Caspian Tern Monitoring Research

In Biological Monitoring, Bird Monitoring, Birds, Research by Administrator

Since the early 1990s, a colony of Caspian Terns – the world’s largest terns – has bred on an island in the Reserve’s restored South Marsh area.

The colony has suffered various disasters over the years, from pesticide contamination during the 1995 floods to raccoon predation in 2000. The 2001-2003 breeding seasons had excellent reproductive success, but were followed by years of reproductive failure (2004) and then no nesting (2005-present).

We monitor the number of breeding pairs and track fledging success. In addition, we patrol the area to try to discourage predators from attacking the colony.


Here’s the full story of the Elkhorn Slough Caspian Tern Colony (updated August 2003)

The caspian tern colony - photo by Bruce LyonBackground

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.

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!


UCSC Professor Bruce Lyon visits the colony.

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.

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.

Capsian Tern Chicks - Photo by Bruce Lyon

Capsian Tern Chicks – Photo by Bruce Lyon

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, over 60 Caspian tern chicks successfully fledged 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!

Caspian Terns - photo by Bruce Lyon

 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.

2004 to the present

Since 2003, there has been no successful breeding of Caspian Terns on the Reserve. In some years, the terns did not attempt to nest on the Reserve at all, nesting instead at the Salinas River Lagoon. In other years, terns attempted to nest, but suffered predation prior to fledging of offspring.