Capture-recapture is a method of census taking which biologists use to measure activity among bird, fish and insect populations. The simplest form is the two-sample model, which is used to estimate the size of a population.
In the two-sample method, a random sample of wildlife is captured. The subjects are counted and tagged for later identification and then they are released. Given an ideal scenario where there are no natural, biological, or human-related disturbances to the population, another sample of the same species is later captured. Based on the number of reoccurring subjects taken in the sample, an estimation of the current population can be obtained.
The capture-recapture method is scrutinized by some for being unreliable and having too many contributing factors to the outcome. Concerns raised involve whether a sample is taken during migrating season, or if the species lives in an isolated area with no other species involvement that would affect population growth. Another underlying concern is that there can be no births or deaths in a sampling period which would affect the outcome of the survey. More modern methods of capture-recapture allow for testing given these provisions and account for population change and the "catch-ability" or individual subjects.