Catching a Break
A new study reveals that California's black oystercatcher population is higher than previously thought.
With their bright orange beaks, cartoonish yellow eyes, and vociferous calls, black oystercatchers are conspicuous inhabitants of rocky shores from Alaska to Baja California. Listed as a species of concern in the U.S. and Canada, it uses its strong bill to pry open mussels, limpets (marine snails), and other invertebrates. Yet while scientists know the number of oystercatchers in Alaska, Canada, and elsewhere, the number of birds in California was a big question mark. So Audubon California's seabird conservation coordinator, Anna Weinstein, along with the U.S. Fish and Wildlife Service, launched the state's first-ever oystercatcher survey this past June. The results were surprisingly positive: More than 150 volunteers counted 175 nests and 1,346 birds, far more than the previous estimate of 1,000. Weinstein spoke with Audubon about the survey and these striking birds.
Why were you interested in studying black oystercatchers?
The black oystercatcher is a focal species of the U.S. FWS, which means the agency is trying to understand and protect it to prevent an Endangered Species Act listing. The birds are completely and utterly dependent on intertidal habitat, gently sloped areas where there's a lot of wave action over rock. In these spots there are lots of invertebrates like limpits and starfish and anemones and mussel beds. It's a rich, rich marine ecosystem. Where you see less intertidal diversity, studies show there are fewer oystercatchers; so the black oystercatcher is considered a strong indicator of the health of rocky intertidal ecosystems.
What did the volunteers, who searched for the birds in 13 of the state's 15 counties, do?
Each participant was assigned a piece of the coast. At each survey point, you plant yourself for half an hour in an intertidal spot on the rocky shore where there will be lots of invertebrates. You scour the rocks with a spotting scope, looking for nests and birds, listening for their really distinctive call. This survey was timed for breeding season, so they tend not to be flying around in groups, unless they're non-breeding. You'll see a pair chasing away individuals or groups because they're defending their foraging habitat with every ounce of their energy, because that's what their babies depend on: the parents catching those fat, beautiful limpits.
Are the nests hard to spot?
They're very hard to spot. They're made up of a ring of shell fragments and pebbles. In California the vast majority are on rocks that stick up from the water, far away from wave action, on little shelves sheltered from wind and rain. You might go over a rock four or five times and still not spot a nest until an adult flies in and gives you a hint to where it might be. You think, I looked there already--how can it be there? It's just amazing. They're very cryptic, and that's part of their success strategy.
Did anything about their behavior surprise you?
I was surprised to see them nesting so close to western gulls, which are a voracious predator of chicks of other species, like the ashy storm petrel. One volunteer scientist hypothesized that black oystercatchers nest near western gulls because they chase away predators like ravens and peregrine falcons. Over the course of this project we developed a real sense that this bird is not as much of a victim as the snowy plover or least tern. As western gulls, California gulls, and peregrine falcons increase, we're seeing real defense on the oystercatchers' part. Their bright orange beaks are long, they're a large-bodied bird, and they're aggressive. They don't just sit there and let their babies get plucked off. It's really nice to see because it gives you a sense that as their world changes, they at least have some tools to cope.
What other risks do the birds face?
Climate change is undoubtedly already affecting them. Water temperatures are increasing, and the distribution and abundance of intertidal species is changing with climate. Ocean acidification is a big question mark, too: What's it going to mean for the hard-shelled organisms that they depend on [since more acidic ocean waters will make it harder for them to form and maintain their shells]? Sea level rise is going to flood out some of the low-gradient intertidal areas they forage in, too.
How might this study help conserve the black oystercather?
These long-lived birds might return to the same site for 20 years, so now that we know where nests are, we can track them for years. And there is tremendous opportunity for outreach and education. During the survey Ron LeValley of Mendocino Coast Audubon observed commercial algae collectors, and he said, 'Hey, can you just not go on these two rocks for another month until these oystercatchers fledge?' And they agreed. California Coastal National Monument, which owns most of these islets and rocks, is very keen on developing interpretive signs, too. This project is also a wonderful way to link up what we know about the bird here with what's happening with other populations--to look, internationally, at how climate change and changes in coasts are shifting the trajectory and destiny of the species. I truly believe that they oystercatcher is here to stay. We have this sprinkling of islets and rocks that can harbor the species forever if we steward them properly, and get a break on climate change.