Bill overhang in corvids

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.

This common raven doesn’t have a particularly big overhang

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.

Literature cited

  1. 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

1 Comment

Filed under Corvid health, Crow curiosities, Diversity, Ravens

One response to “Bill overhang in corvids

  1. Judith Blades

    what a timely article thankyou. I have been recently painting crows and wondering about the correct bill shape!

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