[EvaH (OP)]

Paul asks:

I have been wondering if it were possible to set up a gadget over my rotating clothes line that generates an irritating frequency upwards that discourages birds from flying over my washing and defecating on my washing. It would also need to take into account the wind direction, so that birds cannot sideswipe my washing and upon encountering such a frequency it might frighten the bird into defecating, so the bird needs to be dissuaded from getting too close to the clothes line. I don't want it to harm birds and the strength of the signal can dissipate below any aircraft flying heights, including police or medical helicopters.

What do you think?

[alancalverd]

Difficult!

You need to determine the responsible species. Gulls will generally disperse if they hear the alarm call of a gull, but crows will come to the aid of a distressed crow. The call of a small territorial bird like a wren or a robin will put off other males but may attract females.

AFAIK there is no single frequency or song that prevents overflight, but you could try the mating call of a sparrowhawk -  you might gain a bemused female but she will happily kill everything else in the  garden whilst looking for the male.

Random gunshots are fairly effective in the countryside but may incur the wrath of the police in a built-up area.   

Don't worry about passing aircraft. Birds have extremely sensitive hearing, and pilots wear noise-cancelling headphones.     

[Rasingin]

I have come to try and read and feel very much liked.

[Colin2B]

I have come to try and read and feel very much liked.

Wrong. Spammers are never liked on this forum.
Feeble attempt by the way.

[evan_au]

Is there a specific sound frequency that discourages birds?

I understand that the sound of a shotgun being fired will scare them away (for a while).
- After they realise there are no consequences, they get used to it, and it doesn't annoy them so much.
- It's the cloud of buckshot that reminds them not to come back...

Of course, your neighbours will also be discouraged by the sounds of frequent gunfire....

[Bored chemist]

If there was a frequency that kept birds away then there would be immense evolutionary pressure for a bird to learn to whistle that frequency and thus drive away all competition. (and there would then be a pressure for that "signal" to stop working.

So you can pretty much answer the question (with a good degree of confidence) without actually studying birds.

[Just thinking]

I have been wondering if it were possible to set up a gadget over my rotating clothes line

Most birds are afraid of snakes if you can find one of those toy rubber snakes and somehow have it elevated above the clothesline maybe strap a long stick to the centre of the line pole and have the snake dangling from the top of the stick. I know it works to keep birds off garden furniture.

[TommyJ]

The exact bird species behavior, ‘crowd’ population, sounds, frequencies, and language differ very much.
It’s better to learn about them from reading and of course bird watching.
I’ve had a fight with sparrows, who made my window as a meeting space. Water gun attacks were first fierce and useless attempts. But finally I have learned that it is possible to keep them away with anything blinking. ‘Anything’ is dangerous because magpies can be there fast. Old CD's worked. No sparrows on my window.

[Petrochemicals]

How about a cat sound ? Big hissing tomcat ?

[Just thinking]

How about a cat sound ? Big hissing tomcat ?

I do that to scare feral cats away give them a big hiss sound they poo near my front door and it stinks. I'm sure it will work on birds as well.

[Petrochemicals]

Joke

Furthermore, my mate Barry makes an uncanny sound that repels birds as soon as he opens his trap.

[Liaata]

Not sure about the frequency, but I would go with the sound of predators. Cats for example. They are natural enemies and I think they are hardwired to fear an ascending cat.

[TommyJ]

During this discussion, you could try to tame birds.
But be aware of animal rights activists.