Nesting Behavior

The scientific understanding of dinosaur nesting behavior began with Roy Chapman Andrews's 1921 discovery of intact dinosaur nests in Mongolia. Andrews attributed the elongated oval eggs to the small ceratopsian Protoceratops, the most common dinosaur at the excavation site. When a skeleton of an unusual beaked theropod was found next to one nest, Andrews concluded it had been raiding the eggs, and the dinosaur was named Oviraptor, meaning "egg thief." This misidentification persisted for over 70 years until Mark Norell's team at the American Museum of Natural History discovered a specimen of Citipati, a close relative of Oviraptor, preserved in a bird-like brooding posture atop a clutch of elongated eggs at Ukhaa Tolgod, Mongolia, in 1993 (published in Norell et al. 1995, Nature 378: 774–776). The alleged Protoceratops eggs had belonged to oviraptorid theropods all along, transforming Oviraptor from a vilified egg thief into a symbol of dinosaur parental devotion.

A landmark discovery in 1978–1979 by Jack Horner and Robert Makela in Montana revealed the massive Maiasaura peeblesorum nesting colony, later known as "Egg Mountain." The site contained hundreds of nests with organized egg arrangements, trampled eggshell fragments, and juveniles at multiple growth stages, providing the first compelling evidence for colonial nesting and extended parental care in dinosaurs (Horner & Makela 1979, Nature 282: 296–298; Horner 1982, Nature 297: 675–676).

In 2012, Robert Reisz and colleagues reported the oldest known dinosaurian nesting site from the Early Jurassic (approximately 190 million years ago) of South Africa, where the sauropodomorph Massospondylus produced multiple egg clutches in at least four stratigraphic horizons within a restricted area. This find extended the record of colonial nesting and site fidelity back by more than 100 million years compared to previously known Late Cretaceous sites (Reisz et al. 2012, PNAS 109: 2428–2433).

Dinosaur nests can be categorized into three broad types based on the degree of egg burial and incubation strategy.

Buried nests were employed by early dinosaurs with soft-shelled eggs, as well as by some hard-shelled egg layers such as titanosaurian sauropods and hadrosaurs. The 2020 study by Norell and Wiemann demonstrated that the earliest dinosaur eggs were soft-shelled and leathery, similar to those of modern turtles and many lizards (Norell et al. 2020, Nature 583: 406–410). These eggs were highly susceptible to desiccation and required burial in moist sediment for incubation, relying on environmental heat sources such as decomposing vegetation or solar radiation. The vast Auca Mahuevo nesting site in Argentina, discovered in 1997, contained thousands of titanosaurian sauropod eggs spread across more than one square kilometer of Late Cretaceous floodplain sediments, with many eggs containing embryonic remains including preserved skin impressions (Chiappe et al. 1998, Nature 396: 258–261).

Mound nests are inferred for hadrosaurs such as Maiasaura, which are thought to have constructed elevated earthen structures mixed with plant material. The decomposition of vegetation would have generated metabolic heat to incubate the eggs, analogous to the nesting strategy of modern megapode birds or certain crocodilians. At Egg Mountain, nests were spaced approximately two meters apart, corresponding to the body length of an adult Maiasaura, suggesting organized spatial planning within the colony.

Partially open nests were used by derived theropods, particularly oviraptorids and troodontids. Oviraptorid egg clutches display a characteristic ring or spiral arrangement, with eggs partially buried in a shallow excavation while the upper portions remained exposed. In large oviraptorosaurs such as Gigantoraptor, the ring diameter could exceed two meters, with the central open space allowing the parent to sit without crushing the eggs. Troodontid nests from the Two Medicine Formation of Montana show eggs more tightly clustered toward the center, indicating that the brooding parent could cover the entire clutch with its body for direct contact incubation (Varricchio et al. 1997, Nature 385: 247–250).

The most dramatic evidence for dinosaur brooding comes from multiple oviraptorid specimens preserved in incubation postures. The famous Citipati osmolskae specimen (IGM 100/979, often called "Big Mama") shows an adult crouched over a ring of eggs with its forelimbs spread outward to cover the clutch, closely paralleling the posture of modern brooding birds. A second Citipati specimen (IGM 100/1004) in the same brooding posture was reported by Norell and colleagues in 2018, reinforcing the behavioral interpretation. However, the wide spacing between eggs suggests that oviraptorids avoided placing their full body weight on the eggs, instead using their feathered arms as insulating covers.

Troodontids represent a more advanced stage in the evolution of avian-style incubation. Their nest architecture, with eggs concentrated at the center, would have permitted full contact incubation using the brooding parent's ventral surface, a behavior functionally identical to that of most modern birds.

Jasmina Wiemann's geochemical analyses using mass spectroscopy identified the pigments protoporphyrin and biliverdin in oviraptorid and other theropod eggshells, enabling reconstruction of original egg colors. Heyuannia and Deinonychus laid blue-green eggs, while some troodontid eggs were brown or white with speckled patterns. Non-theropod dinosaurs (sauropods and hadrosaurs) showed no detectable eggshell pigments, indicating plain white eggs. The evolution of colored eggs coincides temporally with the appearance of partially open nests, suggesting that pigmentation served adaptive functions such as camouflage from predators or species recognition when eggs were no longer concealed underground.

