rocky coastline of Monterey County is not for the timid. Waves, swells
and storms batter the outer coast. Few birds are resident here, but
foremost among them is Black Oystercatcher, listed as
BLOY in 4-letter-code banding lingo ("BLOY" became our survey shorthand
for this species). To learn more about this key local bird, the
Monterey Audubon Society and California Audubon sponsored Black
Oystercatcher surveys along the rocky coastline of Monterey County in
2011 and 2012. [Cal Audubon also sponsored similar surveys in other
northern California counties.] In 2011, Monterey
volunteers surveyed almost half of the county's rocky coastline and
counted 127 birds, representing about 55 pairs, plus 18 young from
perhaps 10 or so nests. This constituted about 9% of the 1346
oystercatchers found on statewide surveys in 2011. Monterey's Breeding Bird Atlas
(Roberson & Tenney 1993) had estimated 77 pairs in the county; the
survey of half the habitat suggested that the county might have as many
as 100 breeding pairs.
In 2012, the project was a more intensive reproductive success survey,
requiring volunteers to follow specific pairs and their nesting
attempts over the course of a breeding season (May-August). Some 18
volunteers signed up at the organizational meeting on 19 May 2012, and
12 of them (67%) followed through on their commitment. [Of the six that
dropped out, 3 became discouraged because of the difficulty in finding
pairs and/or nests within their assigned territory; another found the
project too time-consuming). The dozen volunteers on the 2012 project
spent substantial effort watching the breeding efforts of 21 pairs of Black Oystercatcher. [We have some anecdotal data on another 7-8 pairs, see below.]
most industrious volunteer, John G.H. Cant of Fremont, provided full
data on 7 pairs along the southernmost coast of Monterey County (and
arranged to use Big Creek Reserve as his base for each weekend visit).
Most observers followed 1 or 2 pairs for the summer, although Gail
Griffin & Brian Weed would, in tandem, follow 3 pairs at Pt. Lobos.
The purposes of these surveys were to
- determine (nesting success [nests that hatched at least 1 young],
- determine fledging success [nests that fledged at least 1 young], and
- determine, if possible, reasons for nesting effort failures.
Our results follow.
21 pairs laid eggs in 26 nests – in short, 5 of the pairs made
re-nesting efforts after the loss of an initial clutch of eggs. The
statistics on nesting success – with success meaning hatched at least 1
- Per pair: 57% (12 nests of 21 pairs hatched young)
- Per nest: 46% (12 nests of 26 total nests hatched young)
nests for which the number of eggs is known or can be unambiguously
inferred: 5 nests had 3 eggs, 5 nests had 2 eggs, 1 nest had only 1
egg. Thus 2-3 eggs constituted the ‘standard’ nest.
those nests that hatched eggs, the number of young ranged from 1-3
chicks. Nests with more than 1 young often lost one or more young:
three nests which began with 2 young lost one before fledging age, and
one nest with 3 chicks lost one before fledging age. Of a total of 21
young, only 5 lived to or near fledging age (38-40 days), and one of
those disappeared before fledging was confirmed.
pairs re-nested (24%) after loss of the initial clutch of eggs. One of
the renesting attempts succeeded in fledging young; in 4 cases the
pairs lost the second clutch of eggs, and in one case they hatched a
young but it did not long survive. Three of the 21 pairs (14%) were
observed copulating within the breeding territory after the loss of
either the second clutch or the loss of young. However, it none of
those cases were actual nests found after the late-season mating.
photo above © Michael Bolte [adult & chick in Santa Cruz Co., Aug 2012)
photo below © D. Roberson (adults & chick in Pacific Grove on 28 Aug 2006)
is where the story gets grim. Of our 21 pairs, only 4 succeeded in
fledging young. Three pairs fledged one young, and one pair fledged
two. None of these were from a renesting effort. The statistics on
fledging success were:
- Per pair 19%
- Per nest 15%
these 4 pairs that succeeded, two were at nest sites that had young in
2011, and as we assume the same adults were involved both years, were
thus pairs that already were successfully established at an excellent
nest site. Three of the nest sites were on inaccessible islets, and the
fourth on a sea-cliff inaccessible to the public. The four fully
successful nestings were:
just offshore the 'gazebo' at Rocky Shores, the northern end of
Asilomar SB in Pacific Grove (Rita Carratello); one chick fledged.
islet in middle of Stillwater Cove, Pebble Beach; two chicks hatched
but only one chick fledged (Don Roberson; photo of islet, right). This
pair had young in 2011 (Blake Matheson).
