Bird populations are doing gradually better the further away they are from the hottest part of their distribution range, or...

... Success in the north, trouble in the south

Climate change is affecting our fauna in various ways, by influencing distribution range, population size, and migration and breeding phenology. For example, recent studies have shown that European bird species living in a colder climate (that is, generally more northern bird species), are doing relatively poorly compared to species used to a warmer climate (more southern species).

As far as distribution goes, shifts at the poleward limit of the distributional range, or at the upper edge of the altitudinal range, have been documented for many taxa. But beyond changes at range limits, more subtle changes within the ranges of species are also likely, and might have important ecological and evolutionary consequences.

In a French–Dutch–Swedish joint effort we studied how the different populations of 62 common European bird species have developed over 20 years (1989–2008), as recorded in national bird monitoring schemes. France and Sweden were each divided into three equally sized latitudinal belts, and the Netherlands was considered a belt of its own. The following independent factors were used when trying to explain recent population size changes within these latitudinal belts: the thermal distance (in °C) to the thermal maximum of their European distribution (how far the birds are from the warmest part of their range), habitat preference, habitat specialization, body mass, latitude and migration distance.

For the 62 species considered, we found a gradual and linear significant increase in long-term population growth rate along the thermal range, when moving towards a species coolest range limit. Accordingly, the bird populations were doing better and better the further away they were from their thermal maximum (put in another way, the closer they were to the coolest part of their range). This effect was detected beyond other effects expected to affect population growth rates, such as the decline of farmland birds and habitat specialists.

We failed to highlight that long-distance migrants were more prone to decline, as has been shown in several other studies, probably because the studied set of species was restricted, excluding some Afro-Palaearctic migrants that do not occur in all the three studied countries. Also, recent declines in long-distance migrants are most severe in seasonal habitats in Western Europe, whereas we studied a broader array of habitats and countries.

Thus, beyond previously known effects on population dynamics near range limits, we revealed that population dynamics were not randomly distributed within species range, suggesting that European breeding birds are influenced by climate warming, and are experiencing demographic disequilibrium, along their whole thermal range.

Jiguet, F., Devictor, V., Ottvall, R., van Turnhout, C., van der Jeugt, H. & Lindström, Å. 2010. Bird population trends are linearly affected by climate change along species thermal ranges. – Proc. R. Soc. Lond. B 277: 3601–3608. Read the full article HERE.

// Åke Lindström