Presumptive relict reproductive behavior in small parrots

Recent studies of experimentally elicited egg-care behaviors in three species of small, hole-nesting parrots, and of their evolutionary implications are reviewed. A suite of primarily nonthermal egg-care responses are adaptive for nesting in shallow terrestrial scrapes, rather than in cavities. These responses appear to be relics from times before hole-nesting began; as such, they may provide 'windows' into certain nonthermal aspects of terrestrial-nesting practices during ancestral stages. A suite of primarily thermal egg-care behaviors, elicited when nests are exposed to light and view, progress from mere initial curiosity, inspection, and manipulation of fostered eggs by nonbreeding parents, to more extensive manipulation and episodic concealment of a pair's own eggs by crouching over them, to crouching lengthily over eggs with some incubation but nighttime abandonment, to continuous 24-hour incubation. This progression may retrace certain aspects of major ancestral stages of thermal egg care. Stages of primary oocytes and follicles during ovarian development, and of maturing follicles during breeding cycles of some species, may retrace the phylogenetic progression of sizes of ancestral clutches and ripe follicles. These phenomena and the retention of many of the experimentally elicited egg-care responses are attributed largely to the highly conservative evolution of the central nervous system.