- Islet at south end of Garrapata SP; one chick fledged (Terry Hart).
- Steep cliffside at south end of Rocky Pt. (Blake Matheson), an area that was fledged young before; two young fledged.
addition, we have anecdotal information on 7-8 other pairs from Pebble
Beach, Pt. Lobos, or Garrapata SP (let’s call it 7 more pairs; because
different observers were involved, we are not sure of the overlap or
not at Garrapata). Of these 7 pairs, six of them were just noted as
paired up and acting territorial throughout the summer, but we have
almost no information on their nesting efforts. One pair was seen with
nest-building initiation (moving rocks about) but later checks of the
site did not disclose that a nest was even finished (this pair is shown
just below, as the presumed female checks out the nest site that was
never used). As to the others they may have tried to nest and failed,
or they may simply have not tried to nest (perhaps they were younger
birds?). However, in one case a pair at Garrapata was seen with a fledging on
1 August (Blake Matheson), which means it did successfully nest there,
but we don’t know the nest site or anything else about the effort. The
confirmed breeding is excluded from our data for the detailed analysis
of this project, but it is interesting that 1 of 7 other pairs fledged
one young, a fledging rate of 14% (and thus quite similar to our 15%
fledging success rate determined by pairs that were followed for the
REASONS FOR NESTING FAILURES
obtained very little direct information about nesting failures. The
nest or young were there on one visit, and then missing on subsequent
visits. Observers could often draw inferences, though, from the
circumstances. Of the five renesting efforts, three of these were along
the Pacific Grove coastline (Berwick Park, and two pairs at Pt. Pinos)
with heavy use by the public. All three of these pairs lost both
clutches of eggs that were laid, and in each case it was believed that
disturbance of the nesting pair by the public provided an opportunity
for egg predation by Western Gulls. This same scenario was true for the
pair at Hopkins Marine Station and at Pt. Joe, Pebble Beach – likely
human disturbance and egg predation by Western Gull – except those
pairs did not re-nest. Thus five nesting failures can be directly
blamed on human disturbance, leading to egg predation. In the case of
the Pt. Joe pair, one egg was recovered with a hole in it and a dead
chick inside – strong evidence of egg destruction by a strong peck by a
gull (see right).
A further problem with the
nest failures of these 5 pairs in Pacific Grove/Pebble Beach was that
the nests that were lost (including the 3 renesting second nests) were
located either on the mainland (3 pairs) or on offshore rocks
accessible from the mainland at low tide (2 pairs). The locations of
these nests made human disturbance possible. In contrast, the two nests
that fledged young in Pacific Grove/Pebble Beach were on offshore
islets that never began accessible to the mainland at low tide, and
were thus fully protected from humans. The islet in Stillwater Cove is
far from shore (can only be viewed via scope) and is very tall and
steep, so is inaccessible even to humans in boats. At Bird Rock in
Pebble Beach, two pairs failed (one of them after a renesting attempt).
In at least one case, the egg loss was likely due to being crushed by
California Sea-lions that sun and loaf on the Rock, and it seems
probable that disturbance by sea-lions was the cause of all nesting
failures there. Many Western Gulls nesting there would readily take an
egg if the pair were disturbed by the pinnipeds. Pinniped disturbance
is a documented cause of nesting failure in Black Oystercatchers
(Warheit et al. 1984).
and Larry Rose watched an oystercatcher pair at Pt. Joe on a nest on
the mainland shore through incubation and a passage of time (28-30
days) in which the egg should have hatched. Only after that time did
they approach the nest (above) and find that the egg was broken — with
a large hole in it (below; from a gull peck?) and a dead chick inside.
An adult had been incubating the damaged egg for days (photo ©
three breeding efforts at Pt. Lobos State Reserve succeeded in hatching
2-3 eggs, but before the young could fledge, all of them disappeared.
