Some birds no longer migrate, others 'move', some shorten their trips, others lengthen them… climate change is turning everything upside down. We show you some of its most notable effects.
It is estimated that about 50,000 million birds leave their breeding range each year and migrate to warmer latitudes. The reason for your trip is not the winter cold, but the shortage of food. In Central and Northern Europe, a good part of the native birds undertake their migration towards North Africa in search of insects and other food sources that they are unable to find there.
However, it is not a peaceful holiday in the mild African sun, but a daily fight with the local birds with whom they share food. For this reason, although the return in spring supposes enormous wear and tear on the birds, it is worth returning to their breeding places for the mere fact that there the competition for food is less demanding: the days are longer, the insects more abundant and the chances of survival much greater.
But climate change is revolutionizing everything and for many species the new trend is quite different. More and more birds choose to remain in their territories even at the risk of not surviving. The prize is worth it: to be the first to occupy the breeding territories. A significant case is that of storks, which with increasing frequency can be seen parading through the snowy landscapes of Poland and Germany. Climate change has softened temperatures making your stay bearable thanks to the sufficient presence of insects.
Furthermore, as a consequence 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 Central European warblers, as well as for many cranes, who prefer to stay halfway and not continue their journey to the Iberian Peninsula.
Therefore, the observation of the migration patterns of birds provides us with good information on the effects of climate change on ecosystems and on the behavior of many species. In Spain we have a privileged place from which to observe this phenomenon: the Strait of Gibraltar.
A privileged situation
The Strait of Gibraltar is in a strategic situation for the scientific study of migration, as it is here where, according to data from the Migres Foundation, the migratory routes between Europe and Africa of 400,000 soaring birds and several million small birds converge. size (swifts, swallows and planes, bee-eaters, among others).
Changes in distribution areas
Due to global warming, the thermal optima 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 colonize new areas where they were not found before, as long as these are available.
For example, in the case of high mountain birds, there comes a time when they cannot move higher, and something similar happens with those that live in the Arctic tundras, since there is no more territory to colonize in the north. .
In addition, as a consequence of these movements, changes in the average body sizes of some species have already been detected.
Birds and butterflies uncouple their cycles
A study in which Spanish researchers participated, framed within the Montes Consolider project, has already shown that between 1990 and 2008, the average European temperature shifted northward 249 kilometers. To stay in similar climatic conditions, the species should have moved the same kilometers in the same period of time. However, this international study reveals that on average, bird communities in Europe moved north by only 37 km, while butterflies would have moved only 114 km. Therefore, the researchers warn that birds and butterflies are not following in the footsteps of climate change fast enough and that a gap is accumulating that scientists call "climate debt."
This also means that many communities of birds and butterflies that previously lived in the same habitat no longer coincide; So you have to think that many birds that feed on butterfly caterpillars would not have food, and that in general this could lead to less availability of resources for a good number of species.
“The study shows that the climatic debt is greater in birds than in butterflies, and this can lead to imbalances in the trophic relationships that are established between both groups. Furthermore, both European birds and butterflies live increasingly far from their optimal climatic areas and, therefore, under thermal stress that makes them increasingly vulnerable to possible threats ”, explained Dr. Stefanescu, one of the researchers of the project.
Colonization of African species
In the Strait of Gibraltar, some of these processes of changes in the distribution areas of the species have already been detected. There are more than 20 species of birds of African descent that are colonizing the European continent in recent decades. Some examples: blue elanio, moorish buzzard, spotted vulture, borni hawk, saharan corridor, kaffir swift, moorish swift, orange bulbul, trumpeter bullfinch. From an ecological point of view, it could be said that southern Europe is becoming more and more like North Africa.
Changes in migration distances
As a consequence of changes in distribution areas, changes are taking place in the distances that birds have to travel during their migratory journeys. In the case of species that colonize or use territories further north, the distances they have to travel are increasing. For example, Balearic shearwaters have to move further north during the dispersal season, because the schools of fish on which they feed are increasingly north.
Species that stop migrating
In many other cases, birds are shortening migration distances because winter conditions are increasingly favorable and it is no longer necessary to move. Short-haul migrants tend to become sedentary, that is, they stop migrating, and long-haul migrants fail to cross the Sahara.
These are some examples of migrants who shorten their migration or become sedentary: geese, many species of ducks, cranes, wood pigeons, rooks, buzzards, red kites, storks, swallows, robins, thrushes ...
Due to this phenomenon, experts monitoring migration in the Strait of Gibraltar are realizing that some species are slowing down, even though their populations are stable or even increasing. This is because they are staying further north to winter and are no longer traveling 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.
Changes in passing calendars
Global warming is also producing changes in the calendars of biological cycles. Spring is early and the plants sprout and bloom earlier. The phenology of invertebrates, food for many species of birds, is also altered: they begin to be active earlier and have much faster life cycles. All these changes also affect migratory birds and a very general trend has been observed to advance spring or prenuptial migration, as well as breeding dates. In some species we are talking about more than a month of difference compared to the dates of 40-60 years ago.
Some are late
However, some species are unable to adjust their migration calendars and tend to show descending 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, making the food peaks coincide with the moment in which the chickens are brought forward. But as spring has come early, many migratory birds arrive late and do not find enough food to feed the chickens (the food peaks have already passed), and as a consequence the productivity of the population falls: they do not produce enough chickens to maintain the levels populational. An example of this phenomenon is the case of the cerrojillo flycatcher.
Autumn migration changes
Once the reproduction has taken place, which tends to advance as already mentioned, the birds move south to spend the winter: some will do so towards the African Sahel (trans-Saharan) and others towards the Mediterranean environment (pre-Saharan). ). In this case, changes in the passing dates are also detected.
Advancement in migration
In general, trans-Saharan migrants tend to advance their autumn migration, that is, they arrive earlier, breed earlier and leave earlier, probably to take advantage of the good conditions in the African Sahel and also to escape the increasingly hot and dry summers in Europe southern.
For their part, pre-Saharan migrants tend to delay the autumn migration, probably trying to spend more time close to the breeding grounds, and do not migrate unless the environmental conditions are especially harsh (which is increasingly rare, because the winters tend to be more favorable).
Increased migration periods
As a consequence of many species expanding their ranges to the north, the observed migration periods in the Strait of Gibraltar have been extended and the duration of passage tends to increase.
By Victoria González