Migratory birds may seem to be well equipped to cope with the consequences of climate change, because their trait of seasonal migration is of course already an evolutionary adaptation to temperature fluctuations that characterise the seasons of Earth’s temperate climate zones.

But as we have discussed in our special piece about the effects of climate change on temperate zone biodiversity – it’s far more complex, and having evolved under relatively high natural temporal climate variation may actually be a handicap, when trying to adapt to globally rising temperatures under anthropogenic climate change. And birds migrating between Africa and Europe may be ahead of Asian and American species, facing early consequences.

Seasonally migrating bird species may seem to have developed a trait that may place them at a relative advantage to adapt to the consequences of anthropogenic climate change. But in fact their complex behaviour and linked habitat requirements makes these birds extra vulnerable, and European species may be ahead of this global development. Some studies show that birds that winter in Africa are most vulnerable, others state shorter-distance European migratory birds are most affected. Image: Forbes Science.

Already in 2006 a group of four researchers from the Netherlands Institute of Ecology (NIOO-KNAW) reported in a publication in Nature that they could attribute strong population declines for the pied flycatcher (Ficedula hypoleuca) to a climate change-induced trophic temporal mismatch, causing the birds to miss the peak in spring food availability their successful nesting depended on.

The pied flycatcher is a long-distance migratory songbird that winters in tropical (West) Africa and breads in Europe. The researchers write that in a comparison of nine Dutch breeding populations, they found that populations have declined by about 90 percent over the then previous two decades ‘in areas where the food for provisioning nestlings peaks early in the season and the birds are currently mistimed’.

This early food peak was not universal. In areas where the food peak was later early-breeding birds still arrived in time. However the researchers think that as climate change progresses similar temporal mismatches between interacting species will become an increasing problem:

“Mistiming as a result of climate change is probably a widespread phenomenon, and here we provide evidence that it can lead to population declines.”

Spring temperature, breeding and spring arrival date of a pied flycatcher population in the Netherlands from 1980 to 2000.

African-European migratory birds can offer an early climate warning

Now why would a study over ten years old that looks back at the late 20^th century already find such stark climate change impacts? That may have to do with continents-scale climate fluctuations superimposed on the underlying worldwide warming trend.

As we’ve discussed in our special article about possible ‘Sahel greening’ (part 34, this same series) anthropogenic climate change manifests a strong hemisphere warming difference – with the northern hemisphere warming much faster than the southern (mainly because the latter has a larger warming-delaying ocean surface). This hemisphere difference is further amplified over European-African longitudes due to both natural (Atlantic Multidecadal Oscillation) and anthropogenic factors (decreased aerosol pollution over Europe following stricter environmental measures) – both explained in more detail in our Sahel greening article.

As a consequence, between 1971-2000 and 1981-2010 the Dutch average temperatures have risen by 0.42 degrees (per decade) – more than twice the global average and indicative for relatively rapid warming over much of Western Europe.

And it’s not just temperature: in the same timeframe spring fog and stratus cloud cover have declined sharply, leading to substantially sunnier springs – which could be the most important trigger to activate insect life and other dietary species that early songbirds favour.

[For your information European flycatchers eat spiders, ants, beetles, bees, wasps, mosquitoes, moths – in other words anything that’s small and either walks or flies. In that context it is also important to note a general collapse of European flying insect populations, with a 76% measured decline (in Germany) between 1989 and 2016. This flying insect decline is probably caused by land use factors (agriculture, possibly insecticides like neonicotinoids, possibly amplified by inert effects of ‘ecological sinks’ in the landscape) and independent of climate development.]

Illustration of a temporal trophic mismatch following climate change. The ideal moment for the spring food peak is not upan arrival or nesting, but some time between hatching and fletching, when parents need to bring large amounts of food to their offspring. Image by Dutch Centre for Field Ornithology SOVON.

Yes, Africa is rapidly warming. But over recent decades West Europe has warmed faster than West Africa – promoting a temporal climate mismatch for migratory birds

In the same time tropical West Africa (where the pied flycatcher spends its winters) is also warming, but not equally fast. For the local climate development the latest IPCC report (AR5, 2014) refers to an extensive literature study by Jennifer Collins of the University of South Florida that was published in Journal of Climate in 2011. We cite from that publication a few relevant observations, illustrating that although Africa too is rapidly warming – tropical West Africa lags behind (the more rapid) European warming:

“Hulme et al. (2001) noted that throughout the twentieth century, Africa has warmed at a rate of 0.5°C century⁻¹ and from 1987 to 1998, the six warmest years in Africa’s temperature record occurred with increasing intensity making 1998 the warmest. From 1901 to 1995, they report that the Mediterranean coastal countries in northwestern Africa and inland southern Africa warmed at 2°C century⁻¹. However, over the same time period, a few regions such as Nigeria–Cameroon and Senegal–Mauritania actually cooled.”

