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 domestic chicken breeds. Even within a single bird, within a single follicle, a feather can vary drastically throughout an individual's life. Tail feathers are perhaps the most diverse of all feather types, varying not only in size and shape, but also in function. In woodpeckers, swifts, and woodcreepers, for example, tail feathers are often short, stiff, and spine-like to provide support as the bird perches on a vertical surface. In countless other species, tail feathers are long or elaborate and are used to attract a mate, while in others still, they are specialized for aerodynamic control (e.g. forked tails).
Conversely, flight feathers in the wings (i.e. remiges) serve a single, critical function - producing the aerodynamic forces required for flight. In fact, unlike tail feathers, the shape of remiges has only evolved in sexually selected displays in four species! 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 rare and dramatic 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 research, including work I conducted as an undergrad, has been published in the Journal of Experimental Biology. Download it 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
Specialized Shapes for Sonations
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.