Flummery's right - I'll bet they make their own in the empty flowerbed.

Cat litter tray is an ideal size for one... we used to use a washing up bowl but they perched on the side of it and tipped it over all the time.

If you can find one, an old stone sink is ideal.... we have a battered old Belfast sink which we fill with dry soil and mix in some cold ash from the fire as that helps keep mites away