The avian eggshell, a marvel of nature’s design, functions far beyond simply enclosing the life within. This complex bioceramic structure, primarily composed of calcium carbonate, serves not only as a sanctuary for embryonic development but also as a formidable barrier against external threats. Remarkably, it balances the delicate interplay between strength and the necessity for the emerging chick to break free. The fascinating evolutionary dance of avian brood parasitism, where some species cunningly lay their eggs in the nests of others, showcases nature’s ingenuity. These parasitic birds, having forsaken the role of nurturing their offspring, have instead developed eggs that not only deceive host species through mimicry but are also incredibly resilient. Such evolutionary innovations highlight the intricate ways life adapts to thrive, drawing us into a deeper appreciation of the natural world’s complexities.

A team of scientists, publishing in iScience, has unveiled a fascinating evolutionary strategy among obligate brood parasitic birds, which lay their eggs in the nests of other bird species (the hosts) to ensure their survival. This remarkable research effort was led by Dr. Analía López from the University of Buenos Aires and Dr. Raúl Bolmaro from the Rosario Physics Institute, Argentina. The interdisciplinary research team also included Dr. Seung Choi from the Chinese Academy of Sciences, Dr. Yong Park from Seoul National University, Dr. Daniel Hanley from George Mason University, Jin-Won Lee from Kyung Hee University, and Dr. Marcel Honza from the Czech Academy of Sciences. Together, they have shed light on how certain birds have developed incredibly tough and strong eggshells to outmaneuver predators and competitive hosts, a discovery that challenges previous understandings and opens new pathways for biomimicry in man-made materials.

The scientists’ findings point to  intricate adaptative shifts within the eggshell’s outermost layer (palisade layer), where complex grain boundary (GB) microstructures, rather than mere thickness, enhance mechanical and functional performaces. “The intricacy of GB microstructures within the eggshell’s palisade layer significantly increases its resistance to breakage,” Dr. Bolmaro explains, illuminating the sophisticated “GB Engineering” process. These adaptative shifts  underscore a critical evolutionary edge against egg destruction strategies employed by host birds, illustrating a nuanced arms race played out at the microscopic level of eggshell structures.

The team’s comprehensive egg collection spanned North America, Europe, and East Asia, involving a careful examination of various brood parasite-host systems. “Our methodological precision, from egg collection of relevant species to the detailed electron backscatter diffraction (EBSD) analysis, allowed us to meticulously map the GB networks and crystalline orientations,” Dr. Bolmaro shares, highlighting the depth of their examination. This approach unveiled the micro- and ultrastructural patterns contributing to the eggshells’ increased toughness and strength, offering a new lens through which to understand avian evolutionary strategies.

While discussing their methodology to obtain EBSD data, the researchers specifically selected a suit of state-of-the-art statistical methods that minimize the effects of non-independence between species due to their common ancestry. “This phylogenetic comparative statistical framework was crucial to unequivocally determine the microstructural differences between brood parasites’ and their hosts’ eggshells, unpacking the evolutionary advancements of these avian species,” Dr. López adds, emphasizing the rigorous analysis that underpinned their findings.

This study does not merely enrich our comprehension of avian evolutionary dynamics but also suggests intriguing implications for the field of materials science. The principles underlying the natural engineering of these eggshells could inspire the development of new materials that emulate their resilience and toughness, blending nature’s wisdom with human ingenuity.

Through the collaborative efforts of Dr. López, and her colleagues, not only has a fascinating aspect of avian life been highlighted, but the way has also been paved for future innovations in material design and engineering. Their work stands as a testament to the enduring power of cross-disciplinary research in uncovering the secrets of the natural world. Figures:

