Well, I can only ASSUME that they're birds coz no-ones PROVEN to me yet that farm animals can fly. I actually suspect that this mess was caused by a herd of wildebeest that have been grazing exclusively on industrial solvent, plaster of Paris and fish guts. How does everyone else clean this ^#%^$%@*......stuff, off their boats? Three of us with scrubbing brushes and detergent barely made a dent in this. It doesn't help that I'm in Brisbane and, apart from a few showers a january or two ago, it hasn't rained since 1974 and everything is baked on like pottery. At the moment, I have a 37ft adobe cat on anchor. I put a couple of rubber snakes on board, but seeing as though cyclone Tracy has been blowing here since yesterday, they're probably scaring mud crabs on the bottom by now.
Any thoughts?............
Petey
You could try sitting one of those owl bird repellers on your boom. I've seen them used a lot on beach house balconies, but don't know how well they work.
Well Dr Pete I think you should consider yourself lucky your yacht is not resident at Hamilton Island.
Last time I was there (in the 80s) yachts were plagued by white cockatoos landing on their mast tops where they would destroy anything their beaks could get around such as lights, wiring, windexes, expensive wind sensors and they would even chew through halliards.
This was over and above (pardon the pun) their droppings and not mentioning their screeching.
You could try netting, plastic bags, dummy hawks or owls or even hanging a dead seagull in the rigging.
As for getting the crap off, you could try acid (starting with vinegar) or alkaline (starting with bleach) and increasing the strength until something works and softens it up.
Of course try a small area first. I suspect alkaline might be what will work.
Caustic soda available at supermarkets is what you use in dunnies and drains, so give that a try. The stuff has been baking in the sun so oven cleaner might be the go. That could be expensive. Anything that causes bubbling is working but it might work too well and damage paint or gel coat.
I doubt soap or detergent will shift it but will be good for the after clean up.
Fred Bevis looked into this and reported thus in Sydney Amateur's journal:
LETTER TO THE EDITOR February 2012
Dear Sir,
I have been pursuing a detailed analysis and specific testing of methods and objects suitable to deter seagulls from perching on Warana and thereby disrupt their long-range objective of creating another Nauru. The methods explored to this point have been strings with CDs (Beethoven to Beatles tested) strung on a string above the boom, an owl imperiously looking back along the boom, a specifically-designed hawk suspended above the boom which moves in the breeze and the most recent testing of two coiled red-bellied snakes placed on the stern. I am saddened to report none of these tests has met with significant success. The latest snake approach looks very intimidating to humans at first glance and so it is with seagulls as the photo attests.
I am forwarding a copy of this photo to the purveyors of this “bird- scaring” device with a request that they consult the warranty terms as the product is self-evidently “not fit for purpose”.
I have now turned my mind to first principles and have come to the conclusion that what hitherto had not been a problem has been brought about by more seagulls being attracted to the increasing schools of small fish being attacked, from below, by larger fish and the seagulls feeding on the collateral damage from the attacks. The first principles' conclusion is therefore the reintroduction of high concentrations of tin, lead, zinc and copper to antifouling paints to diminish the attractiveness of the harbour to the small fish, their predators and as a consequence diminish the food supply for seagulls.
You know it makes sense.
Fred Bevis
Fred Bevis is correct.
I get swallows occasionally but that's it. Birds just like some boats and not others. Usually they select one that has a high and safe perch. My mate was driven to despair with a shag that loved his boat. The answer was netting that he covered over his ex trawler. He then mounted a temporary perch off the bow a few feet higher than the cabin height. The shag loved it and would sit on it for hours crapping straight into the water.
Who has not seen seagulls sitting on the head of them owls? There is a great video about of rotating mechanism on the cabin of a fishing boat with a seagull hopping over the slow moving arm as it sweeps across.
The swallows and seagulls will always win I use lots of sea water to soften it and scrub with boat cleaner.
I think we have all felt your frustration. The only solution I have found works is the full cover of netting. It can be a pain to put on and pull off every time you want to go for a sail but it beats having to spend half an hour scrubbing the poop off the deck every outing.
