Most parrots do little nest building, laying in a bare cavity. Lovebirds of the genus Agapornis are exceptions, constructing a pad or cup inside the hole. The rosy-faced lovebird cuts strips from bark, leaves or grass with its bill and then tucks them, several at a time, into the feathers of the rump and lower back before flying.
The habit is species-specific within the genus, which is what makes it useful. Fischer's lovebird carries material in the bill; the rosy-faced lovebird tucks. Hybrids between the two, produced in captivity by William Dilger in the 1960s, showed intermediate and ineffective behaviour: they tucked strips that then fell out, and took years to shift reliably to bill carrying. The work became a standard example of behaviour with a heritable component.
The nesting site is often not a tree at all. In Namibia lovebirds frequently occupy chambers in sociable weaver colonies, which are permanent multi-chamber structures maintained by another species entirely.