Australia is home to the world’s only nocturnal kite, the endemic letter-winged kite (Elanus scriptus). The raptor has long been assumed to have large eyes, like an owl, in order to see better than other hawks in the dark, but there is little data to confirm this yet.
Now, an international team of researchers across Australia, Canada and the US have used CT-scanning and modelling to investigate in three dimensions whether the skulls of raptors do show any features correlated with nocturnal behaviors. Among the birds studied was the letter-wing kite
“It has been assumed that this kite was on its way to becoming an owl morphologically because it’s nocturnal,” says project lead associate professor Vera Weisbecker, from Flinders University in South Australia. “If you look it up in Bird Life magazine, or even on Wikipedia, it describes the letter-winged kite as ‘owl-like’.”
The study included two other Elanus kite species, and 13 other raptors from both the Accipitriformes and Falconiformes orders. The former contains hawks, eagles and vultures, while the latter includes falcons.
Within these groups, the researchers compared the size of the orbit diameter, which reflects the relative size of retinal area, as well as the size of the optic foramen, where the optic nerve goes through for collecting visual data, and the size of the optic lobe, for processing visual data.
“If an animal has big eyes and big foramina, that means a very high resolution of visual information going into the brain,” says Weisbecker. “If an animal has really large eyes and small optic nerve, it means it catches a lot of light, but it’s broken down into a high-contrast but low-resolution image.”
Nocturnal animals tend to have really large eyes, while some have lost their eyes altogether. For instance, the kiwi in Aotearoa/New Zealand has exceptionally small eyes, but has a greater reliance on tactile and olfactory information for foraging.
The team also looked at the size of the “wulst” (or hyperpallium), which is a feature of the brain unique to birds that helps to compute binocular vision. This is usually large and well developed in raptors, especially owls.
“The benefit of using 3D data is that not only can we measure the external parts of the skull, like the orbit and optic parameters, but a bird brain is quite flush within the skull, so we can get measurements of those visual processing regions of the brain from endocasts,” says lead author Aubrey Keirnan, also from Flinders University. “We are able to do this without doing any damage to the skull, and it makes these museum specimens accessible across the world.”
What the team found was unexpected. The letter-winged kite’s eyes were no different to the other two closely related kite species – the black-shouldered (Elanus axillaris) and the black-winged kites (Elanus caeruleus) – based on the measured dimensions. All three kite species had larger orbits relative to their optic foramen, which suggests that all Elanus kites can see well in the dark.
So why has only the Letter-winged kite taken to a nocturnal lifestyle?
“The letter-winged kite is very interesting because it avoids towns and settled areas, where other birds opportunistically hunt at night because of the light pollutions,” says Keirnan.
“It could just be shy and be avoiding stepping on the toes of other hawks,” adds Weisbecker.
The main prey of the letter-winged kite is the nocturnal long-haired rat (Rattus villosissimus), which could be another driving factor for the birds’ own nocturnal lifestyle. Species of Elanus hawks might even use auditory cues to localise prey while hovering, but their hearing abilities have yet to be tested.
While more questions about the letter-winged kite’s behaviour remain, this study demonstrates the incredible diversity of eye sizes and positions in raptors.
The team also found that the Pacific baza (Aviceda subcristata) has the largest orbits, and strikingly laterally positioned orbits, giving it a pigeon-like appearance. The spotted harrier (Circus assimilis) had much larger optic lobes than expected, and front-facing orbits far closer together. The spotted harrier and the letter-winged kite have similar food and habitat preferences, and both also use a unique hunting style of quartering.
“This [information] is really important for conservation of rare and elusive species, as without spending lots of money to observe them in the wild, we can get important information out of just the skulls, and make this data digitally accessible to everyone,” Weisbecker says. “By scanning and modelling the brain and skull of museum specimens, we can also learn about other sensory systems, and build a sensory profile for threatened species.”
Keirnan and Weisbecker previously worked together studying the rare night parrot Pezoporus occidentalis, discovering that this nocturnal bird doesn’t have particularly good night vision, and is at risk of flying into fences and wires.