Hawk Eyes Reveal Hunters’ Visual Secrets

A letter wing kite flying in daylight. Photo: Michael Jury (www.mykelphotography.com.au)

The world’s only nocturnal hawk, the Australian letter-winged kite, may not be better at seeing in the dark than its closest day-hunting relatives.

An international study in the Royal Society Open Scienceled by Flinders University’s Weisbecker Lab, revealed that the seldom-seen kite’s visual system is no different from that of its close relatives that are active during the day, challenging many decades of speculation that which the letter winged kite could become more like an owl than a hawk.

“Letter-winged kites hunt at night and must be able to navigate obstacles in the dark while stalking their favorite prey, the long-tailed rat,” explains Associate Professor Vera Weisbecker.

“So it was long assumed that their visual system exhibited adaptations for seeing in the dark, and in particular that they resembled owls in having larger eyes than other hawks and greater image processing in the brain. However, we found that this was not the case.

Three-dimensional reconstruction of the skull of a letter-winged kite, with the reconstructed brain in purple Credit: Dr. Vera Weisbecker.

Dr Karine Mardon, from the National Imaging Facility at the University of Queensland Center for Advanced Imaging, scanned the skulls of most of the birds of prey included in the study, using a CT scan (CT scan) .

First author and Weisbecker lab doctoral student Aubrey Keirnan then compared the 3D reconstructions of the letter-winged kite’s skull and brain with those of other birds of prey.

“Contrary to many anecdotal reports, the eye dimensions of the letter-winged kites were no different from those of two closely related kites of the same genus – the black-shouldered kites and the -black-winged frills.”

“But there was a twist in the story: we realized that all three kite species had larger eye sockets compared to their optic foramen, the hole in the back of the eye socket through which the nerve passes. optical. This is often seen in nocturnal birds, where a small number of nerves relay a lot of input from the eye in a high contrast, low resolution image.

“This suggests that all Elanus kites may be quite good at seeing in the dark and not so good at seeing in daylight – but for some reason only the letter-winged kite has changed to a nocturnal lifestyle.

Associate Professor Vera Weisbecker leads the Morphological Evo-Devo Laboratory at Flinders University.

Dr. Weisbecker says this could also explain many reports that the letter-winged kite is particularly active during moonlit nights.

“It may not be very good at flying in total darkness, unlike many owls.”

Dr Andrew Iwaniuk, from the University of Lethbridge (Canada), who co-led the study, says using anatomical observations to infer the behavior of the species was an innovative approach to understanding a notoriously difficult to understand bird. observe in nature. .

“The species lives in arid, remote Australia, avoids human settlements and is highly elusive. Population estimates vary between 670 and 6700 individuals according to the IUCN, and it is currently listed as Near Threatened. To conserve the species, it is essential that we understand its needs and behavioral abilities, but they are extremely difficult to observe.

“We are extremely fortunate to have the incredible collections of Australian museums available to us to help us understand this bird without needing to find and disturb the species. For example, the letter-winged kite and its relatives might be affected differently by artificial light sources than other hawks.

Dr Jeroen Smaers, from Stony Brook University (USA), says this study is a great example of how studying variation between many species informs our understanding of individual species; an approach that is beginning to show its contribution to animal conservation.

Ms Keirnan says the work on the kite is really just the start.

“Our measurements have shown that many other birds of prey are quite unique in one aspect or another of their visual system. other birds.

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