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. For more than 70 years, the 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. An assessment of the decline in nesting activity in 2013 is available here (pdf).

Although we don’t know if the birds will return, 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.

Nest monitoring and data collection continues

ACR has been monitoring nesting Great Blue Herons (Ardea herodias), Great Egrets (Ardea alba) and Snowy Egrets (Egretta thula) in Picher Canyon since 1967. This colony site is located at the Martin Griffin Preserve, on the northeast side of the Bolinas Lagoon. ACR has also been monitoring nesting herons and egrets at the Bolinas colony site, at the base of the Francisco Mesa adjacent to the Bolinas Channel, since 1990. A subsite, located on the south end of Kent Island at the mouth of Bolinas Lagoon, also has been monitored by ACR since the site was colonized in 2008.

ACR avian ecologists monitor every heron or egret nest detected in the vicinity of Bolinas Lagoon. Active colonies are surveyed once or twice per week, from February through July, and historically-occupied colony sites are surveyed approximately once per month to check if the colony has been re-occupied. On each visit to active colonies, the biologist records for each nest the number of adults and chicks, the nest stage (i.e. incubating, nest guarded [at least one parent remains at the nest], nest un-guarded [chicks left alone]; a measure of intraseasonal timing), and any other pertinent information. Additionally, the biologist notes any observed disturbances, or potential predators observed or inferred, in the vicinity of the nesting site. The high frequency of nest checks allows relatively high precision in determining the outcome for each nest. 
Nesting summaries for recent years are offered below.
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Heronry Monitor Summary for 2017

PICHER CANYON colony:
Picher Canyon was surveyed for nesting Ardeids approximately once per month between March and June, 2017.

  • Great Blue Heron - There were 0 Great Blue Heron nests in Pitcher Canyon in 2017. 
  • Great Egret - There were 0 Great Egret nests in Pitcher Canyon in 2017. 

BOLINAS colony:
Surveys of the Bolinas colony began on 6 Mar and were conducted approximately twice per week until 2 Aug. There were no observed disturbances to the colony by avian or non-human mammals during the 2017 breeding season. However, a Bolinas resident informed the ACR biologist that on 6 May, some people were discharging fireworks, including loud explosions and ‘rocket-type projectiles’ from a property under the colony. No nest failures or abandonments were observed in the days following this potential disturbance.

  • Great Blue Heron - The first active Great Blue Heron nest in the Bolinas colony in 2017 was observed on 6 Mar and the last active nest was observed on 2 Aug. There were 10 Great Blue Heron nests, and all of these appeared to fledge at least one chick.
  • Great Egret - The first active Great Egret nest in the Bolinas colony in 2017 was observed on 23 Mar and the last active nest was observed on 2 Aug. There were 17 Great Egret nests, and all of these appeared to fledge at least one chick. There were the same number of nests in 2017 as in 2016, possibly indicating that colony size is stabilizing following the influx of breeding pairs that appeared to result from the abandonment of the Picher Canyon colony.
  • Double-crested Cormorant - Double-crested Cormorants (Phalacrocorax auritus) nested in the Bolinas colony site for the first time on record in 2016. However, while these cormorants were observed roosting in the colony on multiple occasions in 2017, nesting behavior was not observed.

KENT ISLAND colony:
Kent Island was checked for nesting Ardeids approximately two times per month during the regular Bolinas colony site surveys. Observations of a pair of Common Ravens on and around Kent Island suggested this species was nesting somewhere on the island, but direct evidence of nesting was not observed.

  • Great Blue Heron - No Great Blue Heron nests were detected on Kent Island in 2016. Great Blue Herons were last observed nesting on Kent Island in 2013.
  • Great Egret - There were no Great Egret nests detected on Kent Island in 2016. Great Egret were last observed nesting on Kent Island in 2012.

Download the 2017 nesting summary report (pdf)


Heronry Monitor Reports for 2016 from Science Staff

PICHER CANYON colony:
Picher Canyon was surveyed for nesting Ardeids approximately once per month between March and June, 2016.

  • Great Blue Heron - There were 0 Great Blue Heron nests in Pitcher Canyon in 2016.
  • Great Egret - There were 0 Great Egret nests in Pitcher Canyon in 2016. 

BOLINAS colony:
Surveys of the Bolinas colony began on 5 Feb and were conducted approximately twice per week until 9 Aug.

  • Great Blue Heron - The first active Great Blue Heron nest in the Bolinas colony in 2016 was observed on 16 Feb and the last active nest was observed on 18 Jul. There were 10 Great Blue Heron nests, and of these 7 nests appeared to fledge at least one chick (one of the 10 nests was not detected until late in the season, and thus Proportion of nests successful, below, is calculated from 9 nests).
  • Great Egret - The first active Great Egret nest in the Bolinas colony in 2016 was observed on 21 Mar and the last active nest was observed on 5 Jul. There were 17 Great Egret nests, and of these 5 nests appeared to fledge at least one chick. 2016 was the second year of declines in number of Great Egret nests since the 2014 peak in colony size, which occurred the year after the Picher Canyon colony was abandoned.
  • Double-crested Cormorant - For the first time on record, Double-crested Cormorants (Phalacrocorax auritus) nested in the Bolinas colony site in 2016. The first active Double-crested Cormorant nest was observed on 1 Jun and the last active nest was observed on 9 Aug. 2 Double-crested Cormorant nests were established, and of these 2 nests appeared to fledge at least one chick.

KENT ISLAND colony:
Kent Island was checked for nesting Ardeids approximately two times per month during the regular Bolinas colony site surveys.

  • Great Blue Heron - No Great Blue Heron nests were detected on Kent Island in 2016. Great Blue Herons were last observed nesting on Kent Island in 2013.
  • Great Egret - There were no Great Egret nests detected on Kent Island in 2016. Great Egrets were last observed nesting on Kent Island in 2012.

Download the 2016 Nesting Summary Report (pdf)


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. 

Further Reading:

Since 1990 ACR has monitored 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 disciplinesThe Atlas includes individual accounts of all known heronries in the region (over 150 sites) based on field studies conducted by Audubon Canyon Ranch and the San Francisco Bay Bird Observatory (you can also view the heronries in Google Earth). Numerous photographs by Philip Loring Greene illustrate the report.

Photo Credits: 

  • Len Blumin