Modern birds sit on their eggs and the eggs hatch. The parent's body heat is sufficient — efficient enough that clutch temperature varies by less than a degree. Oviraptors sat on their eggs too — the fossil evidence is clear, with adults preserved in brooding position atop ring-shaped clutches. But the physics didn't work.
Researchers in Taiwan built a life-size oviraptor model based on Heyuannia huangi and a realistic ring-shaped nest, then measured heat transfer. The parent's body heat alone couldn't reach the outer eggs. The ring geometry — eggs arranged in a circle with open space in the center — meant the brooding adult contacted only the inner ring. Outer eggs received insufficient heat for development, with temperature variation across the clutch exceeding what embryos tolerate.
The solution was the environment. In warm Cretaceous climates, solar radiation heated the exposed outer eggs from above while the parent heated the inner eggs from the center. The two heat sources complemented each other geometrically: body heat radiating outward, sunlight radiating inward. Temperature variation across the clutch dropped to roughly 0.6°C — viable for synchronous hatching.
The structural point: oviraptor brooding was a necessary but insufficient condition for incubation. The behavior was real but the physics was incomplete. The bird-like posture was present 70 million years ago, but the bird-like efficiency wasn't. The gap between having a behavior and that behavior being sufficient is itself an evolutionary observation — brooding evolved before it was adequate, and environmental heat bridged the gap until body temperature and nest architecture co-evolved into the sealed, self-sufficient system modern birds use. The behavior preceded its own competence.