You might expect a boat that never moves from its slip or mooring to have critters make their homes on, under or in it. Mussels and barnacles like the hull - especially the nooks and crannies where no scraper can break them free. Depending on the boat's location, sea lions, otters, muskrats, mice, rats and/or cockroaches prefer the deck and belowdecks. Birds go for the folds of the sails and the rigging.
I'm surprised at how fast critters can infiltrate. In our case, we'd sailed in to Mazatlan on May 27 and had actually been using our sails, so we knew no birds were living on board before then. GB flaked and covered the sails as per usual when we arrived in Mazatlan and we noticed nothing out of the ordinary afterwards. But, a mere 18 days later we removed the main sail for hurricane season...and out from one of the folds fell a heap of dried grass, strips of palm fronds, cigarette butts and cotton batting - seems a couple of mangrove swallows (like those little guys in the pic over there on the right, perched on the lifelines of the boat next to us) had wanted to start a family and liked the looks of The Fox so much, they flew up under the sail cover and into the folds of the sail to build a nest. That's motivation.
I enjoy all birds, and mangrove swallows sure are cute little buggers - but they're small enough that they can compete with flying insects to colonize other small spaces above deck - like the voids in our boom. I couldn't risk the birds rebuilding their nest even without the main sail in place; plus, nesting material can introduce insects to the boat, so following the example on a neighboring sailboat I used scrap Sunbrella to cover both ends of the boom. Some boats, like the one pictured above with the green fabric, use webbing and buckles to keep the boom covers tight and windproof against the mast and backstay, but I just used drawstrings and Velcro. Took 2 or 3 hours, start to finish. Heck, these covers also help protect some of the shackles and rope ends still up there. We'll see in a few months how well the boom is actually protected from critter invasion - heh.