Extreme Bird Eggs pt. 2
July 19, 2023
Welcome back to the second installment of our exploration of the most extreme bird eggs out there! For those of you who didn’t read the first part, you can read it here if interested, though it won’t be terribly crucial to anything presented here. Last time, I focused on eggs that were big, small, or strangely shaped, but today, the focus will be on other unusual features of some birds, particularly features that would seem to make the eggs easier for predators to detect, a strong disadvantage for sure.
The three living species of cassowary (Casuarius) probably need little introduction for most readers. A bird shrouded in mystery surrounding aspects from their taxonomy to their diet, one of the most poorly-understood aspects of the cassowary is their reproduction. As per usual, the information here can apply to cassowaries in general, with many known and probably many unknown differences between the species.
A handsome southern cassowary (Casuarius casuarius) that I saw for a few minutes at the National Zoo in Washington, D.C. Image by Donovan Snell.
Cassowaries are quite well-known for their bright green eggs (though, unfortunately, no green ham is included). As stated, relatively little is known about the mating and nesting habits of the cassowary, but it is known that the male is the primary caretaker of the young. The female, as opposed to the male, has no defined territory and is highly polygamous, mating with several males. After mating and laying a clutch of three to eight eggs, she will move on to the next and repeat the whole process. The eggs can range in color from bright, vivid green to a more pale cyan and are laid in a nest constructed by the male out of fallen foliage.
The father incubates the clutch for around two months and raises the chicks for a further nine months. Even in the rainbow of hues that bird eggs can be, the cassowary’s bright green eggs seem to stand out. The cassowary certainly did not evolve in a predator-free environment like the previously-mentioned kiwi, living alongside several large snakes and monitor lizards that could use these large eggs as an easy food source. Sadly, it seems to be something that no one yet has studied much about. The classic explanation is that it simply helps conceal the eggs in green, jungle foliage. However, considering that few other ground-nesting birds have camouflaged their eggs in such a manner, combined with the fact that the cassowary uses darker, duller leaf litter to construct their nest, I do not think this is an adequate explanation. There is likely more going on. It’s possible that it may have something to do with the several potential properties of the chemical biliverdin, a common component in green and blue eggshells and worthy of its own entire article. For now, however, the cassowary’s eggs are quite a mystery.
A male cassowary named “Rocky” with a clutch of eggs at the Australia Zoo. Note that, even though the bird is in captivity, the eggs still seem oddly un-camouflaged. Image by James R. Biggs of ResearchGate.
Continuing with the unusual world of ratite reproduction (tinamous have, in recent years, been shown to be deeply nested within the ratite clade instead of being its sister clade, as previously thought), the 46 or so species of tinamous, spread across nine genera, have all kinds of variations in their reproductive habits, but with the strange commonality that their eggs have varying degrees of glossiness, which, in most species, makes their eggs appear very shiny.
Who doesn’t love tinamous? Though the birds themselves, such as the great tinamou (Tinamus major) at left and, at right, the spotted nothura (Nothura maculosa), may be rather dull-colored, this is more than made up for by their eggs. Left image by Tony Castro on Wikimedia Commons and right image by Dario Sanches on Wikimedia Commons.
Across tinamous as a whole, they have a reproductive strategy similar to the cassowary, with the polygamous female mating and producing clutches with several males per breeding season. The male, as a result, is the sole provider for the offspring. There are several exceptions and nuances to this across tinamous, such as the fascinating case of the spotted nothura (Nothura maculosa), which is monogamous when young but becomes more polygamous with age in both sexes.
All tinamous lay their eggs on the ground with varying degrees of effort put in by the male for constructing the nest. Some tinamou species don’t make much more than a scrape in the ground whereas others construct meticulous creations. In all species, the egg is proportionally rather large, with an increase in proportional size in the smaller species, and clutch sizes are typically large and have eggs from multiple females, with sometimes as many as 16 eggs. The shells of these eggs are strangely thin for ratites, who typically have tough, thick eggshells due to being heavy ground-nesters. It’s even possible to see the developing embryo inside of the egg if there is good lighting. However, the strangest aspect of these eggs is the fact that, as stated, they are typically very vivid in color and, more remarkably, have a very glossy appearance that is often compared to porcelain.
