Pfranz wrote:
>>>Does anyone actually think about killing birds when they use the expression? I use it frequently and I must admit that I don't. To me it's just a figure of speech. It has no direct meaning in terms of killing anything.<<<

But in a way it is exactly that casual--or thoughtless--reference to unnecessary killing which is problematic, and helps makes the term offensive! (The rest of the offense, in my view, is in that word "unnecessary". Of course in some times and places killing birds efficiently was a necessary part of survival. But in most places where the phrase is now common, that hasn't been true for a very long time. In an age of dwindling biodiversity and looming ecological disaster, gratuitous (and often unconscious) reference to killing birds is offensive. IMHO!)

Meanwhile, WW came up with some alternatives gleaned from a Google search. The one I like best is
"grow two trees from one seed"
It has two very nice (in the 'precise' sense of that overused word!) advantages: it stands the proverbial phrase on its head, metaphorizing birth instead of killing; and it shares with the original the aspect of being highly improbable on the literal level.

Hope this modest post manages to hit two nails with one hammer,