San Diego CBC results and local trends in tern numbers

Robert Patton

Thanks Justyn, Phil, & Paul,
for teasing out trends in San Diego CBC numbers.  Following up on Phil's observation of the declining numbers of Forster's and Caspian terns on local CBCs, Shuford et al. (2016. Numbers of terns breeding in California: trends or tribulations? Western Birds 47:182-213) noted significant reductions in inland breeding Forster's tern numbers and colony sites in the state, and less clear but still declining Caspian numbers.  Short-term cause included the prolonged statewide drought in the 2000s and impacts to wetland nesting areas (including loss of island habitat & crash of fisheries at Salton Sea in regard to Caspians).  Long-term influence from climate shift is evident in the resulting more frequent and more severe weather extremes and subsequent impacts on wetlands and preyfish.  Of course many of those inland breeders are our local wintering birds...
On the immediate local front, as Paul noted, the Crown Point/Kendall-Frost Forster's colony had very limited nesting last year.  Other sites that had supported colonies in North County in the past have had only intermittent nesting attempts in recent years (San Elijo) or been abandoned altogether (Santa Margarita, Batiquitos).  Likewise, we've not documented nesting at Sweetwater Marsh for several years now; and at South Bay Saltworks, nest numbers have dropped precipitously (from 419-438 nests in 2001 to only 12 to 53 nests in each of the last 8 years) with limited fledging success.  Again, climate shift plays a role by affecting local sea level rise resulting in more frequent and higher flooding of saltmarsh breeding habitat, as well as impacting availability of preyfish.  Expanding and increasing corvid populations and their predation on eggs and chicks have not helped either.  The adjacent South Bay colony at Chula Vista Wildlife Reserve has held on a bit better, ranging from 67 to 315 nests in each of the last 8 years.
Caspian tern nest numbers in South San Diego Bay have declined from an average of 356 nests each year 1999 to 2015 (range 180-574) to 163 in 2016 to 2020 (range 86-255).
In contrast, due to climate shift affecting sea surface temperatures and fisheries both here and in the Gulf of California, MX, elegant and royal tern breeding numbers in South San Diego Bay (and California) have increased in general but also with breeding groups shifting from colonies in Mexico to California during some years (see Velarde et al. 2015.  Warm oceanographic anomalies and fishing pressure drive seabird nesting north. Science Advances 1:el400210).  The average number of elegant tern nests in South Bay was 4051 each year 1999 to 2015, but jumped to 22750 for the last 5 years.  New colonies were also established in the San Francisco Bay area over the last 2 years.  Local numbers of royals both breeding and overwintering have also increased, with average South San Diego Bay nest numbers increasing from 118 nests each year 1999-2015 to 516 for the last 5 years (with a record 763 nests last year).
Tern, tern, tern...
Robert Patton
San Diego, CA