Picking up the pieces: A clue-based science

Josh De Guzman

San Diego County provides critical nesting habitat for two species of small beach-nesting birds: the endangered California Least Tern and the threatened Western Snowy Plover. During the summer, these animals will create circular depressions in the sand, called scrapes, abutting the oceanfront, and lay their clutch in them. The parent birds will tend to those nests for nearly a month. During that time, they’ll incubate, protect the nest from predators, and forage close by until their eggs hatch. 

However, it’s a tough world out there for these tiny animals. Sometimes, their eggs don’t produce chicks that will fledge. Some of those eggs never even hatch. There are many potential reasons for this: rising tides and flooding, predation, and abandonment by parents are a few of those reasons. It’s a bittersweet feeling here at the San Diego Zoo Institute for Conservation Research, because we’d much rather watch nests succeed, although we can still be hopeful at the prospect of learning from what’s left behind. 

Ever since we have been monitoring the birds, we’ve been collecting hundreds of field samples, such as those non-viable eggs and other remains from all over Southern California, and have begun to analyze them using many different methods.

Examples of the data collected include analyzing the size of the tern and plover eggs and thickness of eggshells. Over time, we hope that these measurements provide an index between egg health (which depends in part on the quality of their diet) and the resources that are actually available during a given breeding season. These would complement the monitoring work we do every day: for example, necropsies by pathologists at San Diego Zoo Global have revealed that many of the dead least tern chicks we’ve recently found have starved to death, indicating that chick mortality is probably the result of poor foraging conditions out at sea, at least in part.

If a lack of food availability causes poor body condition in chicks, how would it affect their parents? If we have an index from opportunistically salvaged samples, this could be one way to provide clues without having to trap live birds and cause disturbance.

Some failed eggs are sent to a toxicology lab also in order to monitor trends in biochemistry, especially in DDT, mercury, and some trace elements. In the future we hope to explore tern and plover DNA, which would help us understand the genetic diversity of birds that frequent our shoreline. We’re currently working with nearly two decades’ worth of samples from all over the region, and that number grows every year.

Of course, this aspect of our research doesn’t come without challenges: it’s based almost completely on opportunity, and because of individual variation, being able to make inferences about whole populations will require many years’ worth of data— the differences which are usually very slight, but may be important. Of course, the results have huge discovery potential, especially considering they’re derived from otherwise forgotten remains. 

Effective conservation begins with understanding current trends and applying that knowledge to strategy. But that takes time. Our end goal for these analyses is to take our monitoring program to the next level by following seasonal changes at a micro-scale, and using those data to detect important trends, and make better-informed conservation planning and management decisions heading into the future.

Josh De Guzman is a Research Assistant with the SDZG Terns and Plovers team, working at Marine Corps Base Camp Pendleton, and at the Institute for Conservation Research.


Few questions

Fascinating! Always happy to read about conservation work and supporting research.
Have you found any overarching trends related to the biochemical assessments and shell thickness in these protected species?

"Our end goal for these analyses is to take our monitoring program to the next level by following seasonal changes at a micro-scale, and using those data to detect important trends, and make better-informed conservation planning and management decisions heading into the future."

What kind of management action currently exist and what kind of new data are you looking to collect/ would help to optimize the management plan?

Good luck and I look fwd to updates on this project.

Reply for Olivia

Thank you for your great questions!
Yes - as we measure more of our archived samples to note trends from a longer history, we will have a better understanding of these trends. However, at least from our preliminary work, egg size (especially egg width) varies among years in ways that are similar to how chicks grow that season. Poor years of chick growth often correspond with years when eggs size averages were small. For eggshell thickness, we are excited as we recently purchased equipment to enable us to make more accurate measurements. In the field, we are collecting some non-viable eggs with a range of eggshell thickness measurements, but including some as low as 0.14 mm, which are often very fragile eggs. However, for healthy eggs, we can only get measurements from those that hatched successfully, and only when we find the shell for those. Obtaining these averages, over time, will provide some minimum baselines for these ranges to note any annual trends. We continue to work on this, and obtaining more accurate measurements. Stay tuned!
For biochemical analyses, we have very small sample sizes to date (very costly, but also, limited samples going back in much earlier in time), but are finding that we do have some good news. The tern eggs we have analyzed indicate that those sites are becoming more healthy over time, with lower loads of some heavy metals and contaminants, but some components that are part of the egg composition (minerals), are decreasing in some cases - the latter could indicate potential changes with the diet of terns (their fish prey) but we continue to study this to see if this is a true pattern (it is quite new) and as funding and samples become available. We hope to have funding to do similar analyses for plovers.
We work with our site partners and collaborators to develop recommendations for conservation and management of the species on land and at sea (for terns). We have specific approaches that we use to deter predators - for example, by removing structures near nesting colonies that could serve as hunting perches for raptors, crows or ravens. We help guide and actively participate in habitat enhancement: beach clean-ups, invasive plant removal, and making suggestions for site managers for rebuilding dunes and reducing erosion. We also use some management tools to help nesting birds, such as setting aside areas with signage and symbolic fencing, using nest mini-exclosures for plover nests in high risk areas (to reduce depredation), and engaging in lots of outreach to help communities learn about regulations and strategies to help nesting birds and to reduce disturbance. We collect data on all aspects: breeding success, predators, and activities that may be considered disturbance. We say 'adaptive management' because we improve strategies annually or within the season to adapt to changes we see to improve recovery. Our research aims to fill in the knowledge gaps to learn about trends, the needs of the species, and to inform adaptive management. Thanks for reading, and come back again soon to reach our updates!

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