Curious to read my popular science take on our recent publication on how crows behave around their dead? Check out my latest article for Biosphere. Then check out all the other awesome authors and contributors to my favorite popular science publication. You won’t regret it. (And congrats to GO for making the article cover! She’s such a gorgeous bird…)
Category Archives: New Research
For most of us, it’s hard to imagine crows being anything but ubiquitous. Here in Seattle, American crows can nest so densely, I once found myself within 50 m of three different active nests. Such is the case for many other parts of the world too, where house crows, jungle crows, or hooded crows are an almost inescapable part of the landscape. Given these species’ success, it might be tempting to assume that all crows welcome human presence and habitat modification. Rules don’t exist without exceptions however, (especially in nature!) as our Corvid of the Month, the Mariana crow, tragically illustrates.
The Mariana crow, or Aga, is endemic to Guam and Rota and is the only corvid native to Micronesia1. In appearance, they bear a striking resemblance to the American crow, only they’re 40% smaller (cue adoring sound effects). Across their range they’re considered critically endangered and as of today, all of Guam’s birds have been extirpated by the invasive brown tree snake, and only about 46 breeding pairs remain on Rota. If that wasn’t alarming enough, their numbers continue to dwindle and researchers at the University of Washington project they could be extinct within the next 75 years2. Unlike Guam, however, there are no brown tree snakes on Rota. So what is causing the drastic decline of this island crow? As my colleague and Mariana crow researcher, Sarah Faegre, is beginning to tease out, the answer may lie in the delicate nature of island food webs, and the unanticipated butterfly effect that started with a few errant snails.
Like our American crows, Mariana crows are generalists and eat a wide variety of foods from insects, to geckos, to fruits and seeds. But adult Mariana crows have one other food source they’ve come to specialize on: the humble hermit crab. Despite the presence of hermit crabs near other species of corvus, the Mariana crow’s frequent predation on them is unique, especially when you look at how they extract them. Unlike most coastal or inland living crows that drop tough objects like clams or nuts onto hard surfaces to open them, the Mariana crow actually uses its bill to peck and break the shell at the seams to extract the vulnerable crab, a process that takes place entirely on the ground and is only shared by two other known bird species in the world (one of which is now extinct). So what does this have to do with wanderlusting snails? As it turns out, everything.
Rota is home to several species of native land and sea snail, though hermit crabs only utilize the larger shell of the sea snail. Critically, these shells are extra hard and apparently impenetrable to even the most determined crow. In the late 1930’s, however, humans introduced the Giant African Land snail which quickly invaded the island. Two major differences between the native and invasive snails are 1) that the invasive snails have thinner shells, and 2) people were anxious to get rid of them. So, naturally, we introduced yet another invasive species (a predatory flatworm) and…it actually worked. By the 1970’s the island was brimming with large, thin, empty shells, ready and waiting to be filled with hermit crabs. Gradually, the crows learned that these shells were possible to peck open and now hermit crabs are an important staple for Rota’s crows.
On its surface, this seems like the making of an ecological disaster turned into a conservation blessing. After all, we successfully controlled an invasive species while simultaneously creating a new food source for a threatened bird. But in our tangled web of introduced species and ecological fallout we must considering the one remaining player: cats. Although further study is needed, Sarah’s work3 suggests that all that extra time adult crows now spend on the ground cracking open hermit crabs may be making them more susceptible to predation by cats.
Couple the effect of cats with habitat destruction and persecution by people and the results project a bleak outlook for crow recovery. But conservationists and researchers like Sarah are working tirelessly to better understand the threats facing this bird and how to solve them. In fact Sarah and her husband, Phil Hannon, recently started a non-profit called Luta Bird Conservation to help raise awareness and conservation funds to better protect this unique crow. At the top of their priorities is funding initiatives that would bring the science of crow conservation to the classrooms of local people, helping to raise both pride and awareness for the plight of this endemic species.
So the next time you look at a crow and experience a slight feeling of fatigue at such a ubiquitous bird remember; not all corvid species welcome the consequences of people and some have suffered greatly from them. Aldo Leopold once said “to keep every cog and wheel is the first precaution of intelligent tinkering.” The lesson from Rota, and so many others, is that the same can be said of not adding any either.
