I am most grateful to Dr Torben B Larsen, author of Butterflies of West Africa (see below), who has emailed the following very clear explanation of hill-topping.
Hill-topping is a well-recognised mate-location strategy. Males fly up and stay on the hilltop - for days on end if necessary. Females fly up if they want to be mated. Males spend their time flying about chasing others of their sex off the best part of the area - usually the very top. From time to time they perch and wait - that is when you see a male Swallowtail sitting on the ground. The male with the best teritory at the top of the hill has the best chance of mating with the occasional female, who knows the "top male" must be strong and thus genetically fit.
My own favourite memories are from the flat Kalahari savannah where Telegraph Hill - all of 60 metres high - attracted virtually all butterflies in the area. In the early morning, hopeful males made their way up. By noon the flat hilltop was a carnival of butterflies whizzing about at maximum speed. During a couple of hours more species could be collected than would have been found during several days' trawling of the surrounding flatlands. Females were only rarely seen following their chosen male down to copulate - but sitting in cop on the top of the hill would lead to extensive mobbing by other males trying to interfere.
In the Kalahari desert there were several species that I only ever found on hilltops. In the flattest part of the Kalahari, a hilltop may be no more than ten metres above the flat, dry savannah - that's sufficient."
Dr Larsen's monumental Butterflies of West Africa has 900 pages and 130 colour plates in 2 volumes. It is published by Apollo Books, Kirkeby Sand 19, DK 5771, Stenstrup, Denmark. Their website is www.apollobooks.com.