Currently, most birds incubate their eggs by sitting on them. In this way they maintain a constant temperature for the embryos to develop. However, one study postulates that the first flying animals were very heavy and could have broken their future offspring by laying on their eggs.
The research, published in the journal Nature, suggests that hatching is a characteristic of modern birds.
Paleontologist Charles Deeming of the University of Lincoln, who led the analysis, says that although many fossils of early birds have been discovered in the last three decades, direct evidence of their reproductive behavior is elusive. The experts estimated the size and load capacity of the eggs of 21 species of primitive birds, in order to determine if hatching their eggs when sitting on them was common in birds that lived in the Cretaceous and Upper Jurassic periods.
The scientists showed that the size of the eggs increased with time, after the birds developed a more open pelvis. The experts measured the width of the pelvic canal to determine the size of the eggs and the maximum passage they could support without breaking.
With this, it was concluded that the load mass of the eggs of the first birds would have been too low to support the weight of an adult. “If that’s the case, those birds can not sit on those eggs without the real threat of destroying them,” says Deeming.
An example of this is the species Confuciusornis, which had a weight three times greater than what its eggs could have supported. Some paleontologists have criticized the study, since this theory is contrary to the evidence that some dinosaurs would have sat on nests in the ground.