Tuesday, May 25, 2010

Iberia - at the crossroads between Europe and Africa

In recent posts, with the brief interruption from migrating Griffons, I have been looking at the Iberian mountains. Today I want to expand this theme a little and gradually get us back to the lowlands. The photographs have all been taken within the last month, barring some of the Blue Rock Thrush which were earlier. The northern Iberian mountains - the Pyrenees and Cantabrians - are the most southerly outpost for some birds that are typical of the forests further north. They belong to what biogeographers call the Eurosiberian zone, distinguishable from the Mediterranean zone to the south. The Eurosiberian, definable by climate characteristics, penetrates Iberia in these mountains and is marked by trees typical of western Europe and not the Mediterranean. Among the species that breed here, in their most southerly outposts, is the beautiful Bullfinch. The photographs (above and below) were taken in the Pyrenees so these are southerly Bullfinches!

But some species, typically those of more open habitats, have managed to penetrate (or at least survive the global warming that followed the last ice age) further south. We already met the Bluethroat (above) in our post of 30 April and another species typical of these mountain shrublands is the Hedge Accentor (below). Broadly speaking these are Eurosiberian birds that find adequate habitat in mountain peaks.
There are, of course, some remarkable Eurosiberian species that are able to live even in the lowland humid forests deep in the south-west, but they are not many. The European Robin is an example (below).

But not all Iberian mountain birds follow this pattern. The Red-tailed Rock Thrush (above and below) is a species that is typical of the chain of mountains that runs from Iberia eastewards to the Himalayas. I call this the mid-latitude belt (MLB) and it has its own set of species. These are not Eurosiberian birds, they are birds of the MLB, which in the west we equate with the Mediterranean mountains.

The closely-related Blue Rock Thrush is also a species of the MLB and it gives us a great comparison, a natural experiment, with its cousin. The Blue Rock Thrush is resident across many rocky areas, hillsides and low mountains in the western part of its range. Blue Rock Thrushes from the north and the more continental eastern parts are migratory. But in the mild west they can survive the winters without migrating. The smaller Red-tailed Rock Thrush is fully migratory, crossing the Sahara Desert to winter in tropical Africa. It seems to obey an unwritten rule which is that in closely-related species the smaller one migrates more than the larger. It could be a signal of competition between the species in some distant past or it might be that the larger species survives the northern winters better - having a lower surface-to-volume ratio which reduces heat loss. The net effect is that a lot of suitable but empty rock thrush territory opens up in the spring. Some may be filled by Blue Rock Thrushes moving up the slopes but most is left vacant. When the Red-tailed Rock Thrushes come in from Africa in April there is not much for them low down where the larger Blue Rock Thrush is by then established in breeding territories. So these birds find most opportunities high up in the mountains, in places that were inhospitable in the winter and could not support any kind of rock thrush.

Thrushes and chats are very versatile and Iberia, with its wide range of climates and habitats, harbours many species. Away from the mountains, at the other end of the climatic gradient, is an MLB species that is at home in the warmest and drier climates within the Mediterranean zone. It is, not surprisingly, among the last to arrive in spring from sub-Saharan Africa. The Rufous Bush Chat (above and below) is thus at the opposite end of the Iberian climatic gradient from its cousin the Bluethroat. They are here with us now and provide us with a good bridge to future posts that will return to the lowlands. Because we have accummulated a lot of material in recent weeks, when fieldwork has been intense, I will speed up the next few posts. So keep visiting!

No comments:

Post a Comment