www.expertafrica.com/country_info/Tanzania/Serengeti_wildebeest_migration.htm

Headed north when I did this if you miss the migration

you miss a lot... awesome animal herd views there...

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Many of the camps are actually closed in Selous in May, besides with the wet you can expect lots of brush and difficult to spot game. Would concentrate on the north and do include the Serengeti and Ngorongoro.

May is still 'low' season, but as June goes into 'peak' season, so expect big difference in prices.

If in May, consider staying in Central Serengeti; if June, the Western corridor.

I've been often in Tanzania and Kenya end-May/early-June and can count only one amazing thunderstorm in Tanzania... lasted maybe 1/hr and that was it. Some roads may get muddy, but won't inhibit safari.

Second question, you wouldn't be missing out on a Tanzania experience, but you'd just be having something very different than most visitors. It's estimated that 99% of the visitors to Tanzania head to the Northern Circuit of Serengeti, Manyara and Ngorongoro, with only about 1% doing the Southern Circuit, which includes places like Selous (pronounced Sel-oo) and Ruaha. To me Selous is a cross between SA, with its bush, and Botswana with its waterways.

In my opinion, you won’t mistake Selous for the northern parks and reserves: game-viewing is less active and the animals appear to be less habituated to humans (probably because Selous gets much less traffic and/or because of the hunting that's permitted in the vast majority of the reserve---about 90%--- to the south). Leopards are hard to see and cheetah are not present. Lions and dogs are likely sightings. On the other hand, Selous offers activities not usually provided in the north: fly-camping, boat safaris, walking safaris (with an armed Tanzanian Ranger) and fishing outings.

Most camps are closed April and May, so you'd need to arrive in June (when deals may be available). At that time of year the bush is very green and water will be found in the bush, making it unnecessary for animals to come to the lakes (where predators would lie in wait in the dry season) for water. Nevertheless you will see a fair amount of game (and lots of bird species) and if you like wild dogs, you're very likely to see them (you won't see them up north). What you won't see are lots of cars and people!

In summary, I think Selous is a place for those who want a less-congested African safari and who might want to do some activities in addition to game rides (doing afternoon boat safaris is a great way to end the day; sunsets are beautiful!). I heard from other visitors that Ruaha and Katavi were great for seeing predators, so I think combining Selous with Ruaha would make for a perfect trip.

Thanks for the replies. So if you could choose, would you choose late May / early June OR October. And south OR north?

Boy oh boy, I think you win whether you decide June or October! OK, just for the heck of it, go June and spend some time in the Serengeti then get back to Arusha and fly to Selous. The weather in June should be pretty good and you'll have experienced the best of both the north and south. The prices may be up in the north but they'll be down in the south. How's that for feeling strongly both ways?

Incidentally, I did a report on Selous that might be of interest to you:

http://www.fodors.com/community/africa-the-middle-east/some-thoughts-on-selous.cfm

Actually, now that I look at it, the dates would be late May. Does that change anything? I really don't want to get there and find its cloudy, or the wildlife is concealed behind tall grass.

I decided to take the plunge, though I've been known to change my mind after reserving a ticket. I'll be arriving in DAR on 5/20 and leaving on 6/2.