Scientific Research and Essays Vol. 2 (8), 263-294, August 2007          
ISSN 1992-2248 © 2007 Academic Journals

Review

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: lkavanau@biology.ucla.edu

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 at each stage. 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 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. Modern birds comprise Stage 6. Mates in Stages 1 and 2 maintained close nest vigilance from nearby, overhead resting sites in vegetation, jumping or parachuting to protect nests from small egg‑predators. Pervasive selection for these practices led to skeletal adaptations for arboreality and integumentary adaptations for thermal insulation and increased drag. These practices and their selective effects are the proposed roots of avian evolution. Feathers and flight evolved with continued selection along these lines, with gliding achieved in Stage 3 and sustained wing‑flapping flight in stage 4.

Key words: Origin of feathers and flight, nesting practices, egg and chick care.