A tube map for pigeon poo – the Guano Line

What if there was a tube map for pigeon poo? Wait, come back, it’s serious. Well, it’s a concept at least based on an ancient idea we’ve rather turned our backs on.
Map of pigeon poo collectors – from the exhibition
Bird poo — properly called guano — is a very good fertiliser and was once collected in vast quantities with people building ever larger dovecotes to keep birds in, and even mining Pacific islands for it. That died out with artificial fertiliser, bu