Lively little blighters aren't they John. A very nice little bird to watch but they seem immune to "baiting" and unpredictable in their wanderings.
But,- enough about the target - I have just noticed the lens data ........?? they will be even harder to keep track of at 500mm. Just use the "Acme Wagtail-stuffed" available from well known auction sites like I do . Nobody has noticed yet :-)