For more than 70 years, Martin Griffin Preserve hosted one of the most significant and well-studied Great Blue Heron, Great Egret and Snowy Egret nesting sites on the West Coast, with as many as 175 breeding pairs of herons and egrets nesting in the tops of the redwood trees in Picher Canyon. In 2013 and 2014 the preserve nesting sites were abandoned in favor of others on the western shore of Bolinas Lagoon and elsewhere in the Bay Area. Although no one knows why the birds left, we suspect avian predators (Bald Eagles) as the likely cause.
Because ACR’s research on herons and egrets throughout the San Francisco Bay area indicate that Great Egrets could reoccupy the heronry in Picher Canyon at any time—recolonization is possible for at least 13 years after abandonment and more likely when human activity is minimized—ACR took the extraordinary step of closing the canyon for two breeding seasons.
While the herons and egrets did not return to Martin Griffin Preserve in 2015, the productivity of the new nests established on the western side of the lagoon since 2013 indicates that the quality of feeding areas and the availability of food in Bolinas Lagoon has continued to be very good. In fact, egrets with successful (undisturbed) nests in the Bolinas colony were able to provision and fledge more young, on average, than egrets nesting in most other areas of the San Francisco Bay region.
Although we don’t know if the birds will return in 2016, ACR will offer limited access to historical nesting areas during the primary colony-site-selection period, in early spring, and will host our full range of public events and naturalist-led hikes in the months of May through July.
Heronry Monitor Reports for 2016 from Science Staff
June 6, 2016: The chicks in most Great Blue Heron nests continue to grow, stretch their wings, and are beginning to venture out of the nest. Nearly all Heron nests are in the unguarded stage, when both parents are absent from the nest for most of the time. As of June 6, at least one Heron chick has apparently fledged.
Things are not going quite so well for Great Egrets. Beginning around June 1, a number of Egret nests appeared to be empty, and by the beginning of this week 9 of 17 nests had apparently been completely or partially lost. On June 3 I observed an apparent Common Raven attack on an Egret nest, which appeared to result in at least on egret chick falling from the nest followed by the attacker, while a second chick clambered clumsily toward another egret nest. On June 6 there was another apparent attack while I was surveying the colony, but I missed the action during the few moments when I was retrieving my camera from the car. Other birders have observed additional Raven activity in and near the colony over the last 10 or so days. According to Peter Pyle there is an active Raven nest on Kent Island. I don’t recall ever observing more than 2 Ravens during my surveys, so I suspect that it is just this resident breeding pair of Ravens that are responsible for the Egret nest losses. The nest losses have coincided with the time at which egret chicks were beginning to be left alone in their nests for the first time.
Some very interesting news is that Double-crested Cormorants (DCCO) have begun apparently nesting in the colony. While they are regular visitors, this is the first time DCCO have been recorded nesting in Bolinas Lagoon. There are 2 nests, which for the last week or so have had an adult laying on them as if they are incubating.
May 20, 2016: There are currently 10 Great Blue Heron and 17 Great Egret nests in the Bolinas/Smiley's colony. There are 2 "waves" of nest ages for Great Blue Herons. The most advanced Heron nests (6) have large chicks which are fully-feathered except for their tail feathers. These chicks are left alone most of the time but so far have not been observed wandering beyond the outer rim of their nests. Of these 6 early nests, 3 have 1 chick, 1 has 2, and 2 have 3. These nests with older chicks were initiated before, and survived through, the severe storms of late March. In contrast, there are also 2 other nests which were initiated after that bad weather and still have small chicks that cannot be reliably counted yet. I have yet to observe chicks on either of the remaining 2 nests; one of these is very well hidden and I only found it 2 weeks ago, so it may have only been initiated recently.
The majority of Great Egret nests now have young nestlings, and most nests with reliably-countable broods have 2 or 3 chicks. In addition to the 17 nests that are occupied on every visit, there are an additional 2 or 3 nests that are occasionally occupied by 1 or 2 birds (i.e. on some surveys these nests are empty).
As far as I can tell there have been no predation events or losses of entire nests with chicks for either species. Ravens and crows are consistently present around the colony, and Bald Eagles are regularly observed over the lagoon.
As of early May there were no active nests and no egret or heron visits to Picher Canyon.
May 3, 2016: The last month or so has seen substantial activity at the Bolinas colony. Most of the 9 Great Blue Heron nests now have large chicks, which have been pleasing visitors who happen to stroll along Wharf Rd while I have the scope set up. The increased activity associated with feeding 2 or 3 rapidly-growing chicks has led to the discovery of a couple more, well-hidden Heron nests in the last two weeks which I had not seen before.
Great Egret nest numbers continued to rise through April, with another 1-3 nests initiated on each of my twice-weekly visits. As of May 2 the colony is up to at least 14 Great Egret nests, with another 4 nests that are likely still active but are only visible when the adults stand up and move around. During the last week of April the first Great Egret chicks started hatching, and now at least 4-5 nests have chicks.
Picher Canyon continues to be unoccupied by any Ardeid species. There is some indication that Common Ravens are nesting in the canyon. Bald Eagles continue to be observed throughout the lagoon by many people, both near the Bolinas colony and over Picher Canyon. It is unclear whether these individuals are nesting somewhere in the area, or are simply hanging out taking advantage of the varied foraging opportunities on Bolinas Lagoon.
