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 co-vary with the morphology of their companion feather to better detect feather movements and position. We explore covariation in filoplumes and primaries across five species of birds that vary in body size, molt strategy, and the functional life-span of their primaries (Green-wing Teal, Ring-billed Gull, Turkey Vulture, Red-tailed Hawk, Red-winged Blackbird). 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 two species with growing primaries, the number and length of filoplumes was 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 was 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 they are sensors, capable of detecting subtle differences in the position and movement of their companion feathers.

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