There is nothing quite so upsetting as arriving at your boat to find it covered in bird droppings. This happened to us regularly because our marina berth was near a clump of trees where starlings and large blackbirds roosted then did their dirty work all over nearby boats. I therefore jumped at the chance to test a line of products designed to deter birds of boats. StopGullUSA (www.stopgull.com) is an international company operating in the US from Florida. They have a dazzling array of devices for boats and even dinghies, but due to space constraints I could only review three products.
Stopgull “Air” has two wind driven horizontal rotating arms, six feet in diameter that can be positioned just about anywhere there is space using any of eight different mounting adapters. There is also a larger eight foot version. I positioned them on my two staysail booms using strap-on adapters but they could just as easily be located on a coach roof, Bimini or main boom, provided there are no obstructions of rigging or lines. The rotating arms deterred birds from landing in the vicinity. The device can be easily removed and folded flat for storage. These rotating arms will keep birds away, but they still won't stop them from perching on the 
rigging and doing their business from up there.
Stopgull “Bimini Sailboat” is a row of twelve plastic candelabra-like rotating pods on a canvas backing material. I strapped a set to each of my davit arms that is a favorite bird perch where I recently had to shoo away a pelican who really knows how to poop! This device could also be mounted on a rail or even your spreaders. Birds cannot land on the pods because they rotate and upset their balance.
Stopgull “keeper” is a sophisticated adaptation of many home made “spiky things” that sailors have always installed on mastheads, radars and TV antennas. It has a holder for multi pronged rods that also rotate and prevent birds landing on the equipment. Unlike the “Air” this holder must be glued permanently on top of the equipment, most likely with epoxy that might be difficult keeping it in place while the adhesive sets on equipment high on a mast.
All products are extremely well made and even include pop rivets and Allen keys as needed for assembly. They succeeded in deterring birds within their area of influence, but they did not protect the whole boat. To achieve this would require a myriad of Stopgull’s different devices.
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HOW TO KEEP BIRDS AWAY FROM YOUR BOAT