Human disturbance is not considered a factor at any site – all on
inaccessible islets or sea-cliffs – so one might guess that mammalian
or avian predation of the chicks was involved. Along the south coast of
Monterey, all 7 pairs followed nested on offshore islets. Two of the
pairs had young in 2011 surveys. Four of the islets also had Western
Gulls breeding, and in one situation a fisherman’s skiff near the rock
could have been disturbing the pair (this also mentioned anecdotally at
Garrapata SP). Two of the nests were lost at the egg stage, and five
failed at the precocial young stage (including a re-nesting effort).
Presumably predation was a factor in most (or all) of these failures,
but we have no direct information.
low fledging success (only 15-19%, depending on one uses nest success
or the success of a breeding pair in a season) was unexpected for most
volunteers (and discouraging). Yet, our statistics are not unlike that
those of other studies. Hatching success across the species' range is
34-70% (Andres & Falxa 1995) — so our local hatching success of
46-57% is right in the middle of that range. Fledging success
rangewide, though, is much lower, and has been found to be only 12–39%
(Andres & Falxa 1995).
are long-lived. It takes about 5 years to reach breeding age, but after
that strong pair ponds are formed. Banded adults on the Farallon
Islands lived 9–15 years. This means each adult may have 4–10+ years to
replace themselves for the population to remain stable. Fledging
success of 15–19% should maintain a stable population — as appear to be
the case here in Monterey County. It is the young chicks (like the one
at left, photo'd in 2012 in Santa Cruz © Michael Bolte) that are
the most vulnerable.
2012 surveys add a significant amount of information to that already
known about Black Oystercatchers in Monterey County (e.g., Webster
1942, Legg 1954, Roberson & Tenney 1993). The young hatched during
a breeding season tend to remain with their parents through the fall
and early winter, but a past year's offspring is evicted from the
parental territory when courtship intensifies again in January–March
(Andres & Falxa 1995). Such young birds may then be encountered on
their own (right — presumed young oystercatcher with still much dusky
coloration on bill, photo Feb 2006 © D. Roberson) or in loose
groups foraging along the rocky shore. Paired adults, though, maintain
territories year-round and enforce them by vigorous chases.
the meantime, breeding success might be improved if vulnerable nest
sites in public areas on the Monterey Peninsula could be better
protected. To those ends, California Coastal National Monument (which
protects the offshore rocks and islets along much of the Monterey Co.
coast) has installed signs at Pt. Pinos (below) in hopes that the
public can be encouraged to avoid disturbing nesting oystercatchers.
|Acknowledgments: First, many thanks to our volunteer researchers
and those who provided information: Patricia Addelman, John G.H. Cant,
Alan & Sheila Baldridge, Rita Carratello, Don Glasco, Gail Griffin,
Rick Hanks, Terry Hart, Robert Horn, Blake Matheson, Carole & Larry
Rose, Kevin Uhlinger, Diane Tan, Bob Tintle, Jennifer Updike, Brian
Weed, and Jackie Weller. Second, many thanks for the support provided
by California Audubon via project leader Anna
Weinstein. Third, we appreciate the institution support provided in
access and partnership with California Coastal National Monument, Pt.
Lobos State Reserve, and Big Creek Reserve. Michael Bolte and Larry
Rose graciously provided photos for this web page. Linda Trocki in
Marin Co., Joleen Ossello in Mendocino Co., and Elise Elliot-Smith and
Jeff Hollenbeck in Oregon provided useful information during our
Photos: All photos © Don Roberson, except those attributed to Michael Bolte and Larry Rose, and used with permission; all rights reserved.
Andres, B.A., and G.A. Falxa 1995. Black Oystercatcher (Haematopus bachmani) in
The Birds of North America, No. 155 (A. Poole & F. Gill, eds.).
Acad. Nat. Sci., Philadelphia, and A.O.U., Washington, D.C.
Legg, K. 1954. Nesting and feeding of the Black Oyster-catcher near Monterey, California. Condor 56: 359-360.
D., and C. Tenney, eds. 1993. Atlas of the Breeding Birds of Monterey
County, California. Monterey Pen. Audubon Soc., Carmel, CA.
K.I., D.R. Lindberg, and R.J. Boekelheide. 1984. Pinniped disturbance
lowers reproductive success of Black Oystercatcher Haematopus bachmani (Aves). Mar. Ecol. Prog. Ser. 17: 101-104.
Webster, J.D. 1942. Notes on the growth and plumages of the Black Oyster-catcher. Condor 44: 205-213.