Reprinted from Scientific Research and Essays, Vol. 2(8), pp.263-294, August, 2007 Available online at http://www.academicjournals.org/SRE ISSN 1992-2248 © 2007 Academic Journals Review and Synthesis Roots of avian evolution: clues from relict reproductive behaviors J. Lee Kavanau Department of Ecology and Evolutionary Biology, University of California, PO Box 951606, Los Angeles, CA 90095-1606, USA. E-mail:
[email protected] Accepted 4 July, 2007 Remarkable, previously little known, relict reproductive behaviors of small parrots are detailed here. Because of the highly conservative nature of brain evolution, neural circuitry for many such primitive behaviors is preserved in functional condition. These behaviors can be elicited by exposing birds to appropriate conditions during breeding cycles. Founded largely on clues from these elicited responses, six main-line avian evolutionary stages are postulated, being described primarily in terms of lifestyles and reproductive practices. The progression began with small, ectothermic, shallow-nesting theropodan ancestors of birds. In warm, equable Mesozoic climates, the eggs were shallowly covered by dry vegetative debris and incubated solely by climatic heat. The following ectothermic surface-nesting ancestors of Stage 2 maintained sustainedly higher incubative temperatures by appropriately exposing and/or covering eggs with dry or moist vegetative debris, and alternately shading them from direct midday insolation with the parental bodies. Rapid double-clutching became adaptive in Stage 3, with the achievement of primitive endothermy. Higher core and incubative temperatures of Stage 4 and continued rapid double-clutching permitted more annual breeding episodes. In more seasonable early Cretaceous climates, sequential single-clutching again became adaptive in stage 5.