We discussed how Diamond (1969) documented immigrations and extinctions on the California Channel Islands by comparing his censuses of the birds of the islands with the birds recorded over 50 years earlier. Disregarding the numbers for San Miguel and Santa Rosa Islands, which were not well censused in 1917, Diamond showed that an average of approximately six bird species became extinct on California Channel Islands between 1917 and 1968. During the same period, an average of approximately five new bird species immigrated to the islands. Diamond suggested that his estimates of immigration and extinction were likely underestimates of the actual rates. Explain why his comparative study produced underestimates of rates of immigration and extinction.