Read time: 10 mins
Bird migration is an astounding feat of endurance and a wondrous phenomena to behold. 55,923 miles from pole to pole – this is the greatest migration distance ever known to the animal kingdom, and is routinely flown by the Arctic Tern every year. These birds live for up to 30 years: if one adds up the total distance they traverse in a lifetime, it’s the equivalent to going to the moon and back more than three times. What a beautiful, extraordinary thing. Experts estimate there are about 10,000 species of bird on the planet*, and roughly 40% of these are regular migrants. But what do rising global temperatures mean for these birds?
*In 2016, research led by the American Museum of Natural History suggested there are actually about 18,000 bird species in the world. The research findings suggest current methods of species estimations are outdated, and focused instead on using morphology to gain an estimate of bird biodiversity. This research goes against the traditional worldwide estimates, thus is controversial and not universally accepted – some criticise the research as an overestimate whilst others support the validity of the new findings.
For those of us who aren’t ornithologists, when we think of bird migration we tend to remember simplified school terms of ‘flying south for the winter’, and view it in black and white terms of moving away from the cold. It’s true, many birds that nest in the Northern Hemisphere move south as winter approaches and in the UK approximately half of our bird population leave us during winter – like the nightingale, swallow and spotted flycatcher who embark on an epic southbound journey across the Mediterranean to Africa. A large proportion of the bird population in North America and Canada are migrants too, routinely moving to wintering grounds in Central or Southern America. However, this migration is more to do with changing resource availability than simply aversion to cold weather – birds can and do survive extremely harsh winters. Whilst this regular seasonal long-distance movement occurs mainly for Northern Hemisphere birds, the truth is bird migration happens all over the world. For example: the carmine bee-eater has a three-stage migration across Africa, journeying from Zimbabwe and Zambia, to South Africa, to Equatorial Africa; great shearwaters move from tiny remote islands in the South Atlantic ocean upwards to the North Atlantic; and many tropical birds migrate from central to outer tropics to breed during the rainy season, returning when it’s dry.
In short, there are numerous kinds of bird migration happening in different directions all over the planet and the topic is still keenly studied by researchers trying to conclude exactly why. Scientists have many theories on the hows and whys of bird migration and navigation (explained here), and it’s a highly complex phenomenon. Why do some bird species migrate but others don’t? Why do some birds of the same species migrate when their comrades choose to stay? Why do birds who winter in tropical areas bother making the arduous journey back North in Spring? Questions like these are exciting mysteries that have many possible answers. Traditional leading theories like the ‘Southern home theory’ which postulates the evolutionary origins of birds expanded from the tropics up northwards into breeding zones are countered by arguments* of gradual dispersal during non-breeding seasons* for better survival chances. Genetic predispositions* and moving away from tropical diseases during breeding time* are just a couple of the many theories science has put forth on why some birds migrate. Unfortunately, bird migration isn’t as straightforward as we would like it to be, so climate change will likely have all sorts of ramifications.
CC is very likely to disrupt every stage of the migration journey: altering conditions in ‘home’ breeding grounds, migration ‘stop-off’ routes and in ‘wintering’ or ‘non-breeding’ grounds. According to research conducted by BirdLife’s international science team, CC will cause migration routes to be longer as suitable climate areas for breeding and wintering move further towards the poles. They estimate that a whopping 80% of European long-distance migrant birds will have to journey even farther, have more stop-offs, and waste more time travelling. For example, if CC trends continue the way they are, the European Bee-Eater will have to travel at least an extra 1,000km by 2070. Longer journeys mean increased mortality rates, as birds suffer increased predation and starvation from both unpredictable food supplies and higher energetic expenditure.
Devastating impacts are scheduled for our next generation, but serious direct effects are also happening now. A changing climate appears to be triggering early migration in some birds, meaning they arrive at breeding grounds too soon compared to others. Migration timing is a delicate and precise thing according to researchers, who have found that birds who arrive at the wrong time by even just a few days are put at a disadvantage relative to resident birds and risk low-resource abundance, missing out on nesting places and food. A University of Edinburgh study looked at 413 species across 5 continents, and found that birds are reaching their Summer breeding grounds on average about one day earlier per one degree of increasing global temperature, and not all species are adapting equally. Long-distance migrants appear to be less responsive than shorter-distance migrants to these changing temperatures, and late arrival in turn is affecting timing of offspring hatching and survival chances. Timing is a big issue: for example, if long-distance migrants manage to adjust their migration schedule and arrive a few days earlier, but the fruiting plants they eat have reached peak abundance a few weeks earlier, there is a mismatch effect called decoupling, where the balance between birds and their ecosystems is thrown off.
