Birds could respond to the consequences of the world’s changing climate by shifting distributions, altering their bodies and experiencing microevolutionary changes
Migratory birds and the increase of global temperatures
Between 1880 and 2012, the average global temperature increased by zero-point-eighty-five degrees Celsius (IPCC Fifth Assessment Report). This warming of the global average temperature has caused changes in the atmosphere, biosphere, cryosphere, and ocean, affecting humans and other animal species that inhabit the planet. Avian species are being affected by the degradation or loss of suitable habitats, the spread of invasive species and extreme weather events such as droughts, sea-level rise, wildfires.
«Migratory birds take advantage of resources separated in space and time, migrating to follow good conditions. They are present on their breeding ground during the peak of resources and absent during periods of scarcity. In the Arctic, during summer, there is a biomass explosion with the permanent light at those high latitudes, which increases the amount of food available for the birds. During that period, they can nest and have enough food and energy to do such activities. But it’s the opposite in winter. The weather during those months is cold, and the conditions are rough, as there is no more light at such high latitudes.These areas experience the polar nights between three and five months. Birds who better track conditions, move southwards to have better food and temperature conditions», says Manon Clairbaux PhD, postdoctoral researcher at the University College of Cork in the United Kingdom.
«Due to the migration, migratory birds are threatened on their breeding ground, on their wintering ground, and during the migration itself in the stopovers they make along their journey. They are vulnerable in different places at different times. Migratory birds are imperiled by plenty of factors such as fishery for seabirds, invasive species, pollution, the destruction of habitats, and hunting. While they face plenty of threats, climate change is like an extra layer, a bonus threat to migrating birds. Climate change can impact their breeding ground and wintering ground in different ways, meaning that they may need to adapt multiple times during the various periods of the year».
The morphological and evolutionary impact of climate change on birds
As endotherms, birds have endogenous heat production, and are thus able to maintain a stable body temperature independent of the thermal environment they find themselves in. This allows them to keep a steady and suitable body temperature in various climate conditions. Their endothermic metabolism is a feature that has allowed the wide distribution of avian species worldwide in aerial, aquatic, and terrestrial habitats (Yahav, 2015). Like other warm-blooded animals, such as humans, their thermoregulatory ability has its limits. They rely on adaptation to avoid overheating and balance their body temperature with that of their surrounding environment.
«Climate change can impact migratory birds in a direct manner by changing their metabolism. With higher temperatures, they can decrease their energy expenditure to try to maintain their body temperature. But it can also impact migratory birds in an indirect manner, influencing their habitats or their preys. Climate change can mutate the phonology, which is the timing of the migration, because migratory birds decide to migrate from point A to point B thanks to environmental cues, such as the changes in light, temperature, and wind. Climate change, by modifying those indicators, can alter the timing of the migration. We have already observed that some birds are migrating earlier, and that’s due to climate change», explained Clairbaux. «It could impact their spatial distribution: birds could migrate less far in the South if the temperature increases at high latitudes. The birds that migrate are connected to their prey and predators, and all those species can be impacted by climate change in different ways. If the manner in which climate change affects their distribution in space and time differs, there could be an imbalance between prey and predators that can change the food web».
Researchers from the University of Turku and Lund University have found that birds respond to these changes caused by the world’s changing climate. They shift distributions, developing phenotypic plasticity of the thermal and metabolic responses to temperature, and experiencing microevolutionary changes in the endocrinological and molecular mechanisms that regulate temperature tolerance or plasticity.
During the last four decades, global warming and increasing summer temperatures have affected the size of North American migratory birds. A team of scientists from the University of Michigan and the Field Museum have demonstrated this through the analysis of a series of 70,716 North American migratory birds’ specimens from fifty-two avian species spanning over four decades. From 1978 to 2016, rising temperatures have induced across avian species, presenting ecological and phylogenetic differences. This is a persistent decrease in the specimens’ tarsus, mass, and PC1, which are the birds’ indices of body size, while wing length has been increasing as a possible form of compensatory adaptation.
Climate change can have a negative impact on the distribution of migratory birds, impacting biodiversity
Researchers from the Universidad Autónoma de Guerrero and the Universidad Nacional Autónoma de México have found that climate change could cause shifts in the distribution of thirty-four species of birds found in the humid montane forests (HMF) ecosystems of Mexico. The study has highlighted the vulnerability of the bird species from Mexican HMF ecosystems to climate change. Their current suitable areas would shrink under climate change scenarios, and the growing network of protected areas in Mexico do not encompass in full its biological heterogeneity: almost half of the analyzed species overlap with less than ten percent of the protected areas in current and future scenarios. Over the past thirty years, the population of migratory birds flying through the key flyways has decreased by half, while in the past decades, there has been a significant change in the geographical distribution of birds. This is due to climate change and land conversion affecting biodiversity and the connected ecosystem services.
Teams of scientists from the College of Environmental Science and Engineering Institute of Hydroecology, the Ministry of Water Resources, and the Chinese Academy of Sciences in Wuhan, China, have analyzed the habitat change and population centroid shift for a diverse series of birds from various areas of China. These birds were from part of the IUCN Red List of migratory birds, twenty-three of them. They found that the habitat area of migratory birds will decrease in the future, with climate change as the chief cause of this reduction, and that the transferred suitable areas may overlap with those inhabited by humans. This will in turn reduce the number of available suitable areas for birds whose migration will become more challenging due to habitat loss and shift of the population centroid.
Government can protect migratory birds from climate change, and the industries
On 7 January 2021, the Trump administration limited the scope of the Migratory Bird Treaty Act (MBTA) through a rule that allowed the MBTA to cover the sole intentional harm inflicted to protected birds. This left the preventable and so-called unintentional injuries and deaths of protected birds to go unpunished, narrowing the misdemeanor punishable under the act and favoring industries and developers.
This contested move was revoked on 4 October 2021 by the US Fish and Wildlife Service with a new rule that restored the Migratory Bird Treaty Act in its previous form. This made companies and individuals liable and punishable again by the federal government for the accidental deaths and injuries inflicted to the specimens from the more than 1,000 avian species covered by this act. A 2020 study by researchers from the Utah State University has highlighted how in the USA, to halt biodiversity loss and protect endangered species, investments in both public and private land conservation are needed.
Listening to scientists
On 2 August 2021, a comprehensive study developed by an international team of seventy-nine scientists was published in the Conservation Letters. Through tracking, phenology and population data, the scientists behind said study were able to identify a hotspot in the North Atlantic extending from ∼41 to 53˚N and 32 to 42˚W. It is used each year by up to five million seabirds from at least fifty-six colonies, a key location for several globally threatened species like the Bermuda Petrel and the Atlantic Puffin. At the time of the publication, the hotspot status as a marine protected area (MPA) was under consideration by the OSPAR Commission (Oslo-Paris Convention on the protection of the North-East Atlantic). On 1 October 2021, the area was designated as the North Atlantic Current and Evlanov Seamount (NACES) MPA by the OSPAR.
Manon Clairbaux PhD
A scientist, researcher, and a postdoctoral fellow within the XROTOR project, studying seabird ecology in a wind farm context at the University College of Cork.