Ancient bird species found in China's Liaoning: Extra-long tail feathers for elaborate courtship – interview with university of Chicago PhD candidate Alexander D. Clark

What did you choose to investigate in this study, and why?
My background is predominantly in extant bird behavior and anatomy. So, to be honest, the length and completeness of the tail feathers of Plumadraco drew me to it immediately. In the study, the first thing we did was to determine if this specimen was indeed a new species – and it was.

Plumadraco belongs to a clade of Mesozoic birds called enantiornithines, and more specifically, to the family/clade Bohaiornithidae. With this new species came in-depth descriptions of the skeleton and the preserved soft tissues, of which Plumadraco has many. The specimen preserves body feathers (head to ankles), wing feathers (remiges), and those incredible elongate tail feathers (rectrices). I chose to put more descriptive emphasis on the tail feathers too since one is entirely complete, which is very rare for fossil birds.

What are the key findings from your research?
I think the most interesting findings are 1) that the length of this species’ tail feathers is a new record for any enantiornithine (the most diverse group of birds during the Mesozoic), and 2) the structure of the distal rackets themselves.

With this specimen, we have a pretty strong argument that not only did probable males try to attract females with feather ornamentation, but they also did so with ridiculous lengths and probably displays. This can be inferred from both feather structure and muscle tissue traces from other enantiornithine genera.

The feathers structures change throughout their length, and these differences would result in varying levels of swaying or flickering should the feathers be moved. The tail feather’s rackets are distally enfeebled, meaning that less than halfway into each racket, the rachis (central support structure) completely ceases. You can see similar patterns in birds today. A classic example are the rump coverts (not tail feathers!) of a male Indian peafowl (peacock). When these feathers are shaken, they “rattle” or “flicker” because of the enfeebled nature of their structure.

It means that 121 million years ago, this male Plumadraco was dragging along a train of tail feathers twice its body length, just to impress potential mates – something we see today in modern birds as well. Pretty cool, if you ask me!

Tell us why you called the specimen Plumadraco bankoorum - what does the name mean?
Plumadraco is simply Latin for “feather” and “dragon”. A toothed, clawed, flying animal with wings sounds descriptive of a dragon to me – so the genus name was fitting.

“bankoorum” is in honor of Winston E. and Paul C. Banko. The former being the father of the latter. I worked with Paul as a wildlife biologist in Hawaii years ago, and we keep in frequent contact and write manuscripts with one another.

Win and Paul both worked for decades as ornithologist and conservation biologists across the Hawaiian archipelago and beyond. Paul and his father were instrumental in forming our understanding of Hawaiian bird ecology, behavior, and conservation, and their work ethic and kindness as humans inspires me to this day. It was nothing short of fitting to name this new bird in honor of the Bankos – both for their lifetime of work as influential ornithologists, and the impact they’ve had on budding biologists like me.  

You suggest that the specimen’s exceptionally long tail feathers may have been used in courtship displays – can you tell us more about what these displays may have looked like?
It’s very hard to say for sure, but the evidence we do have suggests something along the lines of a raising and lowering and perhaps a prolonged raising of the feathers. The feathers themselves suggest differential, or ”eye-catching”, levels of movement if moved up and down. These pumping actions are something we see in the courtship displays of living birds with elongate or ornamented tails.

These up-and-down movements are common across many bird groups for communication, but when accompanied by tail feathers that nearly exceed twice the body length, it’s always for courtship purposes, not necessarily communication (not explicitly to entice mating rights). Additionally, other enantiornithine specimens suggest muscles along the pelvis and tail which raise and lower the tail. There’s currently no strong evidence for lateral rectricial (tail) muscles which would wag the tail side to side.

So, given the length, morphology of the distal ornaments and muscle tissue data from other enantiornithines, it seems plausible to suggest that Plumadraco’s display would either be an up-and-down bobbing of the tail (really exaggerating that length and the rackets at the end), or it would maintain a raised posture to again, show off the length and distal ornaments. 

Describe the type of environment Plumadraco bankoorum may have lived in – how do you think this would have informed its development? What type of pressures would it have encountered?
We know that the area in which Plumadraco lived was dotted with ponds, streams, and lakes. Vegetation wise, we had mangroves and other woody plants that may have formed a canopy, albeit likely not to the extent of a tropical jungle of today. Additionally, it would have been seasonal environment and not just a warm humid paradise all year.

Closed or semi-closed canopy conditions are variables that significantly affect how birds communicate today, particularly with their plumage and their color. In low light conditions and in expanses of vertical structures (trees), it helps to stand out with bright colors and ornaments that contrast in shape with your surroundings. When flying through the trees, the elongate tail would stand out as it’s perpendicular to the surrounding vegetation. The distal rackets additionally form contrasting shapes to the linear backdrop of trees. Both features are present in sexually dimorphic male birds today that try to garner attention, either for communication or courtship displays.

Plumadraco would have competed with the environmental conditions, other enantiornithines for potential food resources, and maybe predatory non-avian dinosaurs (if it was close enough to the ground). However, what we can say is that the threats and pressures of the day were not the main priority that Plumadraco was “worried” about – the priority was likely to mate soon and probably die young. Indeed, the specimen of Plumadraco, despite its crazy long tail feathers, is not fully skeletally mature – but may have nevertheless been sexually mature.

What do you hope your findings might lead to, and what are the next steps for your research?
Oh boy. Well, I hope that this specimen, like everything that I do in my research, will temporarily shift focus away from us and place it onto the natural world. There are far more rich and interesting things out there to study and protect than just what we deem valuable or worthwhile as humans.

Specifically, as with all my paleontological research, it’s a great honor to be able to introduce a new organism, a new individual, to the rest of my species. This individual hasn’t seen the sun in over 121 million years, and now here you are reading about it. That makes me incredibly happy. It’s like mediating a handshake between the general public and a dinosaur.

My research is multi-faceted and deals with paleontology, ornithology, anatomy, and biomechanics. Most of my research is led by questions that arise when doing other research; it’s a fantastic and dizzying cycle but I love it. I hope that with future enantiornithines, in which feathers are preserved, people take their time in looking at the small details and take care when comparing to living birds. The more we learn about enantiornithines, the more we realize that modern birds are not good analogues, so everything must either be well-justified or tentatively stated and not claimed as an absolute.

-Interview edited by PLOS staff for clarity and concision.

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