Birds are alerting us about climate change
It is estimated that about 50 billion birds leave their breeding grounds each year and migrate to other warmer locations. The reason for their journey is not the cold of the winter, but the shortage of food. In Central and Northern Europe, a large proportion of native birds migrate to North Africa in search of insects and other food sources that they are unable to find in Europe.
However, it is not a peaceful holiday under the warm African sun, but a daily struggle with the local birds with whom they share food. That is why, although the return in spring is a huge wear and tear for the birds, it is worth returning to their breeding grounds simply because the competition for food there is less demanding: the days are longer, there is an abundance in insects and much higher chances of survival.
Climate change is revolutionizing everything and for many species having to adapt is difficult. More and more birds are choosing to remain in their territories even at the risk of not surviving. The prize is worth it: to be the first to occupy the territories of reproduction. A significant case is that of storks, who are increasingly seen parading through the snowy landscapes of Poland and Germany. Climate change has lowered temperatures by making their stay bearable thanks to the presence of sufficient insects.
In addition, as a result of this general rise in temperatures, the birds that continue to migrate have changed their flight charts, now choosing much shorter trips. Thus, England has become the new Spain for a growing number of Eurasian blackcaps, as well as for many cranes, who prefer to stay halfway and not continue their journey to the Iberian Peninsula.
The observation of bird migration patterns therefore provides us with good information on the effects of climate change on ecosystems and on the behaviour of many species. In Spain we have a privileged place from which to observe this phenomenon: the Strait of Gibraltar.
The Strait of Gibraltar is in a strategic position for the scientific study of migration, since this is where, according to data from the Migres Foundation, the migration routes between Europe and Africa are made up of 400,000 flying birds and several million small birds (swifts, swallows and airplanes, bumblebees, etc.).
The III International Congress of Bird Migration and Global Change (Tarifa, Natural Park of the Strait, 3-5 September 2018) will highlight studies that allow us to observe from this privileged place of the planet, a veritable hotspot of biodiversity by sea and sky, which there are very obvious changes.
Due to global warming, thermal optimum of many species are increasingly to the north or at higher altitudes. It has already been found that the distribution areas of some birds are increasingly to the north and many of them are colonising new areas where they were not previously found, provided they are available.
For example, in the case of high-mountain birds, there comes a time when they cannot move further up, and something similar happens with those living in the Arctic tundras, as there is no more territory to colonize in the north.
Moreover, as a result of these movements, even changes in the average body size of some species have already been detected.
A study involving Spanish researchers, framed within the Montes Consolider project, already showed that between 1990 and 2008, the average European temperature moved north 249 kilometres. To remain in similar climatic conditions, the species should have moved the same kilometres in the same period of time. However, this international study reveals that on average, bird communities in Europe moved north only 37 kilometres, while butterflies would have moved only 114 kilometres. Therefore, researchers warn that birds and butterflies are not following in the footsteps of climate change at a sufficient rate and that a gap is accumulating that scientists call "climate debt".
This also means that many communities of birds and butterflies that once lived together in the same habitat no longer coincide; so many birds that feed on caterpillars have no food, and that, in general, this could have the effect of reducing the availability of resources for a good number of other species.
"The study shows that climate debt is higher in birds than in butterflies, and this can lead to mismatches in trophic relationships between the two groups. In addition, both European birds and butterflies are living further and further away from their optimal climate areas and thus under thermal stress which makes them increasingly vulnerable to potential threats," Dr. Stefanescu, one of the project researchers, explained.
In the Strait of Gibraltar, some of these processes of change in species distribution areas have already been detected. There are more than 20 species of African-born birds that are colonizing the European continent in recent decades. Some examples: blue elanio, Moorish buzzard, spotted vulture, Lanner falcon, Secretary bird, Pallid swift, Moorish swift, Common bulbul, trumpet bullfinch. From an ecological point of view, it could be said that southern Europe is increasingly resembling North Africa.
As a result of changes in distribution areas, changes are occurring in the distances that birds have to travel during their migratory journeys. In the case of species that colonise or use territories further north, the distances they have to travel are increasing. For example, the Balearic shearwaters have to move further north during the dispersal season, because the shoals of fish they feed on are increasingly to the north.
In many other cases, birds are shortening migration distances because winter conditions are becoming less harsh and there is no longer any need to move. Short-haul migrants tend to become sedentary, that is, they stop migrating, and long-haul migrants do not make it across the Sahara.
These are some examples of migrants who shorten migration or who become sedentary: geese, many species of ducks, cranes, pigeons, rooks, mice, royal doves, storks, swallows, robins, thrushes…
Due to this phenomenon, experts monitoring migration in the Strait of Gibraltar are realising that some species are slowing down, even though their populations are stable or even increasing. This is because they are staying to winter further north and no longer travel to Africa. One of the most typical cases that we have all been able to observe in our fields and cities is that of the white stork.
Global warming is also bringing about changes in the calendars of biological cycles. Spring comes early and the plants sprout and bloom earlier. The phenology of invertebrates, the food of many bird species, is also altered: they begin to be active earlier and have much faster life cycles. All these changes also affect migratory birds, and there has been a widespread tendency to advance spring or pre-natal migration, as well as breeding dates. In some species we are talking about more than a month difference compared to the dates of 40-60 years ago.
However, some species fail to adjust their migration schedules and tend to show downward population trends. This is produced by a mismatch with the calendars of their prey (plants, insects): the birds arrive in spring and occupy the breeding areas, matching the peaks of food with the moment when they take out the chickens ahead. However, as spring has come early, many migratory birds arrive late and do not find enough food to feed the chickens (the food maxima have passed), and as a consequence the productivity of the population falls: do not produce enough chickens to maintain population levels. An example of this phenomenon is the case of the European pied flycatcher (photo).
Once reproduction has taken place, the birds move south for the winter: some will move to the African Sahel (transaharians) and others to the Mediterranean environment (dams). In this case, changes in the dates of passage are also detected.
In general, trans-Saharan migrants tend to overtake autumnal migration, i.e., they arrive earlier, raise earlier and leave earlier, probably to take advantage of the good conditions of the African Sahel and also escape from the increasingly hot and dry summers in southern Europe.
For their part, the Israeli migrants have to delay the autumnal migration, probably trying to spend more time near the breeding territories, and do not migrate unless the environmental conditions are especially harsh (which is getting strangers, because winter conditions tend to be less harsh).
As a result of many species expanding their range to the north, the migration periods observed in the Strait of Gibraltar have been extended and the length of passage tends to increase.