What’s more important for migratory birds is the temperature development during boreal winter months – and there (20^th century) warming seemed to lag further:

“Hulme et al. (2001) note that there is slightly larger warming in the June–August and September–November seasons than in December–February and March–May.”

Collins reaches the following conclusion – about lagging winter warming: “Considering the months June–August, regions in both North and South Africa saw significantly warmer temperatures in the most recent period 1995–2010 than in the period 1979–94. However, for the months December–February, the significant warming was concentrated in the north of Africa.”

Is winter warming relevant for migratory bird departure – or is migration timing written in the stars? In both cases the response of Afro-European species may offer an early warning for expected effects for American and Asian migratory birds under future climate change

Back to European-African migratory birds – and a possible temporal mismatch some species may face under increasing climate change.

Are we right to assume that birds leave their winter grounds too late to catch up with the earlier onset of European springs? Well, in an even older publication back from 2001 and also in Nature, two of the same group of Dutch researchers (Christiaan Both and Marcel Visser, NIOO-KNAW) already hypothesised that for some migratory birds, the difference of the warming speed between their winter grounds and nesting areas could actually be irrelevant, as the timing of their migration may not depend on temperature, but on an independent biological calendar:

“For long-distance migrants climate change may advance the phenology of their breeding areas, but the timing of some species’ spring migration relies on endogenous rhythms that are not affected by climate change. Thus, the spring migration of these species will not advance even though they need to arrive earlier on their breeding grounds to breed at the appropriate time.”

However, as they continue, again in the case of the pied flycatcher, it does try to start nesting a bit earlier – but still arrives to late in the European spring to catch up with the earlier timing of its insect and arachnid food availability peak:

“We show that the migratory pied flycatcher Ficedula hypoleuca has advanced its laying date over the past 20 years. This temporal shift has been insufficient, however, as indicated by increased selection for earlier breeding over the same period. The shift is hampered by its spring arrival date, which has not advanced. Some of the numerous long-distance migrants will suffer from climate change, because either their migration strategy is unaffected by climate change, or the climate in breeding and wintering areas are changing at different speeds, preventing adequate adaptation.”

In order words, the mechanism may be uncertain – but fact is the birds arrive after their trophic optimum, and that leads to measurable population decline. And because the speed of European spring warming in recent decades is globally unprecedented, bird species that migrate between Africa and (West) Europe may offer an early warning for similar effects in Asian and American migratory birds.

Comparing 100 European migratory bird species – some migrate sooner, some do not, and probably the ones that winter in Africa are most vulnerable to climate change

But then of course we have to exclude the possibility that it’s just a flycatcher phenomenon – there’s always one that oversleeps, right? Well, the temporal mismatch is probably a broad phenomenon, leading to various population declines among European migratory birds, although some species seem to adapt better than others.

Long-term climate-bird studies present similar trends. Above graph (study discussed below) shows earlier spring arrival dates over a 140 year dataset for migratory birds in South and Central Sweden.

We have to look at bird comparison studies to develop a better picture. And through the years there are of course several that are of prime interest. The first we looked at was performed by a large international group of researchers led by Lund University and published in Science in 2006 under the title ‘Rapid Advance of Spring Arrival Dates in Long-Distance Migratory Birds’. This study showed that in fact many European migratory birds do indeed advance the timing of their spring migration in response to climate change [so another study in favour of climate (temperature) over circadian rhythm for migration timing] and that it may actually be the long-distance migratory birds that show better adaption:

“[…] we show that long-distance migrants have advanced their spring arrival in Scandinavia more than short-distance migrants. By analyzing a long-term data set from southern Italy, we show that long-distance migrants also pass through the Mediterranean region earlier. We argue that this may reflect a climate-driven evolutionary change in the timing of spring migration.”

However, two years later, in 2008 a small research group led by behavioural biologist Anders Pape Møller of the Université Pierre et Marie Curie published a comparison of 100 different migratory bird species in PNAS that illustrated many European migratory birds have in fact declining population because of lagging behavioural climate adaptation. And this study reached the opposite conclusion about which birds would be most vulnerable, namely the ones that migrate over longer distances:

“Our study clearly showed that European migratory bird species with declining breeding populations in Europe in the last decades (1990–2000) responded the least to recent climate change as reflected by the temporal trend in spring migration phenology, or even delayed their timing of spring migration, whereas species with stable or increasing populations advanced migration.”

Their comparison, titled ‘Populations of migratory bird species that did not show a phenological response to climate change are declining’, included migratory birds that stayed in Europe all year (nesting in the North, wintering further to the South) and birds that migrate over longer distances, reaching tropical Africa to stay during winter months.

In the supporting information (PDF) appendix the researchers show all their data, including the full list of the 100 bird species, whether the date for their spring migration has shifted (days per year from 1970-2000), population size has changed (1 = small change, 3 = large change) and during which time interval (either 1970-1990 and 1990-2000), what their average migration distance is (expressed in latitude change) and whether these birds winter in (southern) Europe or in Africa. Part of this table is shown below:

These data show that almost all the assessed European migratory birds have advanced their spring migration over the last four decades of the 20^th century.

