Month by month
Samburu in February
February is the hot end of the short dry window. At 32.1°C it is the second warmest month of the year, behind September's 32.4°C, with 29 mm of rain over four rain days. It is also the last month before the long rains change the shape of a visit.
If you only want the answer
Warm, dry enough and quiet, with the migrant birds still in residence. February carries the heat of September without September's rainless bush, so game viewing is good rather than exceptional. If the long rains are what you want to avoid, this is the last clean month until June.
The hot half of the short dry window
February's normals are 29 mm over four rain days, 32.1°C by day and 19.3°C at night. January sat at 30.9°C, so the month adds more than a degree of afternoon heat while the rainfall barely moves.
Only September is hotter, at 32.4°C. The difference is what that heat is doing: in September it comes on top of four rainless months and pushes every animal towards the river, while February's heat sits over country that still holds water from the short rains. The heat is the same; the game viewing is not.
The last clean month before the rains
March changes the year. Rainfall doubles to 68 mm and rain days double from four to eight, and April carries 70 mm. February is therefore the last month until June where the weather is predictable enough to plan a tight itinerary around, including anything that involves crossing the river between the two counties.
That matters more here than in a larger park. These are three small reserves, 131 km2 for Buffalo Springs and 239 km2 for Shaba, with Samburu itself put at either 165 km2 or 104 km2 depending on the source, and the driving is sandy river country. Once the luggas start running, a route you planned on a map can disappear for a day.
Birds: the migrants are still here
The Palaearctic migrants that arrive in November stay until April, so February gives you both halves of the list: the visitors, and the dry-country residents the northern reserves are known for. Vulturine guineafowl, Somali ostrich, golden-breasted starling, superb starling, lilac-breasted roller, white-headed buffalo weaver and both red-billed and Von der Decken's hornbill are the ones guides look for.
Species totals for these reserves need a warning. More than 350 species is the figure Wikipedia gives for Samburu National Reserve, without a source attached, and the 450 that circulates in the travel trade has no independent source we could trace. Treat 350 as the number you can defend.