Let’s Hope She Doesn’t Have Twins! (A Guest Post)

By Eric VanNatta

Of all the oddball bird species in our world, the brown kiwi surly waddles in amongst the flock. Found only in the forests of New Zealand, this small flightless bird belongs to an ancient group of birds called the ratites. Joined by ostriches, emus, cassowaries and rheas, the ratites are all flightless and dressed in shaggy feathers. In addition, the ratites have all been linked to a common ancestor (simply referred to as the ratite) that was isolated after earth’s continents shifted apart some 300 million years ago.

A size comparison of the moa and kiwi.
Drawing by Josef Korenski (around 1901)
at Wikimedia Commons.

Originally found throughout the single landmass, future generations of ratites living in the places we now call South America, Africa and New Zealand experienced changing climates and new habitat types. Depending on individual traits, certain birds had better or worse success based upon their ability to survive and reproduce. Most of these species developed longer legs used for running and lost their large wings required for flight, as we can clearly see today in the ostrich and emu. In the islands of New Zealand, the ratite developed into a similar group of species we know as the moas. Similar to the emu and ostrich, the moa was a large bird with powerful legs used for running. Eventually, several million years later, the moa, too, continued to take advantage of different habitats within the island in the absence of mammalian predators. Of the handful of new species that the moa gave rise to, one of them actually began to shrink back down in size; we know it as the brown kiwi. Although moas and kiwis were both exceptional at surviving on the island, humans later drove moas to extinction through over hunting during early island colonization.

However, the unique ancestry of brown kiwis is not the only thing that granted them an oddball award. The size of their eggs and their properties is something that has inspired curiosity in us ever since humans discovered the island several hundred years ago.

Size comparison between an kiwi and its egg.
Photo taken by Hannes Grobe at the Kauri Museum in
New Zealand. Available at Wikimedia Commons.

For a bird that can weigh in between 1.5 and 3.3 kilograms (3-7 pounds), brown kiwis’ eggs are a whopping 0.4 kg (almost a pound)! Due to the tremendous size of the eggs, a female kiwi only has room for one egg at a time. On the extreme end, up to 20% of a female’s mass before laying her egg is from her egg. That’s equivalent to a human carrying a 30-pound baby before birth!

So why the heck do kiwis have such massive eggs? What good could this possibly be? Those are two questions William Calder III set out to ask in his research on kiwi eggs.

Before he started answering questions, he compiled many of the characteristics of kiwis and their eggs. In order to look at these differences, he compared kiwis to other bird species of similar mass (seabirds, chickens, etc.), and he compared kiwi eggs to similar sized eggs from other species (emus).


Size comparison between ostrich,
emu, kiwi, and chicken eggs. Photo
by Zureks at Wikimedia Commons.

To start the laundry list of unique observations, the yolk itself is as large as that of an emu, the equivalent of about 11 chicken yolks. This gives a developing chick plenty of nourishment, and upon hatching it still has excess yolk to last 10 days without the need to forage for food. Incubation time for a kiwi egg takes 75-84 days, which is double the amount of time as comparably sized eggs. This has been hypothesized to be a result of lowered incubation temperature, since kiwis have a lower metabolism and body temperature compared to other birds, but there has been no formal investigation of this. The eggs themselves also appear to have an excess of antifungal and antimicrobial properties to endure the 3-month incubation period.

After comparing these characteristics and taking into account their unique ancestry, Calder supported the scientific understanding that kiwis’ large eggs are simply relics from their past. Generations of the moa likely decreased in size after smaller individuals took advantage of eating small prey and living in forest understory habitats. Although these resources allowed for a change to smaller physical size, there was no reason for their eggs to reduce their size. In fact, fossil collections have shown that kiwi eggs are nearly the same size as those from the 12-kilogram (26 pound) moa. Kiwis historically never had to worry about nest predators entering their burrows since there were none on the islands. Their only predators were large flying birds, so it was advantageous to keep chicks in eggs for a longer amount of time until they were more developed. The large yolk reserve also allows chicks to stay hidden during their first days of exploration, and not have to worry about eating.

Talk about an odd reproductive system and a unique lineage! Who knows, maybe future environments will present opportunities for kiwis to increase their number of offspring. As human development encroaches valuable forest ecosystems, it would be beneficial to increase the odds of the species’ survival. Surely any chance of this will take thousands of years of environmental opportunities, as have the changes from their ancestors, but it wouldn’t be the first time the bird has surprised us!


References: Calder, W. (1979). The Kiwi and Egg Design: Evolution as a Package Deal BioScience, 29 (8), 461-467 DOI: 10.2307/1307538