The Society for the study of flies (Diptera)

Hello,  Perhaps someone can explain the behaviour of flies that I noticed earlier in the summer.  Around a dozen settled on the motor cover of a brushcutter that had been used some time earlier and was still warm.  The time was 10.30 am, there was patchy cloud and little breeze.  The flies all faced the same way,  north, and all within a few degrees of each other.  The insects did not move around on the horizontal surface.  Now and then, one would fly up and then return.  Landing at any angle to the other flies, it would immediately jump round to face north again.  If the brush cutter was turned around by 45 or 90degrees the returning insects all faced as before.

Later in the day I put the machine down in the same place.  This time only a couple of flies landed and wandered about in a random way.

They seemed to be the ordinary sort of housefly, if that's not too vague a description.

So, could magnetism be involved , or is there some other obvious reason?   David