If temporary fencing just means those metal barriers with lumps of concrete on them to keep them stable, like the ones at construction sites, would they even be able to stop you?

I don't see how it would be any different to putting the birdbath up in the first place. Unless you're in a set of flats or something, or the front yard is shared/common property. I'm pretty sure the law specifically states you have "the right to quiet enjoyment of premises", which, even if the landlord isn't the one violating it, should mean you are allowed to take whatever steps reasonable and necessary to ensure that. I said should not does though

(Sorry if this is all obvious or you've already tried this. More of my 2c/what I'd try/do than trying to push you into anything)

Could be someone who doesn't want wildlife hanging around rather than deliberate vandalism. It's wrong that someone's come onto your property though.