Observations on the eyes and ocular system of the Australian raven (Corvus coronoides)

In my neighbourhood in the Perth metropolitan area (https://en.wikipedia.org/wiki/Perth), the Australian raven (Corvus coronoides, https://www.inaturalist.org/taxa/8040-Corvus-coronoides and https://www.graemechapman.com.au/library/viewphotos.php?c=420) is so common that I interact with it every day.

Recently, I encountered a particularly bold adult individual, presumably male (https://www.perplexity.ai/search/in-adults-of-corvus-coronoides-o5VBIG8MRtaJGtucfTFW6A). I took the opportunity to examine its eyes from close-up (a distance of less than one metre).

I noticed that

• the iris (https://en.wikipedia.org/wiki/Iris_(anatomy)) is white, contrasting sharply with the all-black plumage (https://www.flickr.com/photos/ozjulian/2327533593 and https://www.shutterstock.com/image-photo/australian-raven-portrait-64715767 and https://www.alamy.com/close-up-of-australian-raven-image555427782.html and https://www.youtube.com/watch?v=dVCGByUAnx8 and https://dl.id.au/1/g.php?c=1&i=794 and https://www.dreamstime.com/australian-raven-large-corvid-glossy-black-typical-caw-cawing-call-image158034917);
• the degree of dilation of the pupil seemed constant, in contrast to what I have previously observed in e.g. Ploceus (https://www.mpg.de/17701550/bird-pupils-behave-unexpectedly-different and https://www.parrotforums.com/threads/a-parrots-eyes-why-do-their-pupils-pin-and-dialate.102173/);
• there is a considerable degree of binocular vision (https://www.dreamstime.com/australian-raven-victoria-australia-loud-noisy-corvid-populates-countryside-victoria-image160962249 and https://www.gettyimages.com.au/detail/photo/portrait-of-an-australian-raven-in-the-grass-royalty-free-image/1406721638?phrase=australian+raven&searchscope=image%2Cfilm&adppopup=true and https://www.youtube.com/watch?app=desktop&v=2in0vObSWfM and https://stock.adobe.com/au/images/great-moustache-of-australian-raven/149734268?prev_url=detail and https://www.istockphoto.com/photo/black-raven-in-the-wild-gm1368520237-438484344?searchscope=image%2Cfilm and scroll in https://canberrabirds.org.au/birds/australian-raven/);
• the head is frequently and variously moved in various directions, in order to direct the gaze;
• there is obvious movement of the eyeballs within their sockets, as part of the shifts in gaze; and
• extremely rapid blinking occurs every several seconds, by means of the nictitating membrane (https://en.wikipedia.org/wiki/Nictitating_membrane).

My commentary is as follows:

It has been written that the eyes of birds tend to differ from those of primates (https://en.wikipedia.org/wiki/Primate) in being

• fixed in their sockets, and
• laterally, as opposed to frontally, placed on the head.

According to the above view, birds tend to move their heads, not their eyes, in aid of shifting their gaze.

However, my observations of the Australian raven have shown that this species - although it certainly does frequently move its head in shifting its gaze - actually resembles primates to a considerable degree. I refer particularly to

• movements of the eyeballs within the sockets, and
• binocular vision (https://en.wikipedia.org/wiki/Binocular_vision).

The following recalls what I observed: https://www.google.com.au/search?q=australian+raven+close-up+video&sca_esv=708ca891a389fdf3&sxsrf=ADLYWIKUe67CMaLLUO6NoPhFPf0QBVivrg%3A1726103610727&ei=OkDiZsH5K4X2seMP5eH4wAM&ved=0ahUKEwjB_r-EnbyIAxUFe2wGHeUwHjgQ4dUDCBA&uact=5&oq=australian+raven+close-up+video&gs_lp=Egxnd3Mtd2l6LXNlcnAiH2F1c3RyYWxpYW4gcmF2ZW4gY2xvc2UtdXAgdmlkZW8yBRAhGKABMgUQIRigAUiDIVCIB1j6FnABeAGQAQCYAaACoAHaC6oBAzItNrgBA8gBAPgBAZgCB6ACngzCAgoQABiwAxjWBBhHwgIHECEYoAEYCpgDAIgGAZAGCJIHBzEuMC41LjGgB58S&sclient=gws-wiz-serp#fpstate=ive&vld=cid:dd5219bc,vid:Oy-5zAtAmZE,st:0.

However, in this footage I see subtle, split-second fluctuations in the size of the pupil, which I did not notice in my own scrutiny of the specimen in question. This 'eye-pinning' is not nearly as obvious as in https://www.tiktok.com/@blueplanetpets/video/7057245366206565679?lang=en. However, it may deserve further investigation in genus Corvus.

Posted on September 12, 2024 12:00 AM by milewski