Filoplume morphology and number data

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.