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Background
A
breeding colony first nested on an island in the restored
South Marsh area back in 1992. Docent Rick Fournier vigilantly
observed and counted the birds, and got a new graduate student
at Moss Landing Marine Labs, Jennifer Parkin, excited about
them. From 1993-1996, Jennifer thoroughly observed this
breeding colony, which became the subject of her Master's
research, often spending from sunrise to sunset in a blind
built by her husband and father-in-law. By visiting the
island early in the breeding season and putting out a grid
of stakes, she was able to map and watch specific nests
from her blind, assessing the number of hatched and fledged
chicks for each nest.
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Moreover,
by watching with a spotting scope and by collecting
samples from the island, Jennifer was able to discover
what the terns were eating: anchovy, silversides,
shiner surfperch, topsmelt, sculpins, and, surprisingly,
crayfish. |
Crash
of the Colony: 1995
Disaster
struck in 1995, the year torrential downpours caused the
Pajaro River to flood its banks, releasing polluted sediments
into Elkhorn Slough. Far fewer chicks hatched than in previous
years, and of those, only seven fledged. Jennifer collected
eggs and dead birds from the island, and had them tested
for organochlorine contaminants. For comparison, she also
sent in samples from 1994. The results were dramatic: far
higher levels of contaminants such as DDE (a metabolite
of DDT), toxaphene, and PCBs were found in 1995 than in
1994. DDE interferes with the enzyme birds use to deposit
calcium, and therefore results in overly fragile, thin eggshells.
The
evidence remains circumstantial, but it seems likely that
the massive reproductive failure of the colony was due to
effects of pesticides adhering to sediments that washed
off farmed lands. In subsequent years, the Caspian terns
failed to reproduce in the area, nesting in small numbers,
but abandoning their nests after predation events.
Spring-Summer
2000: Hatching success!
In
March, Jen Parkin and Kenton Parker, of the Elkhorn Slough
Reserve, cleared the island of vegetation early in the season,
making it safer against predation. Many Caspian terns were
seen around the reserve, diving and roosting. By May, they
were courting and tending to nests on the island, which
also hosted a great number of roosting pelicans this year,
perhaps helping to discourage predators.
In
June and July, Bruce Lyon, a professor at UCSC, visited
the colony regularly in his floating blind, a contraption
consisting of a wooden donut attached to an inflated tractor
inner-tube, with a homemade tent on top. Wearing flippers
and carrying camera gear, he quietly paddled out to the
island. From just meters away, he took close-up photographs
and video of the birds, which were completely undisturbed
by his presence, presumably taking him for an odd piece
of floating garbage.
Bruce
found the colony to be healthy and active, with birds coming
and going every few minutes, bringing fish to feed the chicks.
Each arrival caused a ruckus, with displays and thefts and
fights, though most parents got the fish to their chick
before neighbors could interfere.
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There
were about 65 active nests, most with one chick and
some with two. Little chicks were comfortably tucked
underneath their parents, sticking out their heads
for a look around, like little periscopes. Bigger
chicks, some with black caps, were training for flying,
running furiously around and jumping up and down,
even though their wings are still tiny. It looked
like there would be lots of successfully fledged chicks
at Elkhorn Slough once again. |
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Summer
2000: Predation
Unfortunately,
the ending to this story is not a happy one. In late July,
Bruce and Jen both visited the terns, and found that all
but a few of the oldest chicks (that could already fly)
had disappeared. Kenton, Jen, and Kerstin Wasson kayaked
out to the island and found raccoon tracks in the mud, and
about forty half-eaten chick corpses, many of them almost
ready to fledge.
Spring
- Summer 2001
After
the devastating raccoon predation of the Caspian tern colony
in Summer 2000, we vowed to make an attempt to better protect
them in 2001. To our delight, we can now report that over
60 Caspian tern chicks have successfully fledged this year
from their South Marsh island.
Our efforts at protected the terns began this year at the
end of March with the building of a hypothetically raccoon-proof
fence around the part of the island used in the past by
the colony. Sarah Connors lead this effort, with the help
of the Salinas CCC crew. In April, the terns arrived, and
apparently decided that the enclosed area was no longer
prime real estate, settling instead on the other end of
the island. In early May, we deployed Caspian tern decoys
within the fenced area. Initially, this attracted some terns
into the enclosure. Within a few days, however, they’d
returned to the other end of the island, no doubt put off
by the frozen stares of their neighbors, or frustrated by
the lack of response to their overtures. By mid-May, hundreds
of adults were on the island, beginning to lay and incubate
eggs in over fifty nests – all of which remained outside
the safe enclosure.
