Sitting on eggs is only for modern birds. Primeval birds did it differently.

Nature reveals the ins and outs of dinosaur-era reproductive strategies, with research showing that prehistoric birds were too heavy to incubate their own eggs:

Most birds today incubate their eggs by sitting on them. To test whether the behaviour was common in birds living in the Cretaceous and Upper Jurassic periods, Deeming and his collaborator Gerald Mayr estimated the size and load-bearing capacity of the eggs of 21 species of primitive bird, including Confuciusornis and Yanornis and one of the oldest of all birds, Archaeopteryx, which lived about 150 million years ago.

Previous work had shown that modern birds evolved a more open pelvis, which would have allowed the size of their eggs to increase over time, says Mayr, who is at the Senckenberg Research Institute and Natural History Museum Frankfurt in Germany. In early birds, pelvic bones were fused, creating a canal that limited egg size. Given they had no fossil eggs to measure, the scientists measured the pelvic-canal width to estimate the likely size and mass of each species’ eggs. They then calculated the load mass of each egg — the maximum weight that they could bear without breaking.

The pair found that for every species tested, the load mass of its eggs would have been too low to support the weight of an adult.

Some species, such as Confuciusornis, had a mass more than three times what the eggs could have supported, according to the study, which was published last month in the Journal of Evolutionary Biology.

Modern birds, by contrast, lay eggs that can often accommodate three times the body mass of the adult, Deeming says. Based on that, the authors argue that contact incubation — sitting on eggs — evolved late in the history of modern birds, possibly after the emergence of a more open pelvis in the past 100 million years or so.

If you’re curious, a Confuciusornis (or “Confucius-bird”) was about the size of a crow, with little claws on its wings.