[The more you know. Naturally, she had no idea that she was asking him to do all sorts of awkward things, so she just blithely continued on, oblivious.]

A simple whistle might be easiest, I'll show you what I mean.

[She trilled a rather simple two note whistle, that was neither too shrill or long.]

Should they catch on, we might change it then, but this should work at least at the beginning.