At the end of May, Nina D’Amore began work as a summer
research assistant. One of her first assignments was to
monitor the tern colony, and she quickly became committed
to attempting to save them from predation. When the first
downy nestlings were visible in late June, she began walking
her dog along the adjacent railroad tracks in the evenings,
hoping that the canine and human presence would discourage
raccoons from foraging in this area, and from heading out
to the island for a meal of tern babies. Since her (rather
large) dog was spooked and hard to control when trains passed,
she searched for alternatives. She found that one can buy
predator urine (bobcat, fox, coyote) in Santa Cruz, and
began spraying it along the tracks to give raccoons the
impression that vicious predators were active in the area.
She also concocted a hot pepper melange and applied it at
intervals along the railroad berm. These mock-predator efforts
continued for the following month and may have helped deter
raccoons from the area.
Nevertheless, one morning in late June, after a few nights
without patrols, Nina discovered raccoon tracks along the
railroad, right across from the island. Horrified at the
proximity of the cheeky creatures, despite all the efforts
she’d put into discouraging them, she realized that
periodic night patrols and predator urine would not suffice
to entirely deter their foraging in the area. She began
nightly patrols, but needed help to sustain the effort.
Fortunately, the Slough Crew of the CCC volunteered to participate.
For almost all of July, nightly patrols were carried out
between Hummingbird Island and the Parson’s Slough
bridge, either by CCC volunteers or by Nina and her friends
and family. These nightly patrols successfully kept raccoons
from foraging near the tern colony. Baited traps were left
at various places along the berm, in case raccoons showed
up, but no raccoons were ever caught.
Meanwhile, as July progressed, the 80 or so chicks on the
island continued to thrive on the diet of fresh fish brought
in by their raucous parents. The nestlings lost their down
and grew feathers, and then began practicing using their
wings. In early August, the first ones began to fledge.
By mid-August, all but a few late-comers were gone from
island, and juveniles could be seen around the Slough, learning
the art of power diving from their parents.
The sight of dozens of newly fledged Caspian terns flying
around the Slough had not been seen since 1994. Hopefully
it will be a common sight again in coming summers!
Spring Summer 2002: Hatching Success!
Once
again we are pleased to report a successful year for our
Caspian tern colony. This year more than 90 tern chicks
have successfully fledged from their nesting sites on South
Marsh Island. In her second year as research assistant,
Nina D’Amore once again took on the task of monitoring the
Caspian tern colony and protecting them from predation.
Fortunately, she did not need to put too much effort into
predator deterrence, since there was no evidence of predator
threats this spring and summer. Our team deployed decoys
in the fenced area and began observing the Caspian terns
nesting on the small island at the end of May. We observed
about 50 nests, and by early June, adults were busily bringing
food to their hungry chicks. As June progressed, 90 nestlings
were busy growing feathers and getting bigger. By early
August, all of the fledglings were gone from the island,
and juveniles were once again seen flying around the Slough,
learning survival skills from their parents. We are delighted
that this has been another successful year for the Caspian
terns at Elkhorn Slough! Hopefully, the sight of newly fledged
Caspian terns will be a common sight in the coming summers.
Spring
Summer 2003 Update
In
November 2002, we took down the fence surrounding the northern
part of the island in the ESNERR South Marsh. Because the
Caspian Terns were not using the area enclosed by the fence,
we thought it prudent to remove the fence to create more
nesting habitat for the birds. Monitoring will be continued.
Adults were first seen in the area on April 16.
The number of adult terns in 2003 seems to have decreased
relative to previous years. The typical number of adults
observed in 2003 was 60 individuals, whereas the typical
number of adults for 2002 and 2001 was 105 and 110, respectively.
Despite the lower number of adults, the number of nests
was estimated to be about the same as in 2002 (approx. 50
nests). We concluded that the overall reproductive success
of the colony for 2003 was high.
Spring
Summer 2004 Update
Fewer nesting pairs settled on the ESNERR islands in 2004. Unfortunately,
these 30 or so nests were abandoned in mid-June, after most had small
chicks, following what appeared to be another predation event comparable to
what occured in 2000. No chicks survived. Once again the Slough colony
experienced reproductive failure.
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