one mammal species was apparently introduced deliberately by biologists. in the summer of 1957, seven california voles ("microtus californicus"), four males and three females, were introduced to bird island with the apparent aim of measuring the population growth rate on an unoccupied land mass. very quickly, voles were discovered to have crossed the of water eastward to brooks island and were reproducing there. within two years, they had spread across the entire island from west to east and had apparently exterminated the entire population of house mice. the biologists believed this occurred because the stress of competition caused the house mice to stop reproducing. today, voles are present throughout brooks island, but most notably in the grassland habitat, where the ground is riddled with burrows. the population has a very high frequency of buff fur color, thought to be a founder effect due to the small original population.