|Willow warbler singing in Africa - 10g but probably headed to eastern Siberia...|
|Barred warblers are always a treat to see: headed to eastern Europe.|
|Spotted Flycatcher, headed to the forests of Eurasia|
|Eastern Olivaceous Warbler - headed to central Asia or south-eastern Europe.|
Now, it's pretty hard to interpret the main effects of region too much - it's quite possible that regional land use pressures are a factor (which is supported to a degree by the additional importance of habitat), but equally regional patterns in climate change also tend to vary together. So let's not worry so much about that for now (though it is far and away the biggest factor correlating with the declines - my guess from knowing the situation on the ground is that to date land degredation is far and away the biggest cause down here, not climate change yet), and focus on those factors to do with timing, because it's here that things get interesting in our part of the world- by watching when our birds leave here, we can help untangle some of the questions around departure / arrival dates. It has been suggested, for example, that it might just be that the birds don't know when to leave here: in most of the world we know that birds can monitor precisely changes in daylength to determine dates (17mins difference 'summer' to 'winter' is the shortest known detected difference by a bird). If the date of migration is 'hard-wired' into the bird (ie if it hatches with a programmed departure date), it takes evolution to vary that feature. That's probably even harder here on the equator, where there is essentially no meaningful day length variation. Here, we believe the only way birds can leave the area at the right time is by, somehow, maintaining a 'clock' throughout the year that counts the days and tells them when to leave (experiments keeping birds in constant environmental conditions demonstrate that this really does happen). What the mechanism of this clock might be, however, we have very little idea, but to maintain such a clock and then change the settings for a different departure date could be rather complicated, meaning these migrants are simply unable to change fast.
|Poor picture of a smart male Pied Wheatear, heading to eastern Europe|
Alternatively, there's a theory that suggests many long-distance migrants might be constrained in their departure date by having to wait for the start of the rains to encourage lots of insects out, so the birds can feed fast enough to fuel for their migration journey. [If that's the case, our early rains this year should be keeping everything very happy and maybe we'll see early arrivals in Europe this year too!] In other words, perhaps birds can happily survive here through the dry season, but they can't get the extra food they need to power a migration until the rains arrive and their departure is limited by when that happens - which doesn't vary as much, or in the same direction as changes have occurred to spring temperatures in Europe. Migration takes a huge amount of effort and energy (we covered some of it in the Wheatear Migration story) and the adaptations that some birds have evolved to undertake these journeys are extraordinary. Not only can they double their body mass with stored fat with no apparent heart-attack risk, but the physiological changes involved in migration are just extraordinary: birds feed like crazy to accumulate fat, but so costly is having excess weight, and so large the costs of migration that as they set off they metabolise some of their digestive system itself, an extraordinary adaptation that means on arrival or during stopover before they can feed effectively they must first rebuild their digestive system. It's not hard to believe, therefore, that they might ave to wait until feeding conditions are optimal before being able to start a migration.
If, like me, you find these adaptations extraordinary, even before you look at how the birds use magnetism, stars, sun and even sound cues to navigate, it's easy to forget how recent all of this has evolved. Although there might well be challenges involved in evolving different migration route or different migration times, evolution is an immensely powerful process. The whole of the current European/African migration system is, for example, a very recent affair: only 12,000 years ago most of northern and central Europe was covered in ice. So the current migrations have all evolved since then, whilst the average evolutionary age of birds involved in migrations (IE, when they last shared a common ancestor) is around a million years. Indeed, one of the most remarkable ideas coming from recent theory about bird migrations is that, once the basic ability to do partial migrations had evolved in birds (probably very soon after birds themselves appeared), it only takes around 25 generations (40 years) for a species to evolve complete migratory behaviour, or complete sedentary behaviour. So migration is actually a highly dynamic capacity of birds, and certainly far from fixed. Indeed, we're already seeing some interesting changes in migratory behaviour in Europe, the best example being Blackcaps which used to spend the winter around southern Europe or into north Africa (some make here here to Tanzania, usually in the forests), but a population has now evolved a completely different strategy involving heading north west from Germany to UK, where winters are getting warmer and there are lots of people who feed the birds. So effective has this migration been, that the population making this movement is rapidly increasing.
|Not all our warblers are migrants - ID this one and I'll buy you a drink... |
(Desperate attempt to get more people to comment, I know!)
Ockendon, N., Hewson, C., Johnston, A., & Atkinson, P. (2012). Declines in British-breeding populations of Afro-Palaearctic migrant birds are linked to bioclimatic wintering zone in Africa, possibly via constraints on arrival time advancement Bird Study, 1-15 DOI: 10.1080/00063657.2011.645798