Will juvenile birds survive the dry future?

by Noémie Engel

More than half of all populations of shorebirds around the globe are declining, and climate change is a big part of the problem. The increase in temperature and the lower water levels in wetlands make it extremely difficult for shorebirds to rear their offspring past their first few months of life. In certain populations of cosmopolitan species, such as in the Kentish plover Charadrius alexandrinus, there are two strategies to withstand climatic adversities: they either migrate to areas where the environment allows for a more successful reproduction, or they become residents of isolated areas with a comparatively more stable climate.

Female of Red-capped plover *Charadrius ruficapillus* cooling her eggs by soaking them with her wet belly, a necessary parental behaviour to withstand extreme heatwaves in Australia. Photo by **Don Hadden**.

Because migratory populations are exposed to far more threats than resident ones, most researchers have focused their efforts on studying the resilience to climate change of migratory populations. However, resident populations, as they do not migrate, are much more exposed to resource overexploitation, habitat loss, and extreme weather events, especially if they inhabit islands.

Piping plover in the USA foraging next to a plastic water bottle. Photo by **Kim Smith**.

In 2007, our group began monitoring the resident population of Kentish plover on Maio, a small island in the Cabo Verdean archipelago situated off the West coast of Africa. After tireless efforts by students, researchers, and volunteers from the local NGO, Maio Biodiversity Foundation, we managed to mark approximately 80% of the population with unique colour-ring combinations for each individual. More than ten years later, and now with an immense database in our hands, we were able to study the demography of this resident population of Kentish plover. After rigorous statistical analyses, what we discovered is somewhat alarming.

Investigator measuring the tarsus length of a Kentish plover female upon capture (left). Kentish plover with coloured rings in the wetland Salinas do Porto Inglês, Maio, Cabo Verde. Photos by Noémie Engel.

We discovered that over the last decade, nest survival and nesting densities have been trending downward. In other words, individuals have fewer nests, and the nests they do produce are surviving less well. Although this sounds bad, it is well known that island bird populations do not reproduce as much as continental populations. It would be worrisome if there were not only fewer successful nests, but also if the adults producing them were surviving less. Therefore, in our most recent study, we wanted to estimate the survival rate of adult and juvenile Kentish plovers on Maio. But more importantly, we also wanted to understand how local climatic conditions affect their survival in the light of ongoing climate change.

Daily nest survival real parameter estimates ± 95% CI for Kentish plovers on Maio, Cabo Verde between 2007 and 2019 plotted across the years from the survival models with annual variation or a linear trend of year. Photo of a Kentish plover chick hatching at a nest on Maio, Cabo Verde. Photo by Noémie Engel, 2016.

The climate of Maio is dry. Seeing raindrops fall during the year is rare, except in summer and early autumn. With the arrival of rain from July to October, seaweed and eelgrass grow, and with them appear coastal worms, small mollusks, and crustaceans- the favorite prey of shorebirds. This is why the successful reproduction of the Kentish plovers largely depends on the availability of water.

Kentish plover eating a worm. Photo taken from **BirdFact.com**

Thanks to the conservation efforts of our team, now the great majority of the population breeds in a large, protected wetland and designated RAMSAR site: the Salinas do Porto Inglês. However, this protected area is not immune to climate change, more specifically to droughts. Cabo Verde historically suffers from cyclic patterns of dry and wet years and the local population and animals are suffering from this. This is why we particularly wanted to study how the survival of Kentish plovers is affected during dry and wet years.

Major habitat types in the Salinas do Porto Inglês in Maio. Panels show: (a) the grassland, (b), the saltmarsh, (c) the semidesert and (d) the salt-extraction. Photos were taken in August 2023 just after the first rainfall. Photos by Noémie Engel.

After crunching the numbers, we discovered three interesting results. First, we confirmed that while the resident population of Maio has low reproductive success, adults have a relatively high apparent survival compared to mainland populations of plovers. Second, we found that individuals in their birth year have lower survival than adults, which is coherent with our expectations. Last and most importantly, we found that juveniles that were born in a dry year are 10 times less likely to survive their first year compared to those that were born in a wet year.

Apparent annual survival rates ± 95% CI of juvenile and adult Kentish plovers on Maio, Cabo Verde under different climatic conditions.

Such effects of droughts on survival are concerning. As weather conditions might become more unpredictable in the future under ongoing climate change, the cyclic patterns of dry and wet years in Cabo Verde will change. Our results provide demographic estimates that indicate which age class to focus our conservation efforts to, particularly in dry years. Nonetheless, the question that arises after this long-term analysis is: will juvenile Kentish plovers in Maio survive the dry future?

Three Kentish plover chicks in the Salinas do Porto Inglês in Maio. Photo by Noémie Engel.

If you're eager to get involved in studying and conserving the Kentish Plover population in Maio, there are two direct ways you can help:

1) Join our research team in Maio by participating in a graduate program or volunteering.

2) Follow and support the Maio Biodiversity Foundation. Check out their events on social media to discover opportunities for hands-on involvement in the flora and fauna conservation of Cabo Verde