A Beak Full

It's a bit confusing to someone like me who has difficulty identifying even the most common birds when a grey wagtail (Motacilla cinerea) has yellow plumage.  I mean, surely that should be a yellow wagtail?

I spent a lovely few minutes watching this female gorging herself on the insects flying just above the water in the stream.  She kept on flying off, I assume to feed her chicks, returning within a few minutes to repeat her hunting.  Her acrobatics were impressive, but unfortunately none of the shots I took of her in the air were in focus.