Do birds detect and respond to forces acting on feathers through filoplumes, which appear to be unique mechanosensory feathers? If filoplumes function as sensors, their morphology should covary with the morphology of their companion feather to better detect feather movements and position. We explore covariation in filoplumes and primaries across 5 species of birds that vary in body size, molt strategy, and the functional life span of their primaries (Green-winged Teal [Anas crecca], Ring-billed Gull [Larus delawarensis], Turkey Vulture [Cathartes aura], Red-tailed Hawk [Buteo jamaicensis], and Red-winged Blackbird [Agelaius phoeniceus]). Filoplumes never extended beyond the coverts and inserted immediately adjacent to the base of their companion primaries, positioning them to detect subtle changes in feather vibration or movement. Far more variation in filoplume number and morphology was due to species differences than to individuals or position in the wing. Across species, filoplume length and number increased with calamus length of primaries. In the 2 species with growing primaries, the number and length of filoplumes were only weakly associated with molting primaries, suggesting that filoplumes were not replaced when their companion primary was replaced. Further, filoplumes associated with a growing primary were not replaced synchronously, leaving others to sense primary position and movement. Finally, filoplume number and length were greatest in Red-tailed Hawks, a species that carries individual feathers for multiple years, but links between filoplume morphology and molt strategy await broader comparative studies. Taken together, the morphology of filoplumes and their replacement schedule relative to their associated primary suggests that they are sensors, capable of detecting subtle differences in the position and movement of their companion feathers.
LAY SUMMARY
How do birds detect and respond to forces acting on feathers, which have no nerves? One idea is through unique mechanosensory feathers called filoplumes.
Filoplumes resemble hairs with downy tufts at their tips. They are found immediately adjacent to every contour and flight feather and are thought to transmit information about feather position and movements.
We explore links between filoplume morphology and flight feathers in five species of birds. We find that, across species, filoplume morphology matches feather morphology as expected if they function as effective sensors. We also find that the replacement of filoplumes is staggered and not strongly tied to their companion feather, which would allow for the overall sensory function to be maintained.
Our findings support the idea that filoplumes may function as sensors of feather position and condition. If these suppositions hold, we predict strong associations between flight ecology, molt strategy, and filoplume morphology.