The eggs become less colorful and lose their gloss over time, usually becoming completely dull by the time the chick hatches. Having an egg that is so poor at blending in seems unusual to say the least, especially for a bird that lays their eggs on the ground. However, this may be of little concern to the tinamou, as most of their potential predators are nocturnal and wouldn’t be able to detect these eggs based on vision anyway. Still, that doesn’t explain why they have such conspicuous eggs in the first place. An early hypothesis was that this was actually the point; having these glossy, easily visible eggs may help the male find his eggs in the dense, dark foliage of the rainforest.
This early proposal is not accepted very much anymore and has been replaced with all kinds of interesting ideas. A popular explanation, put forward by Harley et al., is what has been coined the “blackmail hypothesis.” Though tinamous are certainly an extreme example, they are certainly not the only birds with strangely bright and colorful eggs, with the trait having evolved independently across several different lineages. One odd trend for all these birds, however, is that the parents tend to be dull and much more camouflaged. Therefore, the nest is most hidden from predators when the parent is on top of it instead of when the parent is off. Therefore, by having bright eggs, the incubating parent is coerced into spending more time on the eggs to keep them more protected from being eaten. The benefit of this is that, since the parent is spending more time on the eggs, the eggs are receiving more heat which, in theory, should increase the speed of their incubation. A very interesting idea!
The luster of these red-winged tinamou (Rhynchotus rufescens) eggs at the Delaware Museum of Natural History is no joke. Sure, it makes the eggs beautiful, but it also raises more questions than answers. Image by David Stang on Wikimedia Commons.
Another explanation for these eggs could relate to the reproductive habits of most tinamou species, with multiple females laying eggs in the clutch of a single male. Patricia Brennan has come up with the idea that the glossiness of the eggs serves as a visual cue to females looking for a male to mate with, or, more importantly, his nest to lay in. Brennan has stated that the female tinamou will be more likely to lay her eggs in a nest that already has brighter and glossier eggs as opposed to one with duller eggs. The reasoning behind this is that, since the eggs become duller over time, the female will want to lay her eggs in a nest with more recently laid eggs in order to make it so that all of the eggs in the nest, including hers, will hatch at around the same time. If all the eggs hatch at the same time, that means that all the chicks will receive the same degree of care from the father. Brennan also collected some data that contradicts the blackmail hypothesis. It was observed that predators stole eggs most often when the male was incubating the nest instead of when the nest was left unoccupied, which goes against the idea that the male is coerced into brooding in order to keep the eggs protected. As stated, most of the tinamou’s predators are nocturnal, and non-visual signals such as the sounds and smells of the brooding father may be more of a risk factor than the appearance of the eggs.
The Eurasian hoopoe (Upupa epops) is a classic European bird, though they are also found in many parts of Asia and Northern Africa, their range extending into most of Africa and even into Madagascar depending on if you consider the three species of hoopoe truly separate species. That aside, they are a comparatively rather well-studied bird with an unusual reproductive defense that was only fully understood relatively recently.
The hoopoe has long been associated with military prowess and authority. But beyond their regal appearance lies a pretty gross and odorous defense. Image by Martin Mecnarowski on Wikimedia Commons.
The hoopoe’s mating habits are nothing too out of the ordinary. Seasonally monogamous, hoopoes are capable of making nests in a variety of places but typically prefer cavity nests in large trees or river banks. Clutch size is highly variable, anywhere between four and twelve, which is not uncommon for birds with large ranges. Hoopoes in tropical climates tend to have bigger clutches. The female is the primary incubator, which takes usually between 15 and 18 days. This is a fairly short incubation period, during which the father provides food for the brooding mother as well as for the chicks after they hatch.
