COURTESY PXHERE
I was recently made aware of the island fox (Urocyon littoralis) of the Channel Islands, off southern California. They look like a gray fox, but they are about the size of a house cat. This is an example of “island dwarfism,” an evolutionary phenomenon associated with the small island races of elephants and rhinoceroses.
After the 1990s the island fox numbers began to decline catastrophically. Already beset by canine distemper and rabies, they also began being preyed upon by golden eagles (Aquila chrysaetos). Just as distemper had been previously absent from the islands, so had the golden eagle, and the foxes had no behavioral defenses against this predator.
I have found no suggestion that bald eagles (Haliaeetus leucocephalus) actively antagonize golden eagles, but their diets overlap and they are not found together even though their ranges broadly overlap in the western United States. Both species will prey on small mammals, but bald eagles primarily eat fish, while golden eagles eat a wider variety of mammals.
These species are not closely related, in spite of both being called eagles. However, they are about the same size and have very similar juvenile plumages. Because golden eagles are so rare in the eastern U.S., birdwatchers here have a tendency to mistake juvenile H. leucocephalus for the much less common juvenile or even adult A. chrysaetos. Both juveniles are dark brown birds with white markings on the wings and tail. However, juvenile bald eagles also have whitish feathers on their bellies and scattered around the rest of their bodies. The white feathers are also brighter in golden eagles, gleaming in the light.
If you are close enough, you can see that the juvenile bald eagle has a proportionately larger beak than the golden. It is a third of the length of the head, while that of the golden is proportionately similar to that of a buteo. In fact, golden eagles are closely related to the buteos and other hawks, while bald eagles are related to kites.
The eight Channel Islands are about 20 to 25 miles off the coast today, but the distance was less when sea level was lower during the Ice Age. The four northern islands were then joined together into one large one called Santa Rosae. It was during this time that the island fox dispersed onto the islands. Members of the Chumash and Tongva tribes also colonized the islands while sea level was lower, and after sea level rose again they took the semi-domesticated island fox with them from the northern to the southern islands.
The topography is rugged, with the greatest altitude on Santa Cruz Island: Devil’s Peak (2,429 feet above sea level). This is not optimal golden eagle habit; they prefer more wide open spaces. Furthermore, there are only three native land mammals: deer mice, an endemic spotted skunk, and the island fox. This is not a very interesting menu for a golden eagle. Perhaps consequently, the bald eagle was originally the species native to the islands, at least during the historical period. As they were focused on the abundant fish of the surrounding ocean, bald eagles did not prey on island foxes, at least not to any significant extent.
In 2006 on Santa Cruz Island, the first bald eagles to be born on the Channel Islands in over 50 years hatched. By the early 1960s, DDT and PCBs had eradicated H. leucocephalus on the islands, with the last successful nest recorded in 1950. In the early 1990s golden
eagles dispersed to the Channel Islands. Unlike the bald eagle, golden eagle populations remained stable across the continent between 1966 and 2009, according to the North American Breeding Bird Survey. They were less affected by DDT than bald eagles, possibly due to their diet of mammals rather than fish. Consequently, as western bald eagle populations dwindled, golden eagles filled the vacated places.
The native mammals of the islands were greatly supplemented over the years by the introduction of domestic animals like pigs, which had established feral populations, and wild species like mule deer. The populations of island foxes, already beleaguered by disease, now felt the added pressure of golden eagle predation.
Mainland gray foxes have a head and body length between 35-45 inches, while the island foxes are only 19 inches long, not including the tail. This makes them about the same size as marmots, prairie dogs, and ground squirrels, the usual golden eagle fare across the western U.S.
Within a decade of the arrival of the golden eagles, the island fox populations on all the islands were in freefall. For example, in the six years after 1994 the population on Santa Catalina Islands declined from 2,000 to 135. In the same period on Santa Rosa Island the population went from an estimated 1,500 foxes to 14. On some islands the few remaining foxes were rounded up and placed in captivity to keep them from going extinct. The feral pig populations on the island were eradicated in the early 1990s, removing a prey species from the golden eagle menu and increasing their focus on island foxes.
Between 1999 and 2006, the Santa Cruz Predatory Bird Research Group with help from the National Park Service and the Nature Conservancy trapped and removed all 44 of the golden eagles from the islands and relocated them to distant locations on the mainland.
Although efforts began first on Santa Catalina Island in 1980, between 2002 and 2006 an innovative restoration program released 61 young bald eagles on the northern Channel Islands. In 2007 the species was removed from the national Endangered Species list. By the end of 2014 there were at least 43 bald eagles resident in the Channel Islands.
As for the island foxes, their numbers have bounced back on all the islands. As of 2015, the population was 1,750 on Santa Cruz Island, 15 years after it had been reduced to 70.
Bill Chaisson has been a birdwatcher for over 50 years. He lives and works in Wilmot.
As your daily newspaper, we are committed to providing you with important local news coverage for Sullivan County and the surrounding areas.