Rare Night Parrot’s Unique Lopsided Ears Give It Keen Hearing

CT scans reveal unique asymmetrical cranial anatomy, similar to an owl, may be key to the survival of Australia’s critically endangered night parrot

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A couple years ago, I shared with you a scientific report that Australia’s nocturnal night parrot, Pezoporus occidentalis, cannot see well at night (more here). If this parrot can’t see well, how does it manage to remain so elusive?

The secretive night parrot lives in the arid inland region of Australia, and thus, is very difficult to study in the field. This species is classified as Critically Endangered under IUCN criteria because the number of mature individuals is estimated to be somewhere between 40–500 (best estimate: 200), its population is severely fragmented, and it is likely declining (ref). For these reasons, we just don’t know much about the night parrot’s biology, including how it navigates at night. Although that earlier study that I shared with you concluded that night parrots probably have similar visual abilities to day-active parrots, its skull anatomy was not investigated, leaving the question open as to how does it navigate at night?

One hypothesis is that night parrots may have enhanced acoustic abilities, perhaps like owls or bats. If this is the situation, it is in stark contrast with its sister species, the day-active eastern ground parrot, Pezoporus wallicus, estimated to have shared its last common ancestor with the night parrot around 3 million years ago, as well as with the only other nocturnal parrot, the flightless kākāpō (or owl parrot), Strigops habroptila, which may navigate the night world through scent.

Considering how rare the night parrot is, you might wonder how this study came about at all? It was actually something of an accident. The authors of this study, who are all palaeontologists, recently identified some fossil leg bones as probably belonging to the night parrot. But because there were no skeletons available to compare them with, they had to get creative by studying a rare specimen collected 1861 by ornithologist John Gould. This specimen, held by the British Museum of Natural History, is the holotype, that one specimen used to formally describe the species in the scientific literature.

“In our work we typically study the anatomy of extinct animals, but we also use our skills to investigate living species — in this case one of the threatened bird species prioritized by the Australian Government for recovery over the next decade”, the study’s lead author, palaeontologist Elen Shute, a research assistant at Flinders University, said in a statement.

Dr Shute and her collaborators scanned the holotype to recreate a high-resolution picture of its bones.

“In using CT scanning, we were able to digitally peel back the preserved layers of feathers and skin to reveal the skeleton beneath without any damage to the specimen”, one of the study’s co-authors, palaeontologist Alice Clement, a research fellow at Flinders University, said in a statement.

A close examination of these μCT scans of the night parrot’s skull provided the team with new insights into how a night parrot can navigate without crashing into fences, trees and other solid objects (Figure 1).

The night parrot skull has special features. These include uniquely enlarged exoccipital bones (purple area, Figure 1a) that may amplify sound. The team also identified bilateral ear asymmetry (‘bulge’, Figure 1e,f,g), a structure never before identified in any other parrot species, but which likely enhances its directional hearing, as in many species of owls.

“We were impressed by how far one of the Night Parrot’s ears stick out sideways, and also the sheer size of the external ear region”, senior co-author, paleontologist Gavin Prideaux, said in a statement.

“Examination of the skull showed how the enlarged ear chambers appear to constrain the maximum size of the Night Parrot’s eyes yet they are able to fly up to 30 km at night to forage before returning to roost at sun-up.”

These enlarged asymmetrical exoccipital bones impinge upon other cranial structures, constraining the size of the night parrot’s eye orbitals, suggesting an evolutionary trade-off between hearing and vision.

“By measuring the scleral ring — a ring of bone that sits inside the eyeball — and comparing it to those of other birds, we found that a Night Parrot’s cornea is about as small as it can get while still allowing visually-guided nocturnal flight. A millimeter or two smaller, and they really would be flying blind”, Dr Shute explained.

Hearing is important to night parrots as demonstrated by the success of human listening surveys in detecting night parrot populations. Although nothing is known about the night parrot’s acoustic ecology, the authors propose that its uniquely enlarged exoccipital bones may act as resonance chambers, amplifying the volume of sound transmitted to its inner ears. These exoccipital bones occupy more of the skull than in any other parrot species examined.

“Taking up about one-third of the length of its head, its enlarged ear chambers may act like amplifiers which in turn would increase the volume of sound transferred to the inner ears”, Professor Prideaux elaborated.

“This is probably why ornithologist John Gould described the Night Parrot as having an outsized or ‘bluffy’ head compared to its body — evolution has crammed as much as it can into its skull”, Dr Shute remarked.

Asymmetrical ears are typical to some bird taxa, especially nocturnal birds, but this has not previously been seen before in any species of parrot. In many species of owl, for example, the left and right ears are markedly offset dorso-ventrally and antero-posteriorly, producing a dramatically distorted cranium. But why does a seed-eating parrot need such keen hearing?

In this ground-dwelling granivorous parrot, its ears are optimised to collect sound at the ground level rather than on the wing, as in many owls: its ear slits face forwards and upwards, not downwards. This specialised ear anatomy of the night parrot contrasts with the apparently unremarkable anatomy of its closest living relative, the diurnal eastern ground parrot — and even with that of the nocturnal kākāpō. This enhanced directional hearing could help explain the extreme elusiveness of the night parrot, which is very efficient at detecting the direction of approaching humans, allowing it to remain hidden.

Considering this study’s findings, is it possible that noise pollution may present a special threat to the night parrot? It’s difficult to say at this time because these parrots communicate using relatively low-frequency calls that noise pollution usually does not interfere with, but the potential impacts of noise disturbance cannot be overlooked and thus, may warrant consideration in night parrot conservation strategies as a precaution.

This vanishingly rare and unique parrot is one of 22 Critically Endangered Australian birds that are prioritised by the federal government for recovery.

“We hope ecologists can make the most of the next 10 years to find out as much as possible about the biology and behavior of Night Parrots and work out how many there are, how many locations they survive in, and what we need to do to put their populations on the upward trajectory.”

Source:

Elen R. Shute, Alice M. Clement, and Gavin J. Prideaux (2023). Cranial adaptations of the Night Parrot (Psittaculidae: Pezoporus occidentalis), a cryptic nocturnal bird, Emu: Austral Ornithology | doi:10.1080/01584197.2023.2181185

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