The saguaro forest itself is the habitat. Gila woodpeckers and gilded flickers excavate nest holes in the cactus trunks, which elf owls — the world's smallest owl — and purple martins then occupy. Watching a saguaro at dusk in early summer produces a sequence of tenants.

The desert washes lined with mesquite and palo verde are where the mammals move: javelina in family groups, coyote, black-tailed jackrabbit, and bobcat at either end of the day.

The Rincon Mountains in the eastern district climb from desert to pine forest, and the trails up hold a completely different fauna — black bear, white-tailed deer, and montane birds within a few hours' walk of cactus.

The nights are when the desert lives: rattlesnakes, kangaroo rats, ringtails and the extraordinary insect life. Ranger-led night programmes are the safe way to experience it.

The summer monsoon, from July to September, transforms everything — toads emerge from years underground to breed in temporary pools, and the desert greens within days.

Spring, from February to April, is the best all-round season with wildflowers, comfortable temperatures and cactus flowering in May. Summer daytime heat is genuinely dangerous.

Practicalities

Both districts are within half an hour of Tucson and are visited by scenic loop drives with trails leading off them. The Arizona-Sonora Desert Museum between them is one of the best natural history institutions in the country and the single most useful stop for understanding what you are looking at.

Carry far more water than seems necessary, hike early, and watch where you put your hands and feet — rattlesnakes are common and generally want nothing to do with you.