In recent years, ornithologists have devoted increased attention to insular avifaunas and the problems they face (Scott et al. 1986). Many island species are critically endangered and require human intervention in order to survive. Island species also provide the opportunity to test differing ecological and biogeographical theories (MacArthur and Wilson 1967, Jones and Diamond 1976). However, many remote islands are not visited frequently enough or for periods long enough to provide information on population trends and the loss and/or gain of breeding species. Such has been the case for the Islas Revillagigedo off Mexico (Figure 1).