Rookery Monitoring Research

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


Description of Monitoring
Each year, the rookery comes alive with the cacophony of nesting herons, egrets, and cormorants and their rowdy offspring. Volunteers and ESNERR staff have been monitoring the rookery for over a decade. We track abundance of adults and active nests for the three breeding species, track their reproductive timing, estimate reproductive success, and assess colony impacts to the trees they use.

Changes in Abundance
Each of the three species currently nesting in the rookery has shown different patterns of abundance over time (see graph). One pair of Great Blue Herons first appeared in 1985. By 1992, there were about 30 nesting pairs. In recent years, however, the numbers have dropped to about half that. In contrast, Double-Crested Cormorants only began nesting in the rookery recently, in 1997. Their numbers increased dramatically; in 2001 we had about 80 nesting pairs. Great Egrets first arrived at the rookery in 1992. Since then, their numbers have remained relatively steady, probably in the 50s or 60s. (Counts in recent years on the graph represent only those birds visible from the levee; additional nests were hidden from view in trees further back.) We will continue to monitor abundance of these three species over time.

Reproductive Timing
Our monitoring from just the past few years suggests that there are differences between species, and between years, in the timing of reproduction at the rookery. Herons begin (and complete) nesting first, followed by egrets, and then cormorants. Our data also suggest that breeding begins at different times between years. Combined with data from the nestbox monitoring, the rookery may serve as a sensitive indicator of biological responses to interannual climactic variation at the Slough.

Reproductive Success
While assessing reproductive success from afar is challenging, it is crucial. Each year, we attempt to count average number of juveniles per nest during two staff-led group census day in June and July. Our goal is not precision, but just a rough estimate. This should allow us to detect catastrophic breeding failures, so that we can mobilize efforts to test for DDT or search for other possible sources of problems if they arise.

Consequences to Trees
Our monitoring also highlights dynamic plant-animal interactions. While the birds benefit from the presence of tall trees, the reverse is not at all true. Indeed, the birds appear rather rapidly to have lethal effects on the Monterey pines that host their nests. We will attempt to reconstruct from photos and notes the past patterns of tree colonization and death. For the future, we will record these patterns more carefully. In 2000, Kenton and Kerstin attached forestry tags to all of the Monterey pines in the rookery area. Annual censuses reveal how frequently new trees are colonized, and how long heavily occupied trees survive.

The size and success of this rookery is likely related in part to the quality of wetland habitats in the area. Indeed, the rookery first became established shortly after the restoration of nearby wetlands which provide a rich source of invertebrate and fish prey for both parents and their offspring. We track the rookery size and success as one indicator of the health of our wetlands. We also monitor the timing of reproduction which may be useful for tracking interannual climatic variation or long-term climate trends.

Results 1985-2010
The rookery has been monitored since 1985. General trends in the data include a decrease in herons, increase in cormorants, and fairly steady numbers of egrets over time. The species vary in their reproductive timing. The trees they use are killed by guano cover within only a few years of colonization. In 2008, the rookery moved from the South Marsh Loop area of the main Reserve to the Seal Bend portion of the Reserve, in the eucalyptus grove adjacent to Moonglow Dairy.

How to get involved
If you are interested in carrying out rookery monitoring, contact Kerstin Wasson ( email ). Monitoring the rookery involves periodic surveys of the Seal Bend eucalyptus grove.