Shollenberger Park once had a heron and egret colony which was a successor to a long-established colony. That colony was located in the eucalyptus trees near the wastewater treatment ponds off Lakeville Highway. In 2003 a pair of immature Golden Eagles set up residence near the wastewater colony at a critical time when nest building had just begun and scared all the birds away. Some of the displaced birds may have established the former Shollenberger colony, located north of Haystack Landing and across the Petaluma River from Shollenberger Park in giant eucalyptus trees. Those trees were cut down to make way for the freeway widening so that colony no longer exists.

Two more local colonies were established and were tracked: the Ellis Creek Colony, located in the eucalyptus trees off Lakeville Highway (perhaps a revival of the former colony which was dispersed by Golden Eagles in 2003) and the McNear Colony, which was located in several trees on the eastern shore of the Petaluma River just south of the McNear Peninsula. The McNear Colony is no longer active.

PWA docents are tracking the activity of the remaining colony for the Cypress Grove Research Center (CGRC) of Audubon Canyon Ranch and since 2022 for the San Francisco Bay Bird Observatory (SFBBO). This colony is one of over 70 sites monitored throughout the Bay Area.

Once the birds start to arrive in mid-March, the docents take scopes, maps, phones to the viewing site. Each nest is given a numerical designation while identifying the species occupying it. Every week until late July nests are monitored for the emergence of chicks.

Chick count is made at “Nest Stage 4” per the protocol developed by SFBBO. This is defined as “adults not continuously at the nest (may remain after feeding) and chicks (usually) on a nest platform.” Below is a record of chicks produced at Nest Stage 4 in the colonies.