Pomona Valley Audubon -
Chaparral Naturalist Archives

Growing Up and Growing Old on an Island
By CHARLES T. COLLINS

The Island Scrub-Jay, Aphelocoma insularis, is a larger and darker version of the mainland scrub-jay of local residential areas and back yards. It is confined to Santa Cruz Island. These Island birds are an isolated population which is so genetically distinct that they have recently been recognized by the American Ornithologists’ Union as a full species.

Our studies of these unique birds began in 1974-75. The initial goal was to determine if they showed the very interesting social behavior called "cooperative breeding." This behavior has been well documented for some habitat-isolated populations of the Florida Scrub-Jay, A. coerulescens, and might, we thought, also occur in the Island Scrub-Jays. Cooperative breeding entails young birds remaining on the parental territory for one to five years waiting for an opportunity to replace a resident adult breeder on a nearby territory. During this waiting period they participate in the rearing of subsequent offspring of their parents by sharing in sentinel behavior and territorial defense, chick feeding and care of young recently out of the nest.

By 1978, our data indicated that the Island Jays did not have this cooperative breeding system. They showed the more typical system of monogamous pairs with young that disperse within a month or so of leaving the nest. During this time we also noted that very few of the banded birds we were observing disappeared as would be expected through normal mortality. Although not unexpected on an island with a reduced predator fauna, we none the less began a longer term (little did we know then, how long) monitoring of adult survival of SCI Jays.

Each bird, when initially captured, is equipped with three colored plastic and one U.S. Fish and Wildlife Service numbered aluminum band. By using various combinations of nine colors of plastic bands each jay is given a unique color code recognizable in the field from that of any other jay. Subsequently we have been able, year after year, to visit approximately 55-60 pairs of SCI Jays within walking distance of the Channel Islands

From our long-term monitoring we have been able to determine that the adult SCI Jays do indeed have a high annual survival rate with between 86% and 92% surviving from one year to the next. This is about 10-20% higher than most similar sized songbirds. Mortality is greater during the winter months when the weather is worse and when there is an influx of migrant hawks over wintering on the island. The territory-holding adult jays now appear, on occasion, to live to about 20 years! (Now you see why this study has taken so long!) One bird banded in 1975 as a breeding adult was still present 14 and a half years later. Since our data have also shown that it takes a young bird three to four years to become a breeder, this bird was probably a minimal 17.5-18.5 years old at death. Our calculations show that at least some SCI Jays should reach 20 years; several are currently 14-15 years old. Most, however, die at much younger ages.

Perhaps the most interesting thing to come from this study is the documentation of senescent mortality in an avian species. Even the most recent text books emphasize that annual survival and mortality rates of adult birds is constant with increasing age. We have been able to document that these jays, like humans and some other mammals, show a pattern in which the mortality rate steadily increases with advancing age. This pattern has only recently been documented for any avian species and is particularly clear in Island Scrub-Jays.

Luckily, Santa Cruz Island is a wonderful place to conduct long-term and undisturbed field studies. "Having to go out to the island" several times a year for our twice-annual population surveys and banding activities has hardly been a hardship