When I was teaching undergraduate ecology I always impressed upon my students the idea that the categorisations we use to describe “communities” and “ecosystems” are really loose, artificial attempts to put boundaries around borderless ecological systems. Nowhere is this more true than in coastal ecosystems, where the transition from “sea” to “shore” to “sand dune” to “coastal woodland”, for example, is a blur of overlapping habitat types linked by the movement of organisms, nutrients and energy from one to another.
Birds are especially important linkages in this respect, because they are highly mobile and thus effective at connecting “land” to “sea”. Consider gulls, for example, which may be feeding in coastal waters and on grasslands some distance away, and defecating and being preyed upon in both, resulting in transfer of sea-derived nutrients and energy into terrestrial ecosystems, and vice versa. There’s considerable interest amongst ecosystem ecologists in understanding such transfers; for example, here’s the opening sentences from the abstract from the 2013 paper Donor-Control of Scavenging Food Webs at the Land-Ocean Interface by Thomas Scholar and colleagues:
Food webs near the interface of adjacent ecosystems are potentially subsidised by the flux of organic matter across system boundaries. Such subsidies, including carrion of marine provenance, are predicted to be instrumental on open-coast sandy shores where in situ productivity is low and boundaries are long and highly permeable to imports from the sea.
Here on the coastal beaches of the Kattegat I’ve been intrigued by the behaviour of hooded crows (Corvus cornix), which are acting, it appears, as just such facilitators of the “flux of organic matter” from sea to land.
There are six corvid species in the area, and hooded crows are by no means the most common: there’s at least as many rooks (Corvus frugilegus) and jackdaws (Coloeus monedula), and we often see all three species foraging together on ploughed fields or suburban grassland. That’s not surprising, because like many members of the crow family these species are opportunistic omnivores that eat a wide range of animal and plant material, both living and dead, as well as clearing up human food waste, which I described a few years ago during a visit to Kathmandu.
But hooded crows are the only species that we see scavenging on the shoreline.
On Sunday, for example, I took a late afternoon stroll along the local beach with my binoculars and, as usual, I saw hooded crows in small groups of two or three, sometimes in the company of gulls. As I watched, in quick succession I saw two lesser black-backed gulls (Larus fuscus) paddle onto the beach, one with a large, flapping flatfish in its beak, the other with a struggling shore crab. As the gulls tore apart their respective prey they were quickly joined by some hooded crows that had been hanging around nearby. Once the gulls had eaten their fill the crows moved in and demolished the rest. The crows seem to be particularly adept at getting the last bit of meat from inside crab carapaces.
That’s behaviour I’ve seen a many times since we arrived here in August, crows picking over the remains of fish or crabs or (in one instance) a dead harbour porpoise that had also attracted the interest of gulls.
This focus on relatively large carrion items by the crows is understandable, but relatively rare because it’s controlled by the frequency with which such dead animals become available on the shore. It´s much more common to see the crows working their way systematically along the strandline, turning over seaweed in search of insects, crustaceans, and other small food items. I’ve even seen them hack away at washed-up acorns in the beach. It must be a productive way of finding food because they do it with such regularity.
But there’s a number of things about this behaviour that are puzzling me.
For example, why is it only the crows that work the strandline? Why do we never see jackdaws or rooks, which are at least as common, and equally omnivorous scavengers? They are also just as intelligent as the hooded crows and presumably could learn that this is a good place to find food. Also, are the crows that we see strandline “specialists” that spend most of the time on the beach, and nest in the nearby dune woodlands? Or is there a constant turnover of individual birds from the surrounding countryside to the beach and back? Do the birds learn this behaviour from one another and is it passed down from parents to offspring?
I’d be interested in your comments on these observations, as always. If you’d like to know more about corvid behaviour and ecology, I can highly recommend Dr Kaeli Swift’s Corvid Research Blog.