Globally, seabirds are among the most threatened groups of birds, and climate change is considered one of the top threats to their survival (Dias et al. 2019). In the oceans, increasing water temperatures lead to a more stable stratification of the water column, resulting in less nutrients in the surface waters. This can lead to a decrease in productivity and changes in the food web, leading to lower prey availability that can also affect top consumers (Schell, 2000; Hirons et al., 2001; Carroll 2015). Further, the increasing ocean temperatures can lead to changes in the distribution of species, with plankton and fish species from more temperate zones moving polewards. In the Barents Sea and European Arctic, one of the fastest warming areas on earth, we can already observe pronounced changes in the food web, a so-called “borealization” of the Arctic ecosystem. Adapted to their specific prey, those seabird species that are more widespread are doing better than those that are typical high-Arctic species (Descamps & Strøm 2021) These processes may be helped further by a retraction of sea ice that go along with increasing temperatures in the polar ecosystems (Descamps & Ramírez 2021).
Temperature is also an important trigger for phenology, i.e. the timing of seasonal activities such as the start of reproduction in animals or plants or the migration of birds. Across the globe, in marine as well as terrestrial species, there is a general trend to earlier timing of reproduction ascribed to global warming (Parmesan & Yohe 2003, Parmesan 2007, Poloczanska et al. 2013). A sufficient adaptation to climate change and variability appears particularly important in polar and subpolar regions, where the time window during which climatic conditions are suitable for reproduction is very short and strongly linked to the seasonal peak in temperature and light (Wiegolaski & Inouye 2013). Yet, species appear to react to the warming trend at different speeds, and within interacting species (e.g. predator−prey relationships), one can find more and more mismatches in timing, especially in high latitudes (Wiegolaski & Inouye 2013). Generally, lower trophic level prey species can advance their reproduction more than their predators and can. This leads to mismatches, for example in the timing of seabird reproduction and the peak of prey availability (e.g. Hipfner 2008, Shultz et al. 2009). Recent analyses have highlighted the limited ability of birds in general, including seabirds, to adapt their phenology sufficiently to track the effects of climate change (Keogan et al. 2018; Radchuk et al. 2019). At closer detail, it has been shown that advanced reproduction due to earlier spring onsets can be linked to seabird’s foraging behaviour, with surface feeding species showing responses towards earlier reproduction, while diving seabird species showed a stable phenological response (Descamps et al. 2019).
Rising temperatures also lead to rising sea levels – both through thermic expansion and more water in the oceans. This can be regionally problematic for seabird colonies located on low-lying islands (Reynolds et al. 2015). However, the effects of climate change on seabirds are complex and not limited to effects of temperatures alone. Wind patterns are changing globally due to climate change, with the belts of westerly winds located in the boreal zones moving polewards, which can affect commuting costs for seabirds (Weimerskirch et al. 2012), but also impact migration costs for terrestrial migratory species (Nourani et al. 2017).
Climate change also leads to an acidification of the oceans due to dissolved CO2, which can affect food webs further. Many marine organisms have calcareous shells or skeletons, and these are disadvantaged under rising CO2 levels, leading to changes in marine communities and food webs (Kroeker et al. 2013; Riebesell et al. 2018). The pelagic swimming sea snail Limacina helicina, is among those species known to suffer from ocean acidification (Lischka et al. 2011). These snails have a high fat content, and Black-legged kittiwakes rely on them as a food source during winter (Karnovsky et al 2008). As a result, kittiwake adult survival is reduced in winters with low amounts of pelagic sea snails in the Labrador Sea/Grand Banks area (Reiertsen et al. 2014). Ocean acidification may therefore indirectly affect seabirds through their diet.
Climate change further increases the risks for extreme weather events, such as heat waves and storms. While changes in the food web, and thus food availability and flight costs might lead to more indirect / subleathal effects, e.g. affecting breeding success, extreme weather events such as winter storms have the potential to kill adult birds (Reiertsen et al. 2021; Clairbaux et al. 2021). Due to seabirds being long-lived, their population trends are more sensitive to changes in adult survival compared to changes in chick production, and such impacts on adult survival will have much more severe impacts on population numbers than effects on breeding success. However, subsequent years with breeding failure can contribute and amplify population declines. Studies of which conditions affect seabirds outside of their breeding grounds are therefore important, in addition to causal studies within the breeding season.
References and further reading
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