Which of the following methods for measuring populations utilizes recording signs of animals, such as droppings, tracks, nest counts, etc.?

The method that involves recording signs of animals such as droppings, tracks, and nest counts is known as the Index of Population Size. This approach is useful in estimating the abundance of a species without needing to observe individuals directly. By gathering indirect evidence of animal presence, researchers can infer population density and trends over time.

This method is particularly valuable when dealing with elusive or nocturnal species whose direct observation may be challenging. It allows for the monitoring of animal populations in various habitats, providing necessary data for conservation strategies and management practices.

The other methods listed do not focus on the indirect indicators of animal presence. Visual observation emphasizes direct sightings of animals, while ecological sampling typically involves systematic collection of data through various methods, and trapping techniques specifically rely on capturing animals for study. Each of these approaches serves a different purpose in wildlife research and management but does not specifically emphasize the use of signs as the Index of Population Size does.