Study: Seabird chick survival linked to diet

Caitlin Coomber

You are what you eat, even if you are a seabird. So, what does it mean if a lot of seabird chicks start dying like California least tern chicks did in 2012? Can science link the die-off to diet, and the small forage fishes off the coast? Dan Robinette, an ornithologist, says “yes” and he has the poop to prove it.

At the recent Headwaters to Oceans (H20) conference, Robinette, a researcher at Point Blue Conservation Science, gave us the scoop on what he’s learned about least tern diets and their link to reproductive success, based on a decade-long study of terns at Vandenberg Air Force Base in Central California, as well as new ongoing field work in Southern California, supported by the Ocean Protection Council and California Sea Grant. Here is a distillation of that talk’s highlights:

  • Living off the fat of the land for a California least tern means having an abundance of 1-year-old Northern anchovy and young rockfishes near the bird’s breeding colonies. Without an ample supply of these fishes, the chicks’ chances of survival are low.

  • Least terns don’t benefit from “eating the rainbow.” A high diversity of fishes in the diet usually means the terns are facing shortages of their preferred prey.

  • A diet that includes Pacific saury – an offshore species – is also associated with low chick survival. Its presence suggests the birds are being forced to travel too far to find food. Besides going hungry, the chicks may become easy targets of predation when the parents are away.

  • Chicks that are being fed larval fishes throughout the rearing season also fare poorly. As the chicks get bigger, they should be eating larger prey, not lots of snack bites.

In June, Robinette completed his first round of surveying this year’s breeding least tern colonies in Southern California. The chicks are just beginning to hatch, he says, and the survivorship to date is mixed and still too early to tell.

In coming months, he hopes to begin analyzing seabird data in relation to fisheries and ocean climate data to further investigate the factors that influence seabird populations.

Below is a technical summary of his ongoing project with California Sea Grant:

Use of Estuarine, Intertidal and Subtidal Habitats by Seabirds within the MLPA South Coast Study Region
R/MPA-28 Jun. 2011–Sep. 2014
Dan Robinette, Point Blue Conservation Science, 805.735.7300,
Jaime Jahncke, Point Blue Conservation Science, 707.781.2555, ext. 335,

In this project, ornithologists are evaluating whether the new MPAs are adequately protecting seabirds, specifically pelagic cormorants, Brandt’s cormorants, Western gulls, black oyster-catchers, pigeon guillemots, California least terns and California brown pelicans, and if not, why. To do this, they are compiling and analyzing existing records of seabird populations prior to the establishment of the South Coast MPAs and also conducting new bird surveys at key sites. Last year, scientists monitored seabird breeding colonies, rooting sites and foraging rates on Santa Cruz Island, in La Jolla (where there is also a Brandt’s cormorant colony), at Cabrillo National Monument on Point Loma in San Diego and along the Palos Verdes peninsula in Los Angeles. The MPAs and special closures were established, in part, to protect roosting and breeding seabirds from passing ships, fishing lines and other human activities. As part of this project, scientists will be looking for evidence that the new regulations are reducing seabird behaviors, such as nest abandonment, that indicate disturbance. During the 2012 field surveys, researchers observed low numbers of chicks at all of the least tern colonies monitored. Fecal samples were collected to study whether their poor reproductive status might be due to diet. Findings from this project will be used to enhance and encourage science-based approaches to seabird conservation.

Written by Christina Johnson