Image 1. Left. Eggs of the chalk-browed mockingbird (Mimus saturninus) punctured by its parasite, the shiny cowbird (Molothrus bonariensis). Force versus displacement curves recorded during puncture tests on eggs of the screaming cowbird (Molothrus rufoaxillaris) and two hosts, the grayish baywing (Agelaioides badius) and Brown-and-yellow marshbird (Pseudoleistes virescens). These two parasitic cowbird species (from South America), during the nest visitation and egg-laying events, frequently peck and puncture other eggs already laid in the nest, including other parasites’ eggs. Right. Eggs of the brown-headed cowbird (Molothrus ater) and its host, the red-winged blackbird (Agelaius phoeniceus). Eggs of the common cuckoo (Cuculus canorus) and its host, the great reed warbler (Acrocephalus arundinaceus). Brown-headed cowbird  (from North America) typically removes just one host egg when it lays its own, by either grasp or puncture ejection. Overall, the frequency of egg-puncture behavior by this cowbird is much lower than that performed by the other two congeneric cowbirds. Parasitic cuckoos do not puncture eggs already laid in the nests of their hosts. However, cuckoos usually remove (by grasp-ejecting) and consume 1–2 host eggs before laying their own egg. Instead, most cuckoo hosts have beaks that are too small to grasp-ejecting the parasitic eggs, therefore they often peck and puncture eggs before ejecting them from the nest (puncture-ejections). Center. EBSD maps of eggshell cross-sections for common cuckoos (Cuculus canorus) and its host, the great reed warbler (Acrocephalus arundinaceus). Parasitic cuckoo (and both parasitic egg-puncturing cowbirds from South America) shows higher density boundaries between crystals (or grains), forming a complex network of paths.
Image 2. Eastern Phoebe (Sayornis phoebe) nest with three host white eggs and two brown-headed cowbird (Molothrus ater) eggs. Photo credit: Analía V. López (New York State, USA).
Image 3. House wren (Troglodytes aedon musculus) nest with one shiny cowbird (Molothrus bonariensis) egg. Photo credit: Analía V. López (Buenos Aires province, Argentina).


Analía V. López, Seung Choi, Yong Park, Daniel Hanley, Jin-Won Lee, Marcel Honza, Raúl E. Bolmaro, “Avian obligate brood parasitic lineages evolved variable complex polycrystalline structures to build tougher eggshells”, iScience, December 15, 2023.



Edited graph from: López AV, Bolmaro R E, Ávalos M, Gerschenson LN, Reboreda JC, Fiorini VD, Tartalini V, Risso P, Hauber ME. (2021) How to build a puncture- and breakage-resistant eggshell? Mechanical and structural analyses of avian brood parasites and their hosts. J. Exp. Biol. 224, jeb243016. doi: 10.1242/jeb.243016

López AV, Choi S, Park Y, Hanley D, Lee JW, Honza M, Bolmaro RE. (2023). Avian obligate brood parasitic lineages evolved variable complex polycrystalline structures to build tougher eggshells. iScience 26:108552. doi: 10.1016/j.isci.2023.108552


Dr. Raúl E. Bolmaro – Physicist. He earned his PhD in Physics in La Plata University, Argentina (1987). He spent several research stays in Los Alamos National Laboratory (USA) and DESY, Deutches Elektronen Syncrotron and Helmholtz-Zentrum Geesthacht-Centre for Materials and Coastal Research, Geesthacht, Germany. His research epertisse embrace multiscope materials science techniques, such as texture of materials, x-ray, neutron and synchrotron radiation and EBSD. His main achievements span all over different metals, alloys, biomaterials, ceramics, etc. He is currently Director of the Physics and Micromechanics of Heterogeneous Materials Group, Professor and Senior Resarcher in Instituto de Física Rosario, CONICET, Argentina.

Dr. Analía V. López – Biologist. She earned her PhD in Biology (2021) from the University of Buenos Aires in Argentina. Her primary research has focused on the study of the structural and mechanical features of eggshell material in avian systems of obligate brood parasites and hosts, under a coevolutionary “arms-race” context. Different systems vary in their eggshell pecking destructive behaviors, and such behaviors would act as selection pressure mechanisms that shape the differential evolution of egg phenotypes. To explore issues of behavioral and reproductive ecology in specific parasite–host systems, as well as to investigate evolutionary convergences between independent lineages of obligate brood parasites worldwide, her research work has required a multidisciplinary approach. She is currently collaborating with researchers from Taiwan and Argentina, who are internationally recognized for their contributions in mechanical and (bio)materials engineering. Her work combines a wide array of field behavioral studies, engineering techniques and mathematical modeling, and state-of-the-art Phylogenetic Comparative methods.