Where the herds usually are, month by month.

The migration is not an event with a start date. It is a continuous, roughly clockwise circuit driven by rainfall and the grazing that follows it, and some part of the herd is moving at all times.

Two things people usually plan around are the calving on the southern short-grass plains early in the year, and the river crossings in the north in the middle of the year. They are at opposite ends of both the calendar and the map, which is why the best month depends entirely on which of the two you want.

The herds are typically spread across the short-grass plains of the southern Serengeti and the Ndutu area, on the edge of the Ngorongoro Conservation Area. These plains sit on volcanic ash, which produces short, mineral-rich grass — good grazing for animals about to give birth.

Calving usually peaks around February, and it is heavily synchronised: the bulk of calves arrive within a few weeks of each other rather than spread across the season. Predators concentrate accordingly, which is why this period is known for predator viewing rather than for scale.

The long rains generally fall over this period. The herds begin moving north and west off the short-grass plains as the grazing there is exhausted, and they tend to spread out through the central Serengeti.

These are the quietest and usually the cheapest months. Tracks can be muddy and some can become impassable, and animals are dispersed rather than concentrated — the trade-off for having the place largely to yourself.

As the herds move, the rut takes place, and the noise of it is one of the things people remember. Columns often move through the Western Corridor towards the Grumeti River around this period.

The Grumeti crossings are smaller and less predictable than the Mara crossings later in the year, and they are spread over a longer window.

This is the classic dry season and the period most associated with the migration. The herds are typically in the northern Serengeti, around the Kogatende area, and moving back and forth across the Mara River.

A crossing cannot be scheduled. Herds gather at a crossing point, sometimes for hours or days, and may then turn away and cross somewhere else, or the next day. Anyone promising you a crossing on a set date is promising something nobody controls. What can be planned is being in the right area with enough days to wait.

Elsewhere in the ecosystem this is also the easiest general game viewing of the year: grass is short, water is scarce, and animals concentrate at what water remains.

The short rains usually arrive around November, and the herds generally begin the move back south and east towards the short-grass plains, often through the eastern Serengeti.

It is a transitional month and a genuinely unpredictable one, which also makes it one of the quieter ones.

If you want calving and dense predator activity, look at late January through February. If you want river crossings, look at July through October and allow several days in the north rather than one. If you care more about having few other vehicles around than about the herds specifically, April, May and November are worth considering.

Whichever you pick, the resident wildlife of the Serengeti, the Ngorongoro Crater and Tarangire does not migrate. A trip built around those is good in any month.