[Image above] Many species of cuckoo birds, such as the common cuckoo above, are brood parasitic birds, meaning they lay their eggs in the nests of others. Credit: Birds of Gilgit-Baltistan, Flickr (CC BY-SA 2.0)
Male cardinals are often instantly recognized at bird feeders thanks to their bright red plumage. But female cardinals can sometimes slip by unnoticed due to their unassuming pale brown feathers, which I only learned after a recent zoo outing with a friend.
Unlike me, my friend is one of the many people who got into birdwatching during the COVID-19 pandemic. This recreational activity surged in popularity as people looked to engage with nature after months spent trapped indoors.
While many of my friends shared pictures of rare or beautiful bird sightings on social media, some also shared their surprise at learning about the dark side of bird life, namely brood parasitic birds.
Brood parasitic birds will lay their eggs in the nests of others, sparing themselves the expense of rearing their own young. When the eggs hatch, the parasitic chicks will compete with the host babies for food and nest space.
Host birds have developed various defense strategies against brood parasites, which the brood parasites then evolve to counter, as explained on the webpage of Georgia-based natural history museum Fernbank Science Center. For example, since at least the late 1800s, scientists have reported that parasitic eggshells are often stronger than the host eggs to prevent host birds from puncturing them in rejection.
To date, scientists have typically inferred eggshell strength based on the shell’s thickness. But these inferences “underestimate and trivialize the complex eggshell ultra- and microstructure and its specific functional performances,” researchers write in a recent open-access paper.
The researchers come from several institutions and universities in Argentina, China, the United States, the Czech Republic, and the Republic of Korea. In their paper, they used imaging techniques and conceptual frameworks from the field of grain boundary engineering to understand how structural features may affect eggshell strength, as eggshells are bioceramic materials (mainly calcium carbonate).
The researchers determined that eggshells featuring longer and more complex grain boundary paths were stronger and tougher than eggshells of similar thickness without those characteristics.
Not all parasitic eggshells demonstrated this extra strengthening mechanism, however. It depended on the specific parasitic species and host pair, which determined the likelihood of eggshell puncturing being used as a defense strategy.
While the study investigated the eggshells of a wide range of brood parasitic birds, only a few specimens per species were available. So, “A few more replicative studies will be very useful to test the reproducibility of this research,” the researchers write. They included data on each eggshell within the paper that other groups can use for comparison.
The open-access paper, published in iScience, is “Avian obligate brood parasitic lineages evolved variable complex polycrystalline structures to build tougher eggshells” (DOI: 10.1016/j.isci.2023.108552).