An early start for Cassin’s
April 9, 2020
The ongoing COVID-19 pandemic has unfortunately delayed the arrival of our summer seabird interns, but the birds nevertheless are showing up in big numbers as we progress into the breeding season. The biggest story on the Farallones so far this year is the earliest peak laying on record for Cassin’s auklets. This species is fine tuned to its environment, where lay dates are highly correlated with the spring transition to wind-driven upwelling along the California coast. This upwelling transports nutrients from the depths up to the sun dappled surface waters to form vast blooms of phytoplankton, seeding the base of the food chain with a rich green soup. In response to favorable ocean conditions and an assumed abundance of krill that graze upon this plankton, Cassin’s auklets are arriving in force this year, as evidenced by signs of excavation of their earthen burrows and a chorus of calls after dusk.
We have been closely monitoring the breeding behavior and performance of known-age (banded as chicks) Cassin’s auklets on the Farallones since 1983 by checking the status of roughly 400 wooden and climate smart ceramic nest boxes around the island. The timing of their single white egg is one metric we keep track of, the data of which are visualized below in Figure 1. Over the years we’ve learned that when warmer than normal surface waters are present around the island in the spring, for example during the 1998 El Nino or the 2005 atmospheric blocking event, Cassin’s auklets will delay laying and wait until conditions improve. Thus, lay date for this species can be used as a measure of ocean productivity and health. If lay dates are early, we can assume there is an ample supply of krill around the island to allow for the initiation of breeding. Following delayed reproduction and a near complete reproductive failure for Cassin’s auklets in 2019, peak lay occurred on March 14th this year, the earliest in our long-term record! This bodes well for many other locally breeding bird and mammal species that also depend on krill either directly or indirectly, so long as favorable conditions persist throughout the spring and summer.
By Mike Johns