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The “Capture-Recapture” method is often used in biological research to estimate the population of a species. Consider one particular island, on which a rare species of bird has been found. Biologists capture 200 birds on this island and put a small tag on each bird. The biologists assume that the 200 birds are a random sample of all birds of that species that live on the island.

​The tagged birds are then released, and over the next few days, it is assumed that the birds completely disperse within the bird population on that island. A few days later, a sample of 150 birds is taken. Within this sample, 24 birds that have tags are “recaptured”.

Using this sample of 150 birds, estimate how many birds of that species live on the island.

If the number of birds recaptured with tags in the sample had been 25% higher, then by what percentage would the estimate of the total number of birds living on the island changed?

Number of birds on island

Percentage change




Increase 25%

Increase 20%

Increase 80%

Decrease 25%

Decrease 20%

Decrease 80%

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