Migratory bird populations are declining much faster than resident birds*, and long-distance migrants have steeper decline rates than short-distance migrants*. Identifying exactly why is complicated, but CC is considered a main culprit. Research which looked at 743 estimates on the rate of Springtime advance from 86 years of related studies has found Spring is occurring earlier the farther North you go. This means emergence of the plants and insects that birds eat is happening earlier than the seasonal changes happening at the lower latitudes birds are departing from. For example, as far North as the Arctic, researchers believe Spring is arriving 16 days earlier than it was 10 years ago. There is a fragile balance between the seasons and a lot of different interlinked components like when plants leaf and flower, when insects are most abundant, and when birds breed, nest and hatch. Think of it like the ecosystem food web taught in school – one seemingly small change can impact everything around it, which then impacts everything around it, and so on. Basically, CC is putting the timing of everything out of whack.
“Whatever cues they’re relying on to move northward for spring might not be reliable predictors of food availability once they get there if the onset of spring at these higher latitudes is amplified by future warming,” Post explains. “The springtime emergence of the plants and insects they’ll eat when they arrive is happening faster than the changes at the lower latitudes those birds are departing from.”Eric Post, lead research author
Take the European Pied Flycatcher. In recent years, these birds have responded to the changing climate by breeding earlier, but the key insects that they feed their young have advanced even earlier, also in response to CC. By the time the birds are ready to feed their chicks, the Spring flush of these insects has already peaked, and they catch it too late. Migration and breeding schedules are thought by many to be fixed by an ‘internal biological clock’; for the Flycatchers, research suggests they may be able to adjust this clock, so there is some faint glimmer of hope for the future. However, not all birds are adjusting like this and many are still being held up by a mismatch in seasonal changes North and South. How the European Pied Flycatcher and countless other species will fare remains uncertain.
Global warming is happening fastest in the Arctic, which has a temperature increase of more than twice the world’s average, and climate models estimate the Arctic Ocean will soon be sea ice free each Summer. For arctic-breeding seabird species, this means change: studies have speculated how these birds will likely shift migration from North Atlantic to North Pacific, and others may adopt a high-arctic year-round strategy. CC will modify a range of ecological processes in the Arctic, and results from enforced changes to bird migrations are uncertain.
Weather change caused by CC doesn’t always mean warmer; high atmospheric pressure* in some places is one effect, and this could be negative for many birds like Guillemots, seabirds that look somewhat similar to penguins. High pressure can cause less upwelling, a process where deep cold nutrient-rich water rises to the surface – these birds dive to find food, and when nutrients remain on the seabed there is less food for fish, in turn meaning less fish for birds*. Winds are also changing, which matters for migratory birds because they use more energy flying into headwinds and use tailwinds to conserve energy. 2018 research from the Cornell Lab of Ornithology looking at wind patterns predicts that future climate scenarios will make it harder for North American birds to migrate South in Autumn, but may make it easier for them to return in Spring. Aside from obvious energetic costs, strong winds could deter birds from flying and upset migration timing. According to the lead researcher Frank La Sorte, it’s important to remember these wind changes will not happen in isolation: there will be other global factors for birds to contend with like changes in temperature, precipitation and land cover*.
As the Earth warms, so too the air holds more moisture: a world 4°C warmer than the pre-industrial era has about 28% more water vapour in the air, and this is not evenly distributed across the planet – meaning some areas will see more rainfall, and others will see less. Precipitation models are complicated and not all unanimous, but generally it’s thought that warm areas will become warmer and wet areas will become wetter. Soil evaporation will increase, snowpacks will be reduced, and droughts will be exacerbated even when rainfall hasn’t lessened*. These changes will wreak havoc on ecosystems and resources birds rely on, as well as directly impacting many species by putting increased pressure on thermoregulation processes and the associated endocrine traits. Though some species may be able to adjust, these extreme weather changes could be beyond the scope or speed at which birds are able to do so. Ultimately, this could lead to selection of more capable phenotypes and the extinction of who-knows-how-many bird species*.
These are only a couple of the problems posed by a changing climate to birds. We all know global warming is bad for wildlife, but many of us underestimate or are perhaps unaware just how intricate and delicate the balance held within the natural world truly is. Climate change is disrupting this balance in a myriad of ways, and the time to act is past; we are now on borrowed time, and drastic change needs to happen now.