I can explain the science of it, the wonder of it, or I can show you.
Science will explain that the land of Israel is the bridge between Asia and Africa, perhaps even Europe. Science will explain that as it becomes cold in Europe, huge numbers of birds migrate south to Africa and then make the return trip as Europe begins to warm in the Spring. Science will tell you of air currents and pathways, of bird habits and such. Science can explain the what, but not the why - the why is found in the wonder.
The wonder of it is looking up and seeing huge amounts of birds flying in one direction or another, circling the land as if to show off their beauty, a thanks, perhaps, for our brief hospitality before they move on. Perhaps, just perhaps, they feel the holiness here and want to spend that extra bit of time flying through the air, looking down upon this wonderful land. The why, I think, is a joining of land and sky for that brief period of time, twice a year.
The pilots who fly our skies hate it but the rest of us, I hope, take the time to look up and welcome them. To see the colors, the shapes, the grace.
But sometimes words don't work nearly as well as pictures and so - I offer you the wonder of Israel's skies (with a thanks to @thekotel).
Bird Migration above Israel - Autumn 2011 from Yuval dax on Vimeo.