I hardly notice it when I’m in the field because the bird is either too far or too fussy for me to examine the finer details of its beak, but looking at pictures like this I can’t help but notice that little extra bit of bill that sweeps over this raven’s bottom mandible like an awning. It leaves me wondering, what is this overhang for? A why is it more noticeable on some individuals than others?
When we think of birds with curved upper mandibles we are most primed to think of raptors, which have the most conspicuously curved overbite of any group of birds. This shape give them the tearing power they need to breakdown and consume their prey. But upon closer inspection we can see that actually most birds, even some hummingbirds, have a slight* overbite known as maxillary overhang. Among corvids, common ravens have one of the most noticeable overhangs, though they can be quite visible in crows as well. Unlike raptors though, for most other birds this overbite has less to do with food consumption and more to do with hygiene.1
While they can be good at hiding it under those sleek feathers, wild birds are constantly contending with a suite of exterior parasites like gnats, feather lice, and mites. The presence of these parasites, and the speed at which they can cause real harm if left unchecked, is part of the reason birds dedicate so much time and energy into preening. To remove and kill these pinhead sized parasites, birds carefully comb through their feathers, individually plucking up offenders with their bill tip. Even a small (1-2mm) overhang has been show to provide a significant advantage to birds by providing a backdrop against which the lower mandible can pin and damage parasites. Without such an overhang, parasite loads can be as much as 4x higher. By contrast, the overhang doesn’t seem to play any measurable role in foraging success or efficiency.1
Given the largely carrion based diet of common ravens, it’s possible their more visible overhang provides some advantage when it comes to digging into a carcass. Since it hasn’t been tested, however, I can only guess that if some feeding advantage does exist here, it’s slight. This is supported by looking at other carrion specialized corvids, which don’t seem to show a particular bias in longer overhangs relative to their omnivorous or seed eating corvid cousins, and the fact that even within common ravens overhang length is variable.
To an extent, the longer the overhang the more effective it can be. As anyone who has tried to maintain naturally long nails can attest though, long keratin based appendages are prone to damage and breakage. Bill tips are no different, which explains why there can be so much variation between and within species. The overhang is playing an evolutionary balancing act to arrive at the length that is the most effective and the least vulnerable to damage. That means that not only will some species have shorter or longer overhangs, but individuals within species may, at any point, exhibit more or less obvious tips as due to recent damage. As a result of this variability, I don’t recommend looking to the level of bill overhang as a diagnostic tool for identifying ravens. While it’s certainly useful for crushing bugs, you can’t bank on it being useful for winning that next round of #CrowOrNo.
* Some corvids have more than simply a slight overbite. Unusually long or misshapen bill tips can be the result of a viral infection called avian keratin disorder. Check here to learn more about AKD in crows.
- Clayton DH, Moyer BR, Bush SE, Jones TG, Gardiner DW, Rhodes BB, and Goller F. 2005. Adaptive significance of avain beak morphology for ectoparasite control. Proc. R. Soc. B 272: 811-817
11 responses to “Bill overhang in corvids”
Bernd Heinrich describes the ravens he observed as using this upper bill as a ´chisel´ like tool to harvest strips of meat from frozen carcasses. This description matches what I have observed from raptors with their prey.
While I have your attention: I have heard that crows, and possibly ravens, can see slightly further into the ´purple´, near UV, than we can. Is this so?
And if so, could they see various patterns in each others plumage that is no readily apparent to us?
Thanks for this interesting post. This is something I have noticed over the last ten years with crows around the Seattle area, and some with noticeable variation. Once I saw a crow in the Costco parking lot with a very long overbite I feared for it’s ability to eat and groom. Perhaps this was a avian keratin disorder.
Hi Joey, yes I think that’s likely. It’s probably figured out how to make it work though!
Your post is appreciated with such good info. My mind immediately went to Crossbills, and now I want to know what used a bill has when both upper and lower so long that they overlap.
MERCI, very interesting as usuall, long life to you Madame 🙂
what a timely article thankyou. I have been recently painting crows and wondering about the correct bill shape!
Very interesting to know. I will take a closer look when my “pet” raven comes tomorrow morning to see me.
Very interesting. Crow-or-No is a big one. And you’ve eliminated one of my favorite indicators. But I’m delighted with the information. The other indicator I try to rely on is the wedged tail, but that’s almost impossible to see if the bird is high above in mid-flight. Seems like we just have to get to know the birds over time to decide. In my case I was inspired to write this;
I Am Raven
I don’t take a phone with me when I walk the dog. I enjoy the scent of lilac, watching trees unfurl, and children play. I cringe at the sight of goat-head weeds with their fern-like green leaves and disingenuous yellow blooms. Come fall, the blooms produce a woody fruit with tough spines that pierce the soles of shoes and get loose in the house—a magnet for bare feet.
A cooper’s hawk calls, and a robin rants about its territory in a run-on sentence that never ends. Little dog alarms go off behind five-foot fences, and radios rap. The maw of countless garages features vignettes of oil changes, table games, and the sparkle of a wine glass being sipped at a safe distance.
As Ellie and I round the final stretch of our walk, a large black bird silently soars near. Raven or crow? Two features that distinguish one from the other are impossible to see from a distance. But the three-foot wingspan of this bird, perfectly perpendicular to me and my dog, glides five feet overhead. Hungry to know, I look straight up, and, in that instant, the outstretched finger-feathers and wedge-shaped tail whisper, I am raven.
This seems to help the 2 crows (my attempted murder) stealing eggs from the neighbors hen house to carry the eggs while flying.
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