net to the bottoms of the six posts, and the remaining two zip ties to secure the edge of the net to the entrance posts, halfway up the posts.
Move at least 50 feet away from the trap, but be sure to keep an eye on it. If the seagulls seem wary of your presence, move even farther back.
Run to the trap when a seagull enters the enclosure. Stand near the entrance so the seagull wont escape. Squeeze in to the trap via the entrance, if possible, or remove some of the zip ties and duct tape if there is not enough room to get in. Corner the seagull and catch it by one of the legs.