The Remex Diversity Paradox
// aerodynamics // feather morphologies // sonation behaviors
Feathers are the most diverse integumentary structures on the planet. They range in size from 1-millimeter-long down feathers on baby hummingbirds, to 10-meter-long tail feathers in some chicken breeds. Even within a single bird, within a single follicle, a feather can vary drastically throughout an individual's life. This impressive diversity of feather sizes, colors, shapes, and functions has made birds the successful and beloved group they are today.
Not all feathers vary to the same degree though. Feathers in the wings, for example, are virtually identical between all species (even across 100 million years of evolution!), while feathers in the tail are some of the most dramatic and diverse. How many species can you name that are famous for their beautiful tails? Peacocks, lyrebirds, pheasants, scissor-tailed flycatchers, quetzals, turkeys - even those pesky magpies have an elegant tail! Now, how about elegant wing feathers? No? Can't think of any? There are only four species on the planet with exaggerated wing feathers (two nightjars, one bird-of-paradise, and the Great Argus).
The flight feathers in the wing, also called remiges (or a remex, singly) serve a single, critical function - producing the aerodynamic forces required for flight. Birds rely on these stereotyped flight feathers for all aspects of daily life and their chances of survival decrease dramatically if too many of them get damaged. These intense selective pressures constrain the shape of remiges so strongly that, aside from their size, there's hardly a difference between those in finches (feather A, left) or swans (feather B, left). This striking morphological uniformity is the result of physics - aside from the smallest and fastest species, the laws of aerodynamics and the physical properties of wings are the same for all flying birds.
And yet, paradoxically, some dramatic and rare variation does exist in remiges (feathers C and D, left). If flying birds require these feathers for survival, and if the physical constraints of aerial locomotion are universal, then why does remex shape vary at all?
I began investigating this question as an undergraduate at the Slater Museum of Natural History at the University of Puget Sound. It was there that I first noticed the diversity of odd feather shapes present in pigeons and doves. As a Ph.D. student at the University of Montana, I worked n the Flight Laboratory with Dr. Bret Tobalske, to learn how and why these shapes exist.
My research explores the function of atypical feather shapes, testing their role in sonation behaviors and as aerodynamically specialized devices. My dissertation, entitled "Why the Weird Wings?" can be downloaded for free from the University of Montana's Scholar Works page. Or, you can watch my 50-minute summary of these 10 years of research on my YouTube channel here.
Use the links below to explore the various components of the Remex Diversity Paradox.
Above: a spectrogram of Mourning Dove wing sounds. The first written accounts of the Mourning Dove always included a description of their odd wing sounds. From John James Audubon to Alexander Wilson, the "whistling wings" of these elegant birds have intrigued ornithologists for centuries. Could feather mophology be specialized for sound-production, instead of force-produciton? Listen to the sound below. (Audio by Andrew Spencer 2009)
- XC35100 - Mourning Dove - Zenaida macroura
- Andrew Spencer - xeno-canto.org
Some unique remex morphologies are specialized for producing sounds in flight. These locomotion-induced acoustic signals, known as sonations, likely communicate alarm or are involved in sexually selected displays. But what sorts of morphologies can produce sounds? And how do we distinguish between those sounds that are spurious and those that are communicative?
Right: unusual feather shapes in Columbina doves produce buzzing sonations in flight.
Notches, elongated barbs, narrow feathers, emarginations - these odd feather shapes can be found in more than a third of all pigeons and doves, but why do they exist? Could they influence sound production? Or are they specialized aerodynamic devices? How do they evolve, and why don't they conform to the stereotyped shapes found in all other flying birds?
Left: green pigeons (Treron spp.) have remiges that have intrigued naturalists for two centuries.
Specialized Aerodynamic Devices
Some feather morphologies are thought to produce extra lift during take-off, or increase efficiency while soaring. Could these strange feather shapes in pigeons and doves be similar aerodynamic devices? If so, under what circumstances do such morphologies evolve, and why aren't they present in more species?
Left: this odd P10 feather increases flight efficiency and can be found in a quarter of all columbids.