Common Birds: the case of the Baglafecht Weaver and missing forests

Male Baglafecht Weaver, Mt Kilimanjaro

 If you live on or near an East African mountain, you're very likely to have Baglafecht Weavers in your garden. Like most of the other true weavers, they're a basic black and yellow colour. The first thing to look at in weavers is usually the colour of the eyes and legs: in Baglafecht weavers you'll always see a yellow eye (easy to see against the surrounding black feathers) and pink legs. Males and females differ slightly: males in the population in northern Tanzania and Kenya have only a black mask on the face, with yellow on the top of the head right down to the (black) beak. Females have an all dark head. In northern Tanzania the back of both sexes is essentially black, with some yellow wing edges, in other areas of Tanzania the back is greenish/grey and not as strongly contrasting. Juveniles of all forms are rather greener and lacking in black, but still have the yellow eye. Like other weavers, they weave their nests from grasses in colonies of 5-15 pairs (not usually in very large groups) and males in the breeding season are pretty noisy with their rather scratchy and squeeky song!

Baglafecht weaver nests aren't the neatest of affairs...

Map thanks to the Copenhagen Database

One obvious fact that I could have chosen to talk about would be the famous weaver nests themselves. However, I think I'll save that for a different species in time, since there's so much to talk about there and Baglafecht Weavers hardly have the tidiest woven nest of the family... Instead, the thing I think of whenever I see these birds is the remarkable insight they give us into how species evolve, and some African pre-history. Already above I've hinted that there are some identification challenges with this species, populations in the northern highlands being different from those in the south. And in fact the confusion is even greater than that: there are currently a total of eight subspecies described, many rather distinct. (See some of the variety of three Tanzanian subspecies in pictures on the TanzaniaBirds.net site here) In fact, Baglafecht weavers occur from Nigeria in the west, to Tanzania in the east, north to Ethiopia and south to Malawi. And yet, although it has a very wide distribution, within this area it is far from everywhere, restricted to the higher hills and mountains, some separated from one another by 1000s of kilometres (see map). Now, each of the hills and mountains contains is home to only one of the various races: Northern Tanzania has the race often called Richenow's Weaver, whilst southern Tanzania has Stuhlman's,and each other set of hills has it's own. And it's not just Baglafecht weavers that show this pattern - plenty of other birds other animals and plants also show similar patterns, present on many African mountains, but not in between with slightly different populations in each area. Yet it's pretty puzzling when you think about it: ultimately, all the individuals of one species share a common ancestor so at some point in time all these now isolated populations must have been continuous. But how can that be so, given that they're restricted to mountain forests?

Female Baglfecht Weaver, Arusha

The answer, of course, takes us into the past, quite far into the past, in fact. Just 12,000 years ago northern parts of the world were emerging from an ice-age that had lasted around 10,000 years. Most places currently with temperate climates where then very much colder than today. In tropical Africa, however, the impact was slightly different: it may have been a bit cooler, but the same conditions that brought ice to the north, resulted in drying in Africa. Using pollen collected from deep cores sampled from the bottom of lakes, scientists can identify the plants that lived around those lakes at different times in the past: the deeper you are in the core, the older the sample. And although there are still lots of gaps where there aren't sufficient lakes or people simply haven't had time and money to look, we've now got a pretty good idea of what happened to Africa's biomes during that last glacial period, about 18,000 years ago. Crucially, we can see that the plants currently associated with Afromontane forests occurred at significantly lower altitudes than they currently do, suggesting any of the species around at that time would have had to move. And, of course, there has been not just one ice age, but several over the last million or so year, meaning animals and plants will have been repeatedly moving up and down mountains - a process that has been thought to explain the interesting patterns of distributions among the montane bird communities, at least since Moreau studied birds in Amani.

However, recently advances in DNA sequencing means that we can analyse exactly how different each population is from another one, and there's a handy process called the 'molecular clock' that lets us actually determine to within a few 1000 years how long it has been since two populations were last in contact. (This 'clock' works because there are some parts of DNA that are not really subject to strong selection and accumulate mutations at a regular, and known, rate: roughly speaking a 1% difference in DNA corresponds to a 1 million year split.) Now, I don't think anyone has done this yet for Baglafecht weavers, but they have for other birds like starred forest robins and olive sunbirds. In these (and other) cases, it turns out that most of the populations diverged rather longer ago than we thought: one set of splits occurred around 1.7 million years ago, another around 1.3 million years, and a last just under a million years ago. These times correspond very nicely with periods when we think Africa was undergoing even more extreme drying that during the recent ice ages, suggesting that it's these extreme drying events that allowed the montane forest much lower than today, spreading in an arc that connected all the current fragments.

So by looking at the current pattern of distribution in Baglafecht weavers, we can deduce that Africa once (not even that long ago in geological time!) looked very different than it does today, with low altitude montane forest across much of the central region. And we can realise that our current impacts on global climate are very likely to result in some equally dramatic changes in biomes across the world - who knows where the Baglafecht weaver may have to live in 50 years time!

Main references:

Bowie, R., Fjeldså, J., Hackett, S., Bates, J., & Crowe, T. (2006). Coalescent models reveal the relative roles of ancestral polymorphism, vicariance, and dispersal in shaping phylogeographical structure of an African montane forest robin Molecular Phylogenetics and Evolution, 38 (1), 171-188 DOI: 10.1016/j.ympev.2005.06.001

Elenga, H., Peyron, O., Bonnefille, R., Jolly, D., Cheddadi, R., Guiot, J., Andrieu, V., Bottema, S., Buchet, G., De Beaulieu, J., Hamilton, A., Maley, J., Marchant, R., Perez-Obiol, R., Reille, M., Riollet, G., Scott, L., Straka, H., Taylor, D., Van Campo, E., Vincens, A., Laarif, F., & Jonson, H. (2000). Pollen-based biome reconstruction for southern Europe and Africa 18,000 yr bp Journal of Biogeography, 27 (3), 621-634 DOI: 10.1046/j.1365-2699.2000.00430.x