To pick a few birds of interest, for instance the common redstart (Phoenicurus phoenicurus) first showed a strong population decline that later stabilised. Climatic fluctuations may play a large role, as this bird favours the eastern Sahel region for its winter stay – an area that experienced strong droughts in and around the 1980s.

An example of a bird that shows continued population decline is the northern pintale (Anas acuta), a duck that breeds in northern Europe and splits in winter months in a population that breeds in southern Europe and one that breeds in the Sahel region and Nile Valley.

A migratory bird with even stronger continued population decline is the Black-throated loon (Gavia Arctica) also known as Arctic loon. This aquatic bird breeds in freshwater lakes in Scandinavia and looks for milder regions along Europe’s Atlantic and Mediterranean coasts during winter.

Other long-distance migratory birds with population declines are the spotted flycatcher (Muscicapa striata) and the common redshank (Tringa tetanus) a wader that breeds along intertidal habitats in northern Europe, and that splits in subpopulations during winter months, some birds staying in Europe, others flying to Africa.

Intertidal areas are of huge significance to waders and other aquatic migratory birds. The Wadden Sea that stretches along the entire Dutch North coast is the world’s largest intertidal area – and listed as UNESCO World Heritage. A lesser-known intertidal area in the Netherlands that’s also of large ecological importance is Het Verdronken Land van Saeftinghe (‘The Drowned Land of Saefinghe’) in the country’s southwestern delta – shown in the above video by WeLoveEarth.org. Although a comparatively small reserve, the Saeftinghe reserve is home to the largest Dutch breeding population of redshanks. (For comparison here’s the We Love Earth video of the Dutch Wadden Sea.)

By presenting the above examples we hope to show that it’s not an easy task to distil overall conclusions from available data, because European migratory birds themselves have very different habitat preferences and behaviour and although some trends clearly emerge (earlier spring migration and net population declines) it is another challenge to link those trends to driving causes, of which climate change is one of several candidates.

Slowly settling the science on climate change as a spring migration forcer?

Now a more recent study suggests there is also a similar trend of earlier spring migration over a much longer timescale. These researchers, led by Cecilia Kullberg of Stockholm University, have compared a historical dataset (1873–1917) of spring arrival to southern and central Sweden of 14 migratory bird species to present-day observations (1984-2013).

As they write in their 2015 publication in the journal Ambio over this longer timeframe of 140 years they measure a change in behaviour of spring migration that they attribute to structural climate change. Local geography seems to play an important role too:

“There was a larger change in spring phenology in short-distance migrants than in long-distance migrants. Interestingly, the results further suggest that climate change has affected the phenology of short-distance migrants more in southern than in central Sweden. The results suggest that the much earlier calculated arrival to southern Sweden among short-distance migrants mirrors a change in location of wintering areas, hence, connecting migration phenology and wintering range shifts.”

Lastly, illustrating a global climate change component: although European migratory birds may be ahead in this development, it’s not just a European phenomenon – migratory birds are known to respond globally to natural El Niño and La Niña climate fluctuations and all across Earth’s temperate climate zones long-distance migrants are beginning to show temporal mismatches in their spring migration. For instance a 2017 publication in Nature’s subjournal Scientific Reports shows that 48 North-American passerine species show earlier spring arrival, but that their migration is still outpaced by the vegetation response to spring onset. An earlier study using citizen science to monitor migratory birds along the US Atlantic coast found similar results.

What may further complicate the picture is that climate change not only manifests as an earlier onset of (old) spring temperatures, but also as precipitation changes (both increases and decreases) and in Earth’s temperate zones also as a decrease of cumulative frost. Now vegetation phenology is influenced by all these factors, sometimes leading to paradoxical results that may further amplify interspecies temporal mismatches.

A 2010 publication in PNAS shows this interesting phenomenon around the Tibetan Plateau, one of the most rapidly warming regions in Asia – where plants for the circadian rhythms also depend on very cold winter temperatures. Due to climate change in this area plant spring greening is actually delayed, these researchers find.

Now imagine you’re a caterpillar waking up early in spring sunshine hoping to eat a leave. And then imagine the delayed arrival of a migratory bird, hoping to feed on that local caterpillar population, finding it is starving due to its own temporal mismatch.

If you look at a thermometer it may seem a very slow and steady warming, but when you look at ecology, climate change becomes a very chaotic process. And species that may seem best adapted to cope with these consequences may be among the most vulnerable.

A famous example of a long-distance migratory bird that breeds in Europe and winters in Africa (following the flying insect optimum around the tropical monsoon rains) is the common swift (Apus apus). This very gracefully flying bird, here filmed by We Love Earth in Harlingen harbour (edge of Wadden Sea reserve, Netherlands) only lands to nest – and remains airborne during its entire winter stay over Africa, where it even crosses the equator. According to the PNAS study by Møller et al the swift population showed a small decline at the end of the 20th century.

© Rolf Schuttenhelm | www.bitsofscience.org