The impact of the physical environment on the foraging energetics of shearwaters and the consequences for breeding success

The distribution and availability of food in the marine environment is highly variable over space and time. This creates significant challenges for marine top predators, including seabirds and marine mammals, as they must deploy an efficient movement strategy in their search for food, while navigating an ever-changing landscape of winds, waves and ocean currents. However, oceanographic features, such as tidal mixing fronts enhance productivity, which in turn attracts pelagic and demersal fish. Thus, there is a strong linkage between the dynamic physical environment and the predictability of prey assemblages. Nevertheless, the influence of the physical environment on the distribution of prey in space and time, and the subsequent consequences for marine top predator movement decisions, energetic costs, and offspring success have rarely been studied on a day-by-day basis. For breeding seabirds, their fine-scale movement decisions are typically constrained by the location of terrestrial colonies. This forces them to regularly navigate the variable wind and wave landscape between the terrestrial colony and the preferred offshore foraging areas, often separated by tens to hundreds of kilometres. In doing so they must constantly adapt their behaviour with consequences for route choice, energetic costs, and chick provisioning.
Our project will disentangle the effect of the physical environment on prey availability, movement decisions, flight costs, and chick provisioning using a case study involving Manx shearwaters (Puffinus puffinus) breeding on two colonies within the Irish Sea. Manx shearwaters breeding on Bardsey Island and the Calf of Man forage around a tidal mixing front, namely the Irish Sea Front (ISF). Manx shearwaters are adept fliers and an excellent model species in this context as they use strong winds and waves for low energy soaring but must beat their wings more frequently during low wind conditions, which is energetically more costly. Birds from the two study populations are known to target overlapping foraging areas around the ISF, but operate from opposite sides of the same windscape, resulting in different energetic consequences and foraging strategies. Therefore, variability in wind conditions and the maintenance of ocean fronts are critical to provisioning and successfully rearing chicks. We will monitor the movement decisions and flight costs of Manx shearwaters across two breeding seasons using solar charged GPS and accelerometer tags, and shorter-term high resolution accelerometer tags with inbuilt ECG (to measure heart rate). Accelerometers provide 3-axis acceleration data that measure behaviour and wingbeat frequency which can be converted into estimates of flight costs, along with records of heart rate. Fine scale environmental data will be extracted from across the study area, including wind and waves, while oceanographic models will be used to calculate the position and strength of the ISF over the course of the summers. Changes in the availability of pelagic and demersal prey (e.g. herring, sprat, cod, whiting) will be monitored using a moored echosounder over the course of the summer, while the spatial prey field across the ISF will be measured using ship based echosounder surveys. We will weigh adults and chicks before and after a foraging trip to determine the consequences of movement costs and foraging effort on adult fitness and chick growth.
Using projected changes to the ocean windscape, ocean warming and stratification, we will model the consequences of continued climate change on Manx shearwater movement costs and foraging success. The predicted reduction in the windscape and increased stratification could have disparate effects on the energy budget of breeding Manx shearwaters with consequences for population persistence. This project is critical if we are to understand the impacts of future climate change on these ecologically important species.