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