There are several active dens on the island, and fox kits may be observed during summer months. Small mammals, cached eggs, and perhaps walrus carcasses provide a winter food supply for these year-round residents. Each summer, thousands of seabirds also return to the islands to nest and raise their young. Nearly 250,000 seabirds nest on Round Island. Approximately 135,000 more seabirds nest on the Twins, and the other islands of the Sanctuary also support sizeable seabird colonies.
Round Island is also home to numerous passerine, raptor, duck, and shorebird species.
Including seabirds, more than 100 bird species have been observed within the Sanctuary. Walrus are counted daily at Round Island and other Bristol Bay haulouts to monitor the success of management policies and develop methods for a range-wide walrus census. Researchers have radio-tagged walrus at Round Island to identify other critical habitats and study seasonal patterns of migration. The effects of boat and airplane traffic near the haulout, as well as the Round Island visitor program, are also carefully monitored. In addition to walrus, staff monitor sea lion numbers, seabird populations and productivity, and other aspects of the island's ecology.