Model I birds, the kind that lived during the Age of the Other Dinosaurs, may not have brooded their eggs. Today, birds sit on their eggs in such a way that the adult bird’s down surrounds the ovoids, and warmth from the adult can keep the eggs at a constant temperature. Depending on the bird, you may find additional intersting adaptaitons. For example, Penguins use their own feet as a nest, placing the egg there. One adult broods the egg for a long period (days, in some species) and then swaps with the other adult, with the swapping being very ritualized in some cases. Like this egg swqap between parent Adelie penguins (Tip: this video does not show the actual swap):
Anyway, recent research suggests, controversially, that the early birds were not built to do this, and may have been too heavy.
Here’s the basis of this suggestion. Early birds had a pelvis that was fused, and thus, limited the size of the egg that could be produced. A smaller egg, with its thinner shell, would have been too weak to allow a bird large enough to produce that egg to sit on it without breaking it.
(There are no pertinent actual fossil eggs at this time.)
The controversy arises from the belief among dinosaur-ologists that some non bird dinosaurs were brooding their eggs at this time, so naturally, birds could have done this. However, if you think about it, maybe that assumption would not have emerged to begin with if this bird study was done fifty years earlier.
Here is the abstract:
Numerous new fossils have driven an interest in reproduction of early birds but direct evidence remains elusive. No Mesozoic avian eggs can be unambiguously assigned to a species, which hampers our understanding of the evolution of contact incubation, which is a defining feature of extant birds. Compared to living species eggs of Mesozoic birds are relatively small, but whether the eggs of Mesozoic birds could actually have borne the weight of a breeding adult has not yet been investigated. We estimated maximal egg breadth for a range of Mesozoic avian taxa from the width of the pelvic canal defined by the pubic symphysis. Known elongation ratios of Mesozoic bird eggs allowed us to predict egg mass and hence the load mass an egg could endure before cracking. These values were compared to the predicted body masses of the adult birds based on skeletal remains. Based on 21 fossil species, we show that for non?ornithothoracine birds body mass was 130% of the load mass of the eggs. For Enantiornithes body mass and egg load mass were comparable to extant birds, but some early Cretaceous ornithuromorphs were 110% heavier than their eggs could support. Our indirect approach provides the best evidence yet that early birds could not have sat on their eggs without running the risk of causing damage. We suggest that contact incubation evolved comparatively late in birds.
Deeming, D.C. G. Mayer. 2018. Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs. Journal of Evolutionary Biology. 27 Feb 2018