Legal Eagle

Neither of you will be surprised to know that I am currently on the case with regard to legal issues surrounding benevolent vandalism. Not being the sort to take risks (don't laugh) I feel it is my duty to at least know what the most likely charge would be if we got into hot water.

Right now I must apologise to Jimmy because I didn't take his Health and Safety comment seriously. I think it's spot on, but while we were in the car I couldn't understand why anyone would "hang off" the things we are talking about putting up. You couldn't "hang off" one of those flat sheet street signs either, so it was the "hanging off" bit that threw me. But, the "safety" thing is 100% right. If any of these things cause personal damage to anyone (fall off and hit them etc.) we'd be liable. We'd need to look at Public Liability if we were in any way worried about covering ourselves on this.

Public liability insurance
If members of the public or customers come to your premises or you go to theirs (including if you work from home), you should think about taking out public liability (PL) insurance.

Otherwise we'd be Negligent:

Negligence is a legal concept in the common law legal systems usually used to achieve compensation for injuries (not accidents).

Other than this I'm really struggling to find anything to worry about. These are the only possible clauses or categories under which we might be committing offence, but none of them really cover covert bird box installation.

Graffiti and Fly Posting
But definitions don't include the installation of physical things. It's a graphics crime.
Litter
Well it could be I suppose.
Trespass
It's tinkering with/on someone else's property [Council]. But trespass normally has to have caused measurable damage.
Vandalism (the text is from Wikipedia, but it does give the overall gist)

Private citizens commit vandalism when they willfully damage or deface the property of others or the commons. Some vandalism qualifies as culture jamming or sniggling – it is artistic in nature as well as being carried out illegally or without the property owner's permission. Examples include at least some graffiti art, billboard liberation and possibly crop circles, Criminal vandalism has many forms, graffiti on public property is common in many inner cities as part of a gang culture, however other more devastating forms such as those involved with public unrest, such as rioting, involve the willful destruction of public and private property.

My bold in this because that's what I think we are doing at worst. If we were in Scotland it might qualify as this:

Malicious mischief is an offence under the common law of Scotland. It does not require actual damage to property for the offence to be committed, financial damage consequential to the act is sufficient, unlike vandalism which requires actual damage to property to form the offence, the latter being defined by section 52 of the Criminal Law (Consolidation) (Scotland) Act 1995; the two are different offences.

We aren't going to damage anything directly (this is very important when we design the brackets). But there's something here for me about money. If what we do ends up costing anyone money, or hurting anyone, we are liable for the damages. If someone has to pay to have the things taken down then they will look for whoever put them up to recoup the bill. Other than this I can't see any grounds for trouble.

And I will blame you two anyway.