An intriguing raven population; anyone seen this beak feature? (1 Viewer)

[This will require some background info. and descriptions so it’ll be a bit long.]

As a biologist working for the National Park Service I spent 4 years observing and monitoring the endangered island fox population on San Miguel Island (SMI)--this is one of the 5 islands of Channel Islands NP. The island has a resident population of common ravens. I've always been fascinated by ravens. So, though the foxes were the focus of my work I also liked to keep an eye on the ravens whenever I could. I found the SMI ravens to be somewhat distinct. In particular, I noted a few intriguing and unique features in this population that I have not observed in other raven populations that I've encountered. One of these features is the common occurrence of brown feathers among the population. This feature was variable, ranging from being absent to a mottling of brown to large patches of brown feathers (in fact, I saw some individuals whose tail feathers were mostly or entirely brown, and two ravens whose entire head and neck was brown, similar to that of a male brown-headed cowbird).

However, most intriguing of all was a feature of the beak that was present on most (but not all) of the SMI ravens that I saw (it wasn't until I had been on the island for a while that I finally saw some ravens who lacked this feature). This particular feature occurred on the front (distal) half of the beak--it was (mainly) an arching along the distal occlusal side of the upper beak that may or may not also have some concavity along the adjacent occlusal section of the lower beak. This formed an open gap in this section of the beak when it was fully closed that you could see through when viewing the closed beak in profile. I attached a photo that illustrates this feature so that you can better see what I'm describing (note also the scattered patches of brown feathers).

The gap is variable in size among individuals; the one on the raven in the photo is about at the upper extreme. This is a feature that I've only ever seen on these SMI ravens. I've seen ravens from much of their U.S. range but I never saw it on any of them. Nor did I see it in the raven populations of the neighboring Channel Islands (Santa Rosa, Santa Cruz and Anacapa) and adjacent mainland areas. I've also viewed many raven photos online and over 30 raven specimens in a nearby museum collection and didn't see it in any of these.

I'm especially intrigued re. the origin of this beak feature. Is it a distinct genetically encoded characteristic of this island population or is it perhaps a use-wear thing? SMI is essentially treeless. There are just a few willows that I consider to be trees in a small section of one the canyons on the island. There is also a stand of 4 palm trees that were planted beyond the upper end of a beach some years ago. Other than these, the island is treeless. The SMI ravens predominantly feed on the ground and are commonly seen poking their beak into or digging in the dirt. Is this something that wears away at the beak and thus produces this feature? Interestingly, I didn't see this feature on the handful of fledglings that I saw but it wasn't clear if their beak growth and development was as yet fully completed--if it was, this would add support to a use-wear origin. Seasonally the SMI ravens also eat the fruits of iceplants. Is this beak feature in some way connected to this, either in the manner that the ravens harvest and prepare these fruits or perhaps due to these fruits being somewhat acidic and corrosive (I asked a park botanist if these fruits were slightly acidic and he said that they likely were)? Whatever the cause of this unique beak feature it is an intriguing matter that would make for an interesting study topic as would the SMI population in general.

I’m curious as to whether anyone else has seen this feature in common raven populations in your areas? Anyway, there’s some interesting stuff going on with the SMI raven population. It’s the most distinct common raven population that I’ve seen and I believe it’s well worthy of an in depth study.

I also attached a few photos of one of the raven nests. It was constructed on the wall of one of the island's canyons. Though it's difficult to see, there is one young chick in the nest (on a happy note, this chick successfully fledged). With the island being treeless, possible nesting sites were at a premium, especially in light of the island's fox population.

Have there been any long term studies done on the ravens? Are they isolated or do they mix with the mainland population?

Mono said:

Have there been any long term studies done on the ravens? Are they isolated or do they mix with the mainland population?

Click to expand...

To my knowledge, no studies have ever been conducted on the SMI raven population. Perhaps a contributing factor here is that before I began my work there it appears that nobody was even aware of the distinct features noted in my post (notably the beak feature) and thus the distinctiveness of the SMI raven population. When I began my work with the foxes on SMI these distinct features quickly caught my eye. Enthralled by these I began asking around about what had been learned re. these features and was quite surprised to find out that nobody seemed to be aware of their existence. This was doubly surprising considering that the park has been conducting annual landbird surveys on the islands for quite some time and that many other biologists have conducted various work here. My work was with the foxes so I certainly couldn’t devote the time and effort that would be required to do a proper study of the SMI ravens. I would love to know more about this population so I have contacted other biologists/ornithologists at colleges and universities and told them about this population in the hopes of enticing them or their graduate students into conducting an in-depth study (I even sent an email to noted raven guru Bernd Heinrich). Unfortunately, no luck so far. So, I’ve taken the opportunity to post info. about this population here on a public forum in the hopes that it might interest someone who had the opportunity and resources available to conduct a study.

As for how isolated this island raven population is, here again it will take a proper, focused study in order to determine this. Transmitters and bands will need to be placed on a sufficient sample of the ravens and properly monitored, and observers will need to spend sufficient time watching for departures and for arrivals from elsewhere. From my personal observations the impression was that there wasn’t much interchange with this population (in other words, that it was fairly isolated). My fox work took me all over the island and in the 4 years that I worked on SMI on only one occasion did I observe any ravens leave the island for or arrive from elsewhere. In this instance a few ravens left together from the east side of the island and flew off across the 3 mile channel to Santa Rosa Is. (this is SMI’s closest neighboring island—note that I never saw the distinct SMI raven features on any of the Santa Rosa ravens but it should be noted that I didn’t spend much time working on Santa Rosa since my work centered on the SMI foxes, though I did ask Santa Rosa biologists to look out for them). Also perhaps of significance is that I rarely saw SMI ravens fly over water, period, either over areas just offshore or across coves or bays. In addition, there were some raven pairs who I became familiar with that I know remained resident on SMI during the 4 years that I worked there (this included the pair whose nest is pictured in my post). However, one must keep in mind that this impression was merely based on my opportunistic observations. Any scientist knows that the impression one gets from opportunistic observations may differ considerably from what the actual situation is as determined by a proper, systematic study. Thus the need for a proper in-depth study of the SMI raven population.