One method of observing and recording animal behaviour.
In Ad Lib sampling you record any behaviours that seem interesting or important. It's subjective, and it's not focused or controlled in any way, so usually you would use Sequence Sampling, Focal Animal Sampling, or Scan Sampling.
Ad lib sampling can be useful when you are still getting your bearings, as it lets you see a wide range of behaviors, and learn what is normal and what is not. It can also be useful if you are familiar with the population, and you notice a change in behaviors; it may be useful to keep tracking normal behavior, but it also makes sense to focus on the new and interesting behaviors.