Once at day at least, and there much more nutritional, too.
And a heck of a lot of fun. Through grapes, they all chase .. then after the first grape, swap it for a pebble. They know no difference, attract them all to you showing the pebble, then throw it and see who wins the race to the pebble. The winner gets the grape.
They have genuinely figured this out now, the one that gets to the pebble first comes to me. The rest scuff off..