by Bill Standley
Those that walk along the coastal trails at Estero Bluffs State Park may not be aware of one of the most interesting species of birds that lives year-round in the area, but they have likely heard them. The Black Oystercatcher is one of the noisiest birds along the coast, often even noisier than the abundant Western Gulls cruising the bluffs, but unless you stop to look closely, you may miss them altogether. Black Oystercatchers are relatively large (slightly larger than a pigeon, but not as large as most gulls) and the only shorebirds that are mostly black, and have a large (2-3 inches long) bright reddish-orange bill. While they are noisy, they are quite shy and prefer not be where people are and will flush if you approach too closely.
If you bring along some good binoculars or a spotting scope, you can also see that Black Oystercatchers have yellow eyes with bright orange eye rings, and pink legs with black toes. You may also notice that that only their heads and necks are truly black, with the rest actually being dark brown. While female Black Oystercatchers are slightly larger than males and have slightly longer bills, it is often difficult to discern these differences. There is another trick you can use if you can get a close look: it turns out that you can distinguish males from females by the shape of their pupil! Males tend to have pupils that are almost perfectly round, whereas females have what are referred to as “flecks” that make their pupils more keyhole-shaped.
Young Black Oystercatchers can also sometimes be difficult to distinguish from adults once they reach the same size, but at least for the first year of their life the bill does not turn fully orange.
Black Oystercatchers only occur along the western coast of North America - from Baja California to Alaska - but there are eleven other oystercatcher species in other parts of the world. Black Oystercatchers have been known to hybridize with American Oystercatchers where their ranges overlap in southern California.
Despite their name, the Black Oystercatchers at Estero Bluffs SP don’t eat oysters. Instead, they feed on mussels, limpets and other invertebrates using their large bill to forage among the rocks and to pry open shells. During low tide, they spend most of their time feeding among intertidal rocks and may sometimes forage along small gravel beaches. They can be seen resting or preening on shore or on the larger offshore rocks during high tides.
One of the reasons that Black Oystercatchers are such noisy birds is that during the breeding season (April-August) each mated pair defends a territory along the coast and they use their loud calls to announce their presence and warn off intruders. Pairs appear to mate for life and usually use the same nesting area each year. They lay 2-3 eggs in a shallow scrape located on a protected part of the bluffs or on an offshore rock, and the nest is sometimes lined with small shells or pebbles. Both parents take turns incubating the eggs for about a month. Once chicks hatch, they are able to start walking quickly; they are not able to forage on their own and both adults will provide food. If you watch a nest closely, you can often see the adults bringing limpets or other small prey items to their chicks. The chicks can’t fly until they are about a month old. During this time, because they are very vulnerable to becoming someone else’s lunch, the adults react strongly when potential predators come near. Unfortunately, there are a lot of such predators at Estero Bluffs SP that are glad to snack on oystercatcher eggs or chicks: ground squirrels, raccoons, skunks, and even Western Gulls and Peregrine Falcons.
One of the additional risks Black Oystercatchers face in many of their nesting areas, including Estero Bluffs SP, is the almost constant presence of people enjoying the coast that may disturb nesting birds, often times without being aware of it. Many such disturbances have been recorded at Estero Bluffs SP, usually off-leash dogs, tide-poolers or fishermen climbing on rocks with nesting birds. This can cause the adults to abandon their eggs or chicks (making loud alarm calls), leaving the chicks unprotected. Drones (which are not allowed at Estero Bluffs SP) flown along the bluffs will also cause adults to react as if there was a predator and leave the nest unguarded. In some rare instances, staff from California State Parks will place temporary fencing and signage on the beach or bluffs where there is an active nest that is especially vulnerable to being disturbed.
Black Oystercatchers will only raise one group of chicks each season. They will sometimes re-nest if the first group is lost, and sometimes will even nest three times in a season if both the first and second nests are lost. When lucky enough to survive long enough to learn to fly, the chicks will stay with their parents for several months while they learn to forage on their own. Given all these risks, the reproductive success of Black Oystercatchers is quite low, and nesting pairs are often not successful in raising any chicks within a season. Black Oystercatchers don’t start breeding until they are several years old, and they are estimated to live to be up to 15 years old.
Because Black Oystercatchers are limited by the availability of rocky shoreline habitat, they are much less abundant than many other bird species, and in 2011 the species was designated as a federal “focal species” for priority conservation. Given concern for the species, the California Audubon Society coordinated with multiple state and federal agencies and other interested groups to organize volunteers to monitor as many nests as possible in California for a 10-year period, which concluded last year. The information collected is currently being analyzed and will help agencies decide whether additional conservation measures may be necessary to keep the species from becoming endangered. California State Parks is now partnering with Cal Poly and community volunteers to continue monitoring Black Oystercatcher nests in San Luis Obispo County, including Estero Bluffs SP. The population size of Black Oystercatchers in California is estimated to be 5,000-6,000 and we know that there are at least nine pairs nesting at Estero Bluffs SP.
So next time you are out walking on the bluffs or exploring the shoreline at Estero Bluffs SP, bring some binoculars, watch and listen for this special bird, and be sure to give them some extra room during the nesting season.
