Size and Weight:
One of the enduring mysteries of natural history is why the flying prehistoric birds of the Cenozoic Era never quite matched the size of the pterosaurs, or flying reptiles, of the preceding Mesozoic. The late Cretaceous Quetzalcoatlus, for example, attained wingspans of up to 35 feet, about the size of a small plane--so while the late Miocene Pelagornis, which lived about 55 million years later, was still impressive, its wingspan of "only" about 15 to 20 feet places it firmly in the "runner-up" category.
Still, there's no overstating the size of Pelagornis compared to modern flying birds. This soaring predator was over twice the size of a modern albatross, and even more intimidating, considering that its long, pointed beak was studded with tooth-like appendages--which would have made it an easy matter to dive into the ocean at high speed and spear a large, wriggling prehistoric fish, or perhaps even a baby whale. As a testament to this bird's evolutionary fitness, various species of Pelagornis have been found all over the world; a new fossil unearthed in Chile is the biggest yet.
So why couldn't prehistoric birds match the size of the biggest pterosaurs? For one thing, feathers are fairly heavy, and covering a larger surface area might have made sustained flight a physical impossibility. And for another, bigger birds would have had to nurture their chicks for longer periods of time before their hatchlings achieved maturity, which may have put an evolutionary brake on avian gigantism after Pelagornis and its relatives (such as the comparably sized Osteodontornis) went extinct, probably as a result of global climate change.