Hoopoes earn their extreme position due to something that is not initially obvious in photos but is supposedly unmistakable in person. Since ancient Greek and Roman writings, the hoopoe has been renowned for the terrible odor of their nests, with the nest becoming more “evil-smelling” (as The Breeding Birds of Europe puts it) the longer the female incubates her eggs. In many older sources, this is explained as simply the result of an accumulation of the mother’s feces around the nest. However, the nests of all kinds of birds can have large accumulations of guano around them, and while certainly not pleasant, they certainly don’t have the reputation that the hoopoe has. More must be going on here.
A mother hoopoe feeding her chick. This nest was constructed in a stone wall near a road, showcasing the hoopoe’s surprising adaptability, especially for a larger bird. Apparently, we should all be thankful that we can’t smell this photo. Image by João Manuel Lemos Lima on Wikimedia Commons.
Around the 2000s, it was understood that this foul odor is actually the result of a secretion produced by the mother’s uropygial gland. This gland sits at the base of the tail and, in most birds, produces oils that are important for preening. However, in the hoopoe, these oils have been modified to produce an odorous substance. During incubation, the mother’s uropygial gland enlarges dramatically. Soon after laying her eggs, she will begin carefully dropping the liquid onto her eggs, which stains the normally pale blue shells of the eggs, turning them greenish-gray. The chicks also hatch with an enlarged uropygial gland and are capable of producing stinky oil soon after hatching. The standard reasoning behind the smell, typically described as smelling like rotten meat or cheese, is that it deters predators from attacking the eggs or chicks. However, there may be more to the story than that. The foul odor of the hoopoe’s oil comes from a symbiotic relationship the bird has with certain microbes that live in their uropygial gland and are present in the secretion. There’s some evidence suggesting that these microbes can kill harmful bacteria, which helps protect the eggs from infection. The smell may not even be the point and could be just a side effect. There’s even some evidence suggesting that the male hoopoe will provide more food for his mate if the eggs are more stained, which would imply that the stained condition has some kind of important benefit.
There’s always more to say about the amazing reproductive lives of birds, with many ideas for future articles cropping up in my mind as I was researching and writing this article. Expect more in the future!
Sources
Davies, S. J. J. F. Grzimek's Animal Life Encyclopedia (2nd ed.). Farmington Hills, 2008.
Hayeck, Hattie. “Everything you need to know about the Southern Cassowary.” BBC Wildlife, accessed July 3, 2023. https://www.discoverwildlife.com/animal-facts/birds/facts-about-southern-cassowary/
Khanna, D. R. and P. R. Yadav. Biology of Birds. Discovery Publishing House, 2005. https://books.google.com/books?id=fDblIChi7KwC&pg=PA130#v=onepage&q&f=false
Cabot. J, C. Carboneras, A. Folch, E. de Juanaca, F. Llimona, and E. Matheu. Handbook of the Birds of the World. Vol 1: Ostrich to Ducks. Lynx Edicions, 1992.
Brennan, Patricia L. R. “Clutch predation in great tinamous Tinamus major and implications for the evolution of egg color.” Journal of Avian Biology, 2010. https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1600-048X.2010.04999.x
Hanley, David, Stéphanie M. Doucet, and Donald C. Dearborn. “Blackmail Hypothesis for the Evolution of Conspicuous Egg Coloration in Birds.” The Auk, 2010. https://academic.oup.com/auk/article/127/2/453/5148444
Pforr, Manfred and Alfred Limbrunner. The Breeding Birds of Europe. Croom Helm, 1982. https://archive.org/details/breedingbirdsofe02pfor/page/n7/mode/2up
Perrins, Christopher. Firefly Encyclopedia of Birds. Firefly Books, 2003. https://archive.org/details/fireflyencyclope0000unse/page/n7/mode/2up
Soler, Juan J., M. Martín-Vivaldi, J. M. Peralta-Sánchez, L. Arco, and N. Juárez-García-Pelayo. “Hoopoes color their eggs with antimicrobial uropygial secretions.” Naturwissenschaften, 2014. https://link.springer.com/article/10.1007/s00114-014-1201-3#citeas