What Is a Nesting Colony? Fossil Sites Where Many Animals Bred Together

The distinctive feature is repetition in space and in time. Many nests occur at the same stratigraphic level, meaning they were made in the same season or a short span of seasons, and they are spaced at roughly consistent intervals, frequently around the body length of the adult, which is the spacing modern colonial nesting birds use and which implies the animals were tolerating neighbours at a fixed distance rather than avoiding each other. Several sites show nests stacked at multiple levels separated by sediment, meaning the same ground was used again in later seasons, which is the fossil equivalent of colony site fidelity. Nests themselves vary, with some being scraped bowls, some rimmed mounds and some simple depressions, and the eggs within are frequently arranged deliberately rather than dropped, in rings, spirals or paired rows depending on the group.

Such sites can contain an unusual combination of material, and each element supports a different inference:

• •Eggs, whole or crushed, whose shell microstructure identifies the group that laid them and whose porosity indicates whether they were buried or brooded
• •Embryos inside unhatched eggs, which link an egg type to a species and reveal the state of development at hatching
• •Hatchlings and juveniles of more than one size class, which indicates how long the young stayed at the nest
• •Adults, occasionally preserved over a clutch in a brooding posture, which is direct evidence rather than inference
• •Eggshell fragments trampled into the sediment, which indicate hatching occurred at the site rather than the eggs being transported
• •Sediment structure showing the nest was dug, mounded or covered, which indicates the incubation method

The inference chain from these sites to behaviour is careful and the strongest cases combine several lines. Joint limb bone development in hatchlings is one, since young with poorly ossified joints could not have walked effectively and therefore had to be fed, which was the central argument for parental care at one famous North American site and which gave the animal its name. Worn eggshell trampled within the nest bowl is another, since it implies young remained present after hatching. Size classes of juveniles at the site imply an extended stay. Adults preserved in brooding posture over arranged clutches, known from several specimens of small feathered theropods, establish that at least some groups sat on their eggs. Against this, some groups show well-developed hatchling joints and no evidence of extended residence, so care was clearly not universal and the picture varies substantially between groups.

Eggshell is a mineral tissue with a structure specific to the group that produced it, and it is studied as a subject in its own right with its own naming system, since an egg found without an embryo cannot be assigned to a species and is instead given a name in a parallel classification. Shell thickness relates to the size of the egg and to the mechanical demands of incubation. Pore density is the informative measure, since eggs buried in sediment or vegetation need high gas exchange through many pores while eggs incubated in open air lose water too fast if porosity is high, and measuring it therefore indicates the incubation method without any other evidence. Shell surface ornament varies between groups. Pathological shells, with extra layers or malformed structure, occur and indicate stress in the laying animal. Together these turn an isolated egg into a source of information about an animal nobody can identify.

The interpretation of these sites is contested in specific ways. Association is not the same as nesting together, since eggs concentrated by water transport or by repeated use of favourable ground over long periods could produce a similar pattern, and establishing that nests are contemporaneous requires careful stratigraphy rather than a single surface. Attributing eggs to a species is difficult without embryos inside them, and mistaken attributions have been corrected embarrassingly, with the best-known case involving a species named for apparently stealing eggs that turned out to be sitting on its own. Modern analogies cut both ways, since crocodilians and birds both nest and care in different manners and represent the bracketing relatives, which means either pattern is plausible for any extinct group and the fossils rather than the analogy must decide. And preservation bias favours environments that bury nests rapidly, which are not necessarily where most nesting happened.

Many nests at one level, spaced at roughly an adult body length, imply animals tolerating neighbours at a fixed distance in the way colonial birds do. Poorly ossified hatchling joints, trampled shell and multiple juvenile size classes together indicate young that stayed and were fed. Care was clearly not universal, and one species named for stealing eggs turned out to be sitting on its own.