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The Intergovernmental Panel on Climate Change predicts that sea levels will rise by up to 0.82 m in the next 100 years. In natural systems, coastlines would migrate landwards, but because most of the world’s human population occupies the coast, anthropogenic structures (such as sea walls or buildings) have been constructed to defend the shore and prevent loss of property. This can result in a net reduction in beach area, a phenomenon known as “coastal squeeze”, which will reduce beach availability for species such as marine turtles. As of yet, no global assessment of potential future coastal squeeze risk at marine turtle nesting beaches has been conducted. We used Google Earth satellite imagery to enumerate the proportion of beaches over the global nesting range of marine turtles that are backed by hard anthropogenic coastal development (HACD). Mediterranean and North American nesting beaches had the most HACD, while the Australian and African beaches had the least. Loggerhead and Kemp’s ridley turtle nesting beaches had the most HACD, and flatback and green turtles the least. Future management approaches should prioritise the conservation of beaches with low HACD to mitigate future coastal squeeze.

To maximise fitness, parents should optimise their investment in each breeding attempt. When there are multiple carers, the optimal strategy may also depend on the relative timing of their investment, with coordination of care hypothesised to maximise its efficiency and reduce predation risk. The aim of this study was to test the hypothesis that carers coordinate provisioning as an antipredator measure that reduces the time that a brood’s location is advertised to predators (‘predation hypothesis’). We presented predatory and non-predatory model birds to provisioning long-tailed tit Aegithalos caudatus parents and helpers, predicting that coordination would increase, and carer activity near the nest would decrease following predator presentation, relative to controls. First, carers reduced provisioning rates and took longer to resume provisioning following the predator presentation. Second, contrary to predictions, we found no significant change in any metric of coordination following predator presentations, relative to controls. Moreover, following predator presentation carers spent more time near the nest, resulting in greater near-nest activity compared to controls. In conclusion, although provisioning long-tailed tits are sensitive to perceived predation risk, our findings do not support the prediction of the predation hypothesis that carers adjust coordination behaviour in response to that threat.Significance statementParental care improves offspring condition and is often necessary for their survival but may also confer risk. In birds, provisioning a brood may advertise its location to predators. When multiple individuals provision the same brood, they are hypothesised to coordinate their visits to limit the time that a brood’s location is advertised. This hypothesis has not yet been experimentally tested, so in this study, we investigated whether carers increase their level of provisioning coordination in response to elevated brood predation risk by presenting predatory and non-predatory model birds near 22 long-tailed tit Aegithalos caudatus nests. As anticipated, carers stopped provisioning during predator presentation. However, contrary to expectations, carers did not coordinate more in watches following predator presentation, but instead spent longer near the nest before provisioning, suggesting an increase in risk aversion or vigilance behaviour.