Capture-tag-recapture is a popular method for estimating a population of animals like birds, fish, or large mammals that make long range movements.

For this activity:

• Take a large number of white beads and place them in a jar.
• Draw out a handful and count them. This is the number in the first capture.
• Place in the jar a number of black beads equal to the number of white beads captured. These are the tagged beads.
• Thoroughly mix the beads so that the black beads are evenly dispersed in the jar.
• Draw a second handful of beads and determined the fraction of this sample that has been recaptured.

How can this be used to estimate the total population?

To take an example, say that you remove 27 white beads and you tag them by replacing them by black beads. In the second capture, you remove 42 beads, with 6 of them black.

What does that tell you?

• The number of tagged black beads is 27.
• The number of tagged black beads is about 6/42 = 1/7 of the total number of beads.
• Thus the total number of beads is about 27 X 7 = 189.

This census method works well for animals that thoroughly explore a habitat. The time between the capture and the recapture should long enough so that the the inhabitants of the population have had time to explore the habitat. However, thisinterval of time should not be so long that the inhabitants have a significant chance of dying or that a sizable new population has been born.