|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|
|Female Baglfecht Weaver, Arusha|
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!
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