This is a more common problem that one might think. As a gardener, I’ve seen many gardens with trampolines covered in bird faeces and I’ve seen many customers fix this problem too. One customer simply tied cable ties along the edge of the trampoline with the ends sticking upwards, much like spikes. This will stop the birds from sitting on the trampoline.
Another way I’ve seen is when a plastic toy snake has been put on the trampoline which surprisingly worked really well and another customer used it as an opportunity to build a couple of scarecrows with her children and plot them on either side of the trampoline which worked really well too. When it comes to keeping birds away from your garden there seems to be a few cheap and effective ways. I will write a full article soon and update a link here when I’ve written it.