Survival of sparrows over their first year of life was measured on a large university campus. The nests of 20 breeding pairs were located by careful search, representing less than 1% of all the breeding pairs on campus. Chicks in each of these nests were given a unique set of colour bands on their legs so that they could be identified individually once they had fledged from the nests. A total of 60 chicks were banded, of which only four survived to the following year. The researchers estimated the survival rate in the population as 4/60. Do the 60 chicks constitute a random sample? What consequences might this have for the estimate of survival rate?