Pair of small, slim birds. No bigger than an adult human thumb. They were largely grey with white underside and yellow on top and bottom of the head. The kept flying into and out of the hollow on a gumtree. This appeared to be a tag-team effort. James Lehane was also with me and in one of his photo's observed one of the pair coming out of the hollow with egg-shell fragments in it's beak. So possibly cleaning out the hollow before/after use?