かもめに糞を落とされた！って英語でなんて言うの？

"I have bird droppings on my head"- 'bird droppings' is a nicer way of saying 'bird poop'. You could use it if the occasion doesn't allow for informal word choices or slang words.

I hope this helps :)

"The bird "pooped" on my head."
"It was so funny, the bird "crapped" on his head."

1. A seagull did his dirty business on my head.
   The expression "dirty business' is an expression for pooped.

   2. A seagull crapped on my head!
      The word "crap" also mean pooped.

In the UK we tend to say poo and pooed, whereas in the USA they use the word poop and pooped.

1. In the UK, some people believe it's lucky :-)
2. But really it's unlucky, so other people say, "It's unlucky."
3. 'To make someone's day' = to make the day 'Perfect'.
   "It would make my day if you accepted my invitation."

There are many euphemisms for Excrement or faeces: IE waste matter ..from the bowels!
It seems some words are taboo and should not be used in genteel conversation
Crap / shit /poop / poo / doo-doo etc etc
I like to use the more humorous... ..."took a dump!"
("Toilet humour" is rather popular in the UK;-))

In some cultures, it is considered lucky if a bird poops on you, so consider yourself lucky.