March 22, 2016: Great Blue Herons have continued to occupy the colony near downtown Bolinas, with 7 nests in the incubation stage. The heavy rains beginning around Mar 10, which battered and wilted the early Douglas Irises and other flowers that bloomed during the warm days of February, appears to have done little to deter the Herons from their nesting attempts on the Bolinas side of the lagoon. Elsewhere in the SF Bay Area, other heron and egret species are busy with their nesting: Great Egrets, Cattle Egrets and Snowy Egrets have been busy building nests in Santa Rosa, and Black-Crowned Night-Herons have begun hatching in Oakland and possibly Santa Rosa. Great Egrets have continued to be regularly seen foraging around the shores of Bolinas Lagoon, and finally, around the weekend of Mar 19, Great Egrets were first observed in the Bolinas colony. Three neighboring pairs were all actively building, and another 2-3 individuals were roosting in the colony. One Great Egret was observed trying to pull a stick from the underside of a nest where a Great Blue Heron was incubating, without the heron appearing to care. Also, from across the lagoon at Pitcher Canyon I could see several more egrets on the back side of the trees which could not be observed from the Bolinas Wharf. Kent Island may give us a better view of these birds.
One or two Bald Eagles have been observed visiting the Bolinas colony on multiple occasions, at times flying and perching amid the heron nests. There continues to be no evidence of heron or egret nesting in Pitcher Canyon.
March 1, 2016: As February progressed there was increased activity and an initiation of nesting behavior by Great Blue Herons at the Bolinas Colony. The beautiful weather early in the month was met with 4-5 occupied territories in the tall Bishop Pines lining the channel of the lagoon that separates Kent Island from Downtown Bolinas. However, the strong south and east winds that came along with some mild rain in the middle of the month blew right in to the colony, driving the Herons down into the more-sheltered branches below where the nests are located, and onto the mudflats of Kent Island. At the close of the month, 3 nests appeared to be in the incubation stage, and another 6 nests were occupied by single Herons or pairs. Pairs were observed copulating, and numerous birds were seen pulling sticks from last year’s nests to add to their own.
A group of approximately 40 Snowy Egrets continues to forage together at various places around Kent Island, offering excellent observation from the Bolinas wharf. Smaller numbers of Great Egrets have also been foraging around the lagoon, especially in the tidal flats between Kent Island and Pine Gulch Creek. An immature Bald Eagle was observed on at least 3 occasions by multiple people during February, though for the most part it has kept to the East of Kent Island, away from the Heronry. There continues to be no observed Heron or Egret activity at the historic Pitcher Canyon colony, though it is still somewhat early in the season to expect to see Great Egrets occupying territories.
Heronry Monitor Reports for 2015 from Science Staff
ACR science staff continues to monitor the Picher Canyon and adjacent Bolinas Lagoon heronries. We update this page as monitor reports become available.
July 13, 2015, Bolinas Lagoon Colony: The season continues to wind down. The Great Blue Herons are starting to fledge, and I saw some taking some short and longer flights today, which is always a wonderful sight. There are still 14 Great Egret nests. The three nests with older chicks are still active, and 6 nests have recently-hatched chicks. The rest are incubating.
June 30, 2015, Bolinas Lagoon Colony: Six Great Blue Heron nests with chicks remain. There are still several Great Egrets incubating (11 nests) and a few new nests pair bonding (4 nests). The two nests with chicks are still active as well.
June 16, 2015, Bolinas Lagoon Colony: Things are moving right along on Bolinas Lagoon. There are five Great Blue Heron nests, all with large unguarded chicks. There was one failure at a heron nest that started later in the season. There are 22 Great Egret nests, split equally between birds incubating and nests with older chicks. We had 3 failures and one new nest attempt. Again this week there was one Snowy Egret in the trees near the colony. The pair of ravens was around, but I didn’t see any predation, although they did chase a turkey vulture over Kent Island. It seems all the chicks at the colony this week are now too big for raven predation. No other activity elsewhere on the lagoon.
May 23, 2015: Following last year’s abandonment of the heronry at Martin Griffin Preserve, ACR is continuing to monitor heron and egret nesting activity around Bolinas Lagoon. No nesting has been observed at Martin Griffin Preserve this season.
However, herons and egrets are nesting again this year at the colony near the town of Bolinas. Great Blue Herons began nesting at this site in early February and reached a peak of nine nests, which is the same number of heron nests at this colony as last year. Great Egrets first appeared in mid-March and, as of 20 May, increased to 26 nests, which is fewer than the 34 nests established last year. In mid-May, most nests were in their last days of incubation, which is normal. Chicks have been observed at one Great Blue Heron nest and one Great Egret nest, and more are expected soon.
At the end of March, an adult Bald Eagle flew into the colony and raided several nests. The eagle was seen eating eggs out of one Great Blue Heron nest. Most of the nests that had been established at this time failed, including five Great Blue Heron and five Great Egret nests. However, four of the five Great Blue Heron pairs re-nested, and several additional Great Egrets nests were established, after visitation from the eagle decreased around the middle of April. Because of the recent harassment of the colony-site by a Bald Eagle, the number of nests initiated by Great Egrets in 2015 may not exceed the number of nests established in 2014.
ACR’s commitment to heron and egrets began in 1962 when the organization was established to protect the heronry on Bolinas Lagoon at Martin Griffin Preserve. In 1990 ACR expanded its scientific efforts to include the monitoring of heron and egret nesting sites throughout the Bay Area. In 2005, ACR published a 236-page Annotated Atlas and Implications for Conservation of Heron and Egret Nesting Colonies in the San Francisco Bay Area, summarizing 15 years of heron and egret monitoring in a format accessible to professionals in many different disciplines.
- Len Blumin