Bird poo stains on fabric.

Ink stain remover.

I suppose it depends what the bird had been eating!

Vinegar is recommended but you could try white tablet soap which I find better than any of the proprietary stain removers.

Is the chair cushion cover removable? You could try dampening, rubbing with white tablet soap, don't rinse it then launder with a biological detergent.

No, it isn't removable. It is a kind of canvas over foam. The whole cushion (seat, back and headrest in one) fixes onto the chair frame with velcro. It is too big to to put the whole thing into the washing machine. I don't have any ink stain cleaner. I'll try the white tablet soap, but I can't take it off and launder it.. I've just been to look at it , and now that it is dry most of the marks look fainter, so there is hope. Another clean might get rid of them, or make them unnoticeable at least. Thanks for the replies.

I found another deposit on a window, behind the blind. I don't know how the bird got in there! This had dried on by the time I found it, so took some cleaning off.

A baby blackbird got shut into our conservatory last year; it pooped all around the floor behind the furniture but fortunately not on the furniture. It must have hopped in there, we didn't realise, locked up and went out.
It was still alive, I had to pick it up in a towel to put it out but don't know if it survived after the shock.

Its father was waiting outside for it, calling to it so I hope he fed it and it survived.

Honestly, my go to stain remover is Fairy Liquid, diluted with water, on a white cloth, applied ASAP
It has worked on various 'dubious' stains on my cream sofa
Good luck

After another go with Dove moisturising face soap (the only bar soap in the house) the marks are much fainter. If I didn't know they were there, I wouldn't notice them.