Did other birds copy owls' state-of-the-art navigation system?

A new harrier biology study says these large daylight-hunting hawks have evolved an auditory system similar to owls

Thu, 15 May, 2025 - 12:05

Richard Collins

Bird profiles are elegant and streamlined. Having external ‘pinnae’ isn’t an option... protruding ears like ours would cause noisy ‘drag’ when moving through the air. They are, therefore, tucked away beneath contour feathers.

Owls, however, do things differently. Looking more like blunt-nosed World-War-One biplanes than Concorde, they have flat disk-like faces. Compensating for the lack of ‘pinnae’, the faces, like parabolic antennae, help an owl detect the faintest sounds.

Our human eyes ‘see us’ through the day, but we must sleep at night. Most owls do the opposite; they work the night shift. Having dethroned sight from its privileged sensory position, they rely more on their hearing.

Barn owl (scréachóg Reilige / Tyto alba). Picture: Mike Brown

The ‘sound shadow’ of its head is crucial to a barn owl’s navigation. Its ears are positioned as far apart from each other as possible. Depending on its source location, a sound may arrive at each ear at slightly different times and with differing intensities. An owl’s right ear is positioned higher up on the head than the left one, so as to maximise this time-lag.

There are other adaptations. An owl’s ‘cochlea’, the fluid-filled tube in the inner ear, is unusually large. A finch’s cochlea is less than 2mm long. An owl’s one can reach 9mm. Like a sound-frequency analyser, this organ helps the bird interpret noises.

The barn-owl, a stay-at-home bird, carries a detailed mental map of its local patch. Every nook and cranny is known to it intimately. With its state-of-the-art avian navigation system, it can locate a mouse, or a rat, rustling through leaf-litter in the dead of night. Even in total darkness, this silent killer needs just two sound ‘fixes’ to pin-point a rodent’s location and strike it with unerring lethality.

It has been surmised that harriers, large daylight-hunting hawks, might have similar hearing skills. We have two species in Ireland. The marsh harrier, virtually extinct here now, once bred in wetlands throughout the country. It was common to see one flying to and fro, two to six metres above the ground, ‘quartering’ the dense wetland vegetation. The hunter tilts its head from side to side occasionally, as people do when they are hard of hearing. Is the harrier, likewise, listening for the sound of a potential victim?

Australian swamp harrier. Picture: Mike Lee, Flinders University/SA Museum

According to a paper just published, it is.

Alberta has vast prairies. On a visit there, I once saw a Northern harrier, a close relative of our species. Sara Citron, of Alberta’s Lethbridge University, and colleagues at Flinders University in Australia, are studying harrier biology.

Sara Citron, a PhD student in the University of Lethbridge's Department of Neuroscience

The facial discs of these raptors, though not as prominent as those of owls, may play a similar sound-gathering role. The birds have unusually large ear openings, suggesting that enhanced sound detection is important to them. Crucially, the brain regions involved in working out the directions of sounds, are highly developed. The researchers conclude that harriers have, indeed, a second string to their sensory bows. Using both hearing and vision, they "have evolved an auditory system similar to owls".

What a remarkable example of convergent evolution in animals separated by 60 million years of evolution!