|Vachellia and Senegallia species in massed flower near Longido, Jan 2012|
The other interesting observation is that the only record I have of the bulk of the butterflies stopping to feed, rather than just a few, is in the bush north of Meru, where they're apparently completely covering the Vachellia and Senegallia species that are in flower over there at the moment. Intersting observation that - I was up in the Loliondo area a couple of weeks ago and the flowering event was truly stunning - V. tortilis and S. mellifera in amazing abundance. How widespread is that? Does that have anything to do with the moment? I'm not sure, if you're in the bush tell me how many flowers you've got around you!
The other obvious things that might explain movement patterns is weather - in particular how fast the land is drying out, and what the wind is doing. Now, whilst some have simply been flying downwind and most of the butterflies I've been told about are not flying into the wind, but they're not simply flying with the wind everywhere, there is some movement to one side or another, so it does seem like they're not just passively blowing along in a random search for food - there is some direction to it as well. Check the wind charts here and compare with the interactive map if you need convincing...
So what is going on? Well, my best guess at the moment is that conditions were pretty good in the Maasai Steppe over the last few months - we had plenty of rain in November / December, so lots of food. Now it's drying out very, very fast down there, and food plants on which to lay eggs are vanishing, so a movement began. What the precice trigger was, I have no idea - I doubt there's any coordinating event, but there might have been some initial condition that suddenly hit across a very wide area - perhaps air pressure combined with drying? I don't know but it might be worth havinga little look at some data. Then once moving they headed initially north out of the Steppe until they saw the massif of the Pare Mountains blocking the way which alsomay have served as a sort of collecting point. Then they headed along the ridge NW, and were also blown west by a prevailing north-easterly wind. As this wind turned easterly near Kili, so did the butterflies and on they went, etc. I'm sure wherever they find suitable food plants on their way they'll be laying eggs (so we'll probably have another emergence in about a month - generation time for Belenois is about 30 days), and my guess is that's they'll probably just keep moving and laying along the way until they die. Though it would be interesting if they arrived en mass somewhere as they are reported doing in parts of southern Africa. The only way to tell is if everyone sends their records to me - no rewards beyond that warm feeling associated with being part of something... I think we should be able to combine everything into a note for some journal East African or Lepidopteran focussed - I think we've already got more information on this current movement than I can find out about any previous ones in the region and please let's get more!
Thanks to everyone for contributing so far. Keep the records coming in - and if you're just on your way back from safari, tell me where you've been this last week and what you've seen!