Multiple fossil localities demonstrate that dinosaurs nested in large aggregations. At Egg Mountain, successive nest-bearing horizons separated by sedimentary layers indicate that Maiasaura herds returned to the same nesting grounds over multiple breeding seasons. The Massospondylus nesting site in South Africa preserves clutches in at least four distinct stratigraphic levels, providing the oldest evidence (approximately 190 million years ago) of both colonial nesting and site fidelity in any terrestrial vertebrate.

In 2019, a colonial nesting site of oviraptorids was reported from the Gobi Desert of Mongolia, with 15 nests and more than 50 eggs preserved in close proximity, representing one of the clearest examples of colonial nesting behavior in theropod dinosaurs. A 2021 study from Patagonia, Argentina, reported a massive Mussaurus patagonicus nesting site containing over 100 eggs and more than 80 skeletal specimens organized into age-segregated groups, demonstrating that complex social behaviors including herd structure and colonial breeding were present even in Early Jurassic sauropodomorphs.

Evidence for post-hatching parental care takes several forms. Trampled eggshell fragments in Maiasaura nests indicate that hatchlings remained in the nest for extended periods. Juveniles found together in nests ranged from approximately 30 centimeters (hatchling size) to nearly 90 centimeters in length, suggesting that young Maiasaura grew substantially while still receiving parental attention. The very name Maiasaura ("good mother lizard") reflects this interpretation.

A Protoceratops nest containing 15 juveniles without accompanying eggs was reported by Fastovsky and colleagues in 2011, indicating that the young remained together in their nesting burrow for weeks after hatching. In Psittacosaurus, a five-year-old subadult was found associated with 24 smaller juveniles, raising the possibility of cooperative care by older siblings rather than parents alone (Meng et al. 2004, Nature 431: 145–146).

A 2008 study by Varricchio and colleagues proposed that the large clutch sizes (22–30 eggs) found in oviraptorid and troodontid nests were consistent with paternal care systems, where a single male incubated eggs laid by multiple females. This reproductive strategy parallels that seen in modern ratites such as emus and rheas, where the male assumes sole responsibility for incubation and chick rearing.

Greg Erickson and colleagues developed a method to estimate dinosaur incubation periods by counting von Ebner lines—daily growth increments in embryonic tooth dentin. Their analysis of Protoceratops embryos yielded an estimated incubation period of approximately 83 days, while the large hadrosaur Hypacrosaurus required roughly 173 days (Erickson et al. 2017, PNAS 114: 540–545). These prolonged incubation times are more similar to those of modern reptiles than to the comparatively rapid incubation of birds. Troodontids showed an intermediate incubation period of approximately 74 days, falling between predicted values for birds and crocodilians of equivalent body mass, consistent with their intermediate phylogenetic position.

Recent fieldwork by Patrick Druckenmiller and colleagues in northern Alaska has revealed that dinosaurs bred at extreme polar latitudes exceeding 80 degrees north. Tiny bones and teeth of hatchling hadrosaurs (Ugrunaaluk), ceratopsians, and troodontids demonstrate that these dinosaurs nested and hatched their young in the Arctic rather than merely visiting during summer months. Given the approximately six-month incubation period estimated for Ugrunaaluk and the limited window between snowmelt and the onset of polar darkness (over 80 consecutive days without sunlight), Druckenmiller and Erickson concluded that the young were too small and the season too short for long-distance migration. Instead, parent dinosaurs likely tended their hatchlings through the polar winter, foraging on residual vegetation under conditions of continuous darkness (Druckenmiller et al. 2021, Current Biology 31: 3469–3478).

The 2020 discovery that the ancestral dinosaur egg was soft-shelled fundamentally reshaped understanding of nesting behavior evolution. Norell and Wiemann's phylogenetic analysis demonstrated that hard, calcified eggshells evolved independently at least three times within Dinosauria: once in theropods, once in sauropods, and once in ornithischians (specifically advanced hadrosaurs). Early dinosaurs were therefore restricted to moist nesting environments where soft eggs could survive without desiccating. The independent evolution of hard shells in multiple lineages unlocked the ability to nest in drier environments and to leave eggs partially exposed, a prerequisite for the development of contact incubation and, ultimately, the sophisticated brooding behaviors seen in modern birds.

This evolutionary trajectory can be summarized as follows: ancestral soft-shelled eggs in buried nests gave way to hard-shelled eggs that could tolerate exposure, which in turn enabled the emergence of mound nesting and, among theropods, partially open nests with direct brooding. The appearance of eggshell pigmentation in theropods represents a further adaptation to the visibility of exposed eggs, likely driven by selection pressures related to predation and possibly brood parasitism.

Dinosaur nesting behavior research has been transformative for understanding the origin of avian reproductive biology. Features once considered uniquely avian—feathered brooding, egg coloration, contact incubation, and extended parental care—are now known to have deep roots in non-avian theropod dinosaurs. At the same time, many dinosaurs exhibited decidedly non-avian reproductive strategies: burying soft-shelled eggs like sea turtles, enduring reptile-grade incubation periods, and potentially abandoning nests after egg deposition. This mosaic of traits underscores that dinosaur reproductive ecology was neither simply reptilian nor simply avian, but a diverse and complex array of strategies shaped by phylogeny, body size, environment, and ecology over more than 160 million years of evolution.