As for getting it off, I have always been ok with just water and scrubbing brush so you must have endured a period of saturation bombing if it is that hard to shift.
Thanks for everyone's advise. And Cisco, I know the joy of white cockatoos. Grew up on a farm. I remember many times having to take the shotgun off dad whilst reminding him that they are a protected species. He really wanted to get a couple of shots off to find out just HOW protected they were.....
Answer....... Live aboard, downside off this is they will crap all over your house ! 
I've tried everything, the best is the snake, or plastic bags to creat noise.
The worst was the net, because it's a pain to use each time you sail, followed by the owl. The seagulls saw it as threat, so continued to attack it, poopoping all over the place on the process.
Tie the snake to the foredeck, one is usually enough.
Michael.
Just maintain a constant supply of hot chips on your neighbours yacht. That should attract them away! 
We also have a seagull problem down here in Tassie.
Our old timber boat comes out of the water each year and a few years ago we painted the cabin so left the netting off when she went back in the water as we didn't want to walk all over the soft paint, and guess what....the seagulls didn't go near her for two weeks!!
After some head sratching we realised that the boat has to 'smell right' for them to sit on it. If you clean the boat regulary you get alot less crap, the build up is expenetional.
So apart from fresh paint what else do they not like the smell of? Well after some experimentation it happens to be vicks vapour rub!!!
We made some little holders (one jar lid on top of another, bigger on top and help apart by a spacer) and it kept the birds away!
Unfortunately with a 38 x 14ft boat there was always a spot where the smell would not be and you had to put up with the smell yourself!!
So in conclusion, we use netting with a homemade rotator on the radar dome and it works very well. With practice I can take the netting off/on in about 5min but am looking at putting a small spinnaker bag at the base of the light pole to bunch the net into when using the boat to save time.
this is a pet hate of mine too. There has to be a better way.
Having a tri then a cat means plenty of deck to scrub.
Apologies if if posted this here before, it's being floating in my head for a while.
Humour me for a minute........
Small solar panel hooked up to a battery. Timer circuit that discharges a pulse possibly through a capacitor, through a loop of uncoated stainless fishing trace which is clothes pegged onto the staunchion.
Bird sits on handrails, next pulse goes through, bird flies away.
no wait, edit that, bird sits on handrails, next pulse come through, bird ****s itself, then flies away.
back to square 1.
No, i'm sure a few volts up the *** would be a deterrent, that or you come down the boat and see a line of smoking, bald, charcoal colored comorants looking back.
Hi drpete, I have installed two of the large plastic owles one hanging from each end of the boom and swinging in the brease, as much as it hasnt completely stopped the bird $#%* they have most certainly reduced the amount of %$#* that i have to clean off the top of the boat. i also used to have a net but proved to much of a pain in the bum, both putting on and taking off. but i am more than happy with the owles, which i paid $30+ for each and then saw last week at Bunning for $20 each
Ben405
Ive tried plastic bags,cd's owls,netting,flags etc All with varying degrees of success and inconvenience. I'm thinking of tieing tree branches to the mast cause Ive never seen a seagull sitting in a tree
The most success I ever had was just a simple string along the boom. Friends have crazy spinning devices set up too. It helps if all boats around you have something setup so birds don't hang out in the area.
I have also had the instance where a Possum has been found on an Etchell that was on a mooring. We found it whilst hosing out the boat. The Possum was not very happy with us.
What!! Was he eating the seagulls that dared to venture on his domain??
Mmmmmmmmmmm camp on the boat with a 12 gauge shot gun![}:)][}:)]
I sold my 40 ft'r just for that........... got tired of cleaning bird shrit for hours! The smell run in your nose for day!
What!! Was he eating the seagulls that dared to venture on his domain??
Well after a three day sail with friends (scrubbing bird **** most of the time) I can safely say that the baking soda and vinegar doesn't work. I can also say that the vast bulk of my boat is now stained beyond the ability of any sort of cleaning to remove. Anger..... building....