If you wish to contribute directly to Mariana crow conservation, I encourage you to send Luta Bird Conservation Inc. a check at:
- Faegre, S. (2014) Age-related differences in diet and foraging behavior of the critically endangered Mariana Crow (Corvus kubaryi) (Masters thesis; University of Washington). https://digital.lib.washington.edu/researchworks/handle/1773/27571?show=full
Breaking news: Crows probably have sense of numerical competency (a.k.a. they can count)! Ok, so this isn’t breaking news, and it’s not exactly true, but it makes for a nice headline as evidenced by the number of articles that have shown up in my inbox this week regarding a new study. As early as 1950, Otto Koehler, a German animal behaviorist, showed that captive Western jackdaws would only turn over enough boxes to obtain the corresponding number of treats they saw him hide (up to around six). Parrots too, have shown that they can solve problems requiring the ability to count to around six1. So what makes this new study so special? It’s not so much that researchers showed that crows can discriminate quantities but how.
By presenting trained carrion crows with computer screens that showed two quantities of either matched, or mismatched dots, researchers were able to demonstrate that the birds could correctly indicate if the quantities were the same or different, despite the dots being of different sizes and arrangements2. While that’s in and of itself cool and of value, the main finding what that it’s actually individual neurons that are recognizing and responding to these different quantities.
Why is that so cool? Because that’s basically how our own brains begin to understand numbers too, despite our brains being, in some ways, really different. Take that in for a minute: Our human brain, and a crow (a bird!), process numbers in a very similar way. For a scientist, the neon sign illuminating “convergent evolution” immediately lights up. The researchers did not show, however, that that they could count in a strict sense like us, meaning the neurons were responding to numbers relative to each other and not to stand alone values. So perhaps jackdaws or carrion crows are different in this respect, or Koehler’s experiments were testing a different kind of problem solving ability that better teased this out. Still, crows prove once again what magnificent animals they are and their relevance in understanding our own evolution as humans.
1) Pepperberg, I.M. (1999) The Alex studies: Cognitive and communicative abilities of Grey Parrots. Cambridge, MA: Harvard University Press.
2) Helen M. D. & Andreas N. (2015) Neurons selective to the number of visual items in the corvid songbird endbrain. PNAS DOI: 10.1073/pnas.1504245112
After I lay down my handful of un-shelled peanuts it’s only a matter of minutes before I can hear the “thwap!” as the steller’s jay hits the cable wire that runs above my balcony’s railing. It balances there for a moment before descending onto the pile I’ve offered it. Quickly, and with obvious purpose, it springs down the railing and picks up a nut. Its mohawk feathers bounce as it snaps and cocks its head around in various direction. After only a few seconds, I hear the sound of rejection; the distinct hollow tap as the nut is returned to the railing. The jay repeats the same process with two more nuts before abruptly flying off with one that, to my eyes, appeared identical to the first two.
Certainly most manner of corvids engage in some kind of choosy behavior though I don’t think any of them go about it with as much frenetic spunk as jays do. So the questions arise: Why are they being so picky? What do they know about the rejected nuts that my eyes can’t see?
According to a new study1, Mexican jays are actually ‘weighing’ nuts during this process. By using specialized slow motion cameras, researchers showed that those snappy head movements are actually a way for the birds to get a tactile feel for the the nut’s weight and listen to the sound the peanuts makes as it rattles in its shell. By providing nuts that were visually similar but different in mass, the researchers were able to show that jays could consistently select nuts with the most nutmeat density. A further test showed that large shells that were altered to contain only one nut were typically selected first, only be be rejected, while single nut shells were accepted. This suggests that jays either have a sense for how much nut should weigh (and thus reject nuts that contain less than they should) or that the correlation between hollow sounds and nut density lead to the ability to choose denser nuts.
So the next time your visiting jay delights you with its sassy head snaps remember; it may simply be amusing to you but for jays, it’s an impressive product of evolution that helps keep them alive.
1) Piotr G. Jablonski, Sang-im Lee, Elzbieta Fuszara, Maciej Fuszara, Choongwon Jeong, Won Young Lee. Proximate mechanisms of detecting nut properties in a wild population of Mexican Jays (Aphelocoma ultramarina). Journal of Ornithology, 2015