Final answer:
Researchers hold lighting conditions constant in experiments with pigeons and lice to eliminate the influence of light on the organisms and focus on other variables being studied.
Step-by-step explanation:
Researchers decided to hold lighting conditions constant in their experiments with pigeons and lice in order to eliminate the influence of light on the behavior and physiology of the organisms. By controlling the lighting conditions, researchers can focus specifically on the effects of other variables they are studying, such as the impact of different treatments or environments. This helps ensure that any observed changes or results are due to the specific variable being tested.