Green-rumped Parrotlet (Forpus passerinus) pairs that are in the courtship and nest prospecting mood have several more distinctive behaviours beyond allopreening. In the following video (courtesy of Rae Okawa), a parrotlet pair is duetting outside their chosen box. Watch for the behaviors identified by Waltman and Beissinger (1992), including 'head bowing', 'tail fanning', and 'nipping bouts'.
Up next: An intruder at the nest
Waltman JR, Beissinger SR (1992) Breeding behavior of the Green-rumped Parrotlet. Wilson Bulletin 104(1):65-84 (Available as a free pdf via SORA)