Hypersaline spray increases habitat heterogeneity and nesting density in an island-nesting seabird

Seabirds introduce aquatically-derived nutrients into their terrestrial nesting environments, often leading to vegetative overgrowth that degrades nesting habitat suitability over time. In this study we capitalized upon the process of salt suppression that naturally occurs in salt-spray plant communities in order to reintroduce habitat heterogeneity (the mix of open substrate and plant cover) on a common tern (Sterna hirundo) breeding colony in the Gulf of Maine. In 2019, we randomly assigned hypersaline spray and control treatments within 10 experimental blocks across the site, and recorded plant community and seabird reproductive responses. We compared the habitat heterogeneity resulting from salt treatment to estimates of optimal habitat heterogeneity measured within productivity monitoring areas at this site during 2016-2020. We observed an average reduction of approximately 24% plant cover in hypersaline plots relative to control plots (t(18) = -5.56, p < 0.001), constituting a substantial increase in nesting habitat heterogeneity. Common tern nesting density was 80% higher in treatment plots relative to controls (t(18) = 2.6, P = 0.020), yielding 88% more chicks hatched per square meter (t(18) = 3.3, P = 0.004). We suggest that the application of hypersaline spray may serve as an effective and practical habitat management technique in management-dependent systems such as tern nesting colonies.

Automated summary

Seabirds introduce nutrients into nesting environments, leading to vegetative overgrowth. This study reintroduced habitat heterogeneity by using salt suppression in a common tern breeding colony and found that salt treatment resulted in reduced plant cover and increased nesting density and chick hatching.