 How does the darkness affect the robots' mobility?
At midday some few of the blue sun's rays filtered through to the jungle floor, but now, late afternoon on the planet, the shadows were long and gloomy. The planet's double moon had risen and he could see a robot rolling slowly across the clearing in his general direction, blasting indiscriminately at whatever mind impulses came within its pickup range, birds, insects, anything. The twin moons cast pale, deceptive shadows that wavered and danced across the jungle floor, hiding debris that tripped him and often sent him sprawling into the dark. Behind, the robot crashed imperturbably after him, lighting the night with fitful blaster flashes as some winged or legged life came within its range.
 Their mobility is not affected, but it is harder for them to aim their blasters.
 It makes it harder for them to differentiate people from other animals.  Their tracking of animals is unimpeded, but they still have plans to contend with.  Their mobility is not affected, but it is harder for them to aim their blasters.  It makes smaller signals from insects more distracting.