I'm trying to send my sister who lives in the UK, a birding app, but when she tries to load it onto her iPhone, it complains and says it was purchased in the US, and cannot be loaded in the UK. What kind of nonsense is this? What's the solution?

You can currently only gift content (apps, music, gift certificates) to people whose account is based in the same country as yourself - so if you and your account are not UK-based then you can't gift to a UK account holder. You can either try resending it to somebody who is in the US with a US account (resending) or try contacting iTunes support and see if they will refund or credit you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption