(Cross-posted to my tumblr and blog: to see the original version with embedded images, click here.)
As a lot of you will already know by the time of reading this, a recent publication in the journal Evolution demonstrates that the four-winged, tiny dromaeosaur Microraptor was at least occasionally in the habit of ingesting fish. Scales and portions of several disarticulated skeletons of the osteoglossiform Jinanichthys were found fossilized in the specimen's gut. The authors do not discount the possibility that the fish were scavenged, but considering other known habits of the four-winged wonder, it seems at least reasonably likely that it caught the fish itself - and unlike previous known gut contents, it couldn't have taken this meal in the trees (though it's hard to know what the nay-sayers will claim next...).
This newest study is another piece in the puzzle of Microraptor's ecology - a puzzle which is looking more complete as time goes by. With over 300 undescribed specimens, at least three perserved meals (all different), a color study, conflicting scleral ring studies, and a myriad of biomechanical research, we now know a tremendous about this animal (at least for being dead 120 million years).
But can we extract a reasonable approximation of its diet and lifestyle from the available information? To answer that, one option is to look towards modern birds. But first, let's look at what we do know.
We know that Microraptor ate birds, and unlike the recent piscivory paper, the bird skeleton was found whole and partially-articulated in the dromaeosaur's stomach, indicating that it may have been eaten whole. Dead things are not often scavenged whole, so it seems fairly probable that this was a clear predation event (though it may or may not have occurred in the trees - as many people have since pointed out, predation on arboreal animals does not arboreal make).
We also know, from a 2010 SVP abstract by Larsson et al, that a specimen of Microraptor with evidence of a mammal dinner has also been found. This specimen preserves fragments of bone, including pieces of skull and a whole foot, that most closely resemble the semi-cursorial mammals Eomaia or Sinodelphys.
Taken the dietary information that we know so far, it seems that Microraptor was not particularly specialized to be preying on any one sort of prey animal, but was more of a generalist. Xing et al argue that the reduced serrations and mildly forward-pointing teeth on the anterior dentary indicate some minimal adaptations to a piscivorous lifestyle, but it clearly lacks the more pronounced piscivorous adaptations that we can see on animals like spinosaurids and crocodylians, not to mention a myriad of modern piscivorous birds. Microraptor was probably eating a lot of different things, likely just about anything it could get its teeth on.
As most of you already know, Microraptor is also now known with high probability to be a uniform iridescent black, possibly with a glossy blue or green sheen. The authors argue that the glossy black coloration was likely used as a method of communication (either social or sexual).
And here's where it gets interesting. Among modern avialans, we can construct a sort of Venn diagram between birds that are glossy black and birds that are generalist predators. And within the center of this diagram, we get birds that have a few pretty universal traits.
1. Sociality: A glossy black coloration is definitely used as a communication device in modern birds, since it tends to be the best way to spot a member of the same species across a wide distance. Many birds that congregate in large flocks tend to be black, and Microraptor is known from an incredible number of specimens - this could be preservation luck, but it seems more like that there really were just a damned lot of the things. This in itself isn't evidence for sociality, but taken together with the rest, it certainly seems pretty plausible.
2. Omnivory: Perhaps unsurprisingly, the vast majority of social birds that are generalist predators are also semi-omnivorous. If there are a damned lot of you, and you're traveling together, you're going to want to learn to eat just about anything that you find. It makes more sense from the perspective of the ecosystem, too: a large group of an animal that eats only a specific type or size of other animals will easily deplete the prey populations when they pass through. Reduced serrations on the teeth (typically indicative of a predatory lifestyle: the serrations help the animal to "hold on" and tear the flesh of prey) are also consistent with a certain degree of omnivory. I'd posit that amongst those 300 undescribed specimens, at least a couple of them might preserve evidence of past meals of seeds or insects.
3. Intelligence: Yes, I know, the "hyperintelligent raptor" trope is incredibly old and boring, and it's probably obvious that Microraptor is nowhere near as smart as the modern world's smartest birds - some of which are also glossy black omnivores - like corvids. The fact remains, though, that a combination of social behavior and omnivory often results in (or from) an intelligence level that's greater than that of its ancestors. One working theory of how this tends to go is that a population descended from a largely herbivorous ancestor will begin incorporating more meat into its diet to facilitate the protein needed to aid in brain growth. Omnivory also might somewhat result from greater intelligence, as an animal with more smarts will capitalize on its ability to figure out more things to eat.
So, taken these three factors, what's a good modern analogue for the Microraptor?
Meet... the great-tailed grackle.
Or, rather, grackles in general, and even more generally, icterids as a family. I personally find them to be GREAT analogues to Microraptor for a variety of reasons.
Icteridae, the blackbird family, contains a variety of passerine birds including new world orioles, grackles, meadowlarks, and blackbirds. They have a very interesting variety of plumage: most meadowlarks and orioles are very brightly-colored, often sporting extravagant yellow and orange plumage. Others, like most grackles, are entirely black and very glossy. Looking over the different groups, an interesting pattern emerges: the more brightly-colored icterids, like most orioles and meadowlarks, tend to be largely insectivorous, supplementing their diets with fruit and nectar. This would make sense, since the carotenoids needed to produce colorful plumage are found almost exclusively in fruits and the insects that eat them. Colorful orioles also tend to be largely solitary during most months, forming seasonal pair-bonds and sometimes migrating in loose flocks.
Grackles, on the other hand - and many other black glossy blackbirds - are much more social animals, foraging in sprawling, noisy flocks that utilize large numbers to detect predators, which they often "mob" by way of defense. Grackles are also highly omnivorous birds, and will eat a variety of seeds and plant matter in addition to frogs, minnows, small fish, eggs, and insects. Grackles are also fairly (though not extraordinarily) intelligent, and can mimic a wide variety of sounds and calls.
This correlation between color, sociality and diet in icterids definitely isn't perfect, but it seems far too strong to be a coincidence. And indeed, if you look to other glossy black birds around the world, you'll likewise find a significant degree of omnnivory and social behavior in those animals: corvids and starlings are other prime examples. Among corvids, the fully glossy black ones (like crows and ravens) tend, on average, to be more omnivorous (and also more intelligent) than the brightly-colored ones (like jays).
I do not know with certainty how universal this correlation is, or indeed, whether it's been explored to any significant degree in the literature. Social recognition and all-black coloration in birds, at least, is a fairly well-established link; the link between a partially frugivorous diet and colorful plumage is as well. As far as Microraptor is concerned, though, an interpretation of the animal as a glossy-black, social, semi-intelligent, opportunistic omnivore seems currently well-supported by the available information.
What this means, ultimately, is that more Microraptor specimens need to be analyzed, hopefully culminating in an eventual Microraptor monograph! There is still so much to be learned from our beautiful little glossy biplane.
Larsson, Hans, Hone, David, Dececchi, T. Alexander, Sullivan, Corwin, Xu, Xing. "The winged non-avian dinosaur Microraptor fed on mammals: the implications for the Jehol biota ecosystem" "Program and Abstracts. 70th Anniversary Meeting Society of Vertebrate Paleontology October 2010" 114A.
Jingmai O'Connor, Zhonghe Zhou, and Xing Xu (2011). "Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds". Proceedings of the National Academy of Sciences of the United States of America 108 (49): 19662–19665.
Lida Xing et al. (2013). "Piscivory in the feathered dinosaur Microraptor". Evolution (early view). doi:10.1111/evo.12119.
Li, Quanguo (9 March 2012), "Reconstruction of Microraptor and the Evolution of Iridescent Plumage", Science 335: 1215–1219, doi:10.1126/science.1213780