Owls: silent flight is it all the same?

Many owls have adapted features for silent flight mainly due to the serrated leading edges on their wings; other reasons are thought to be the soft downs on their wings as well as trial feathers (Weger & Wagner 2016, Bachmann et. al 2007, Kun et. al 2012).

These serrated edges are located on a few primaries and alula feathers (Fig1.) and reduce noise frequency below 2kHz (Weger & Wagner 2016, Kun et. al 2012, Bachmann et. al 2007). Although differences in these serrated comb-like edges are present between species, due to different activity times meaning diurnal and nocturnal owls are bound to predate different organisms (Weger & Wagner 2016).

Fig1. Labeled diagram indication location of serrations (from Weger & Wagner 2016 p.3)

This difference in prey would indicate different hunting methods. Such as nocturnal owls locate prey by bi-aural acoustics and hunt silently as well as slowly due to increased drag by the serrations allowing undetected approach (Weger & Wagner 2016). Whereas diurnal hunting owls that do not hunt in the cover of dark, the reduced flight speed from serrations would impede their hunting fitness thus you see lesser developed serrations (Weger & Wagner 2016).

Therefore, the silent flight is not an adaption that is observed in all Strigiformes species as serration formations vary in occurrence to the time of activity in the owls, but is seemingly advantageous in nocturnal species.



Reference:

Kun, C., Qingping, L., Genghua, L., Ying, Y., Luquan, R., Hongxiu, Y., Xin, C. 2012, "The Sound Suppression Characteristics of Wing Feather of Owl （Bubo bubo）", 吉林大学仿生工程学报：英文版, vol. 9, no. 2, pp. 192-195.

Jaworski, J.W. & Peake, N. 2013, "Aerodynamic noise from a poroelastic edge with implications for the silent flight of owls",Journal of Fluid Mechanics, vol. 723, pp. 456-457.

Weger, M. & Wagner, H. 2016, "Morphological Variations of Leading-Edge Serrations in Owls (Strigiformes): e0149236", PLoS One, vol. 11, no. 3, pp. 1-18.

Bachmann, T., Klän, S., Baumgartner, W., Klaas, M., Schröder, W. & Wagner, H. 2007, "Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia", Frontiers in zoology, vol. 4, no. 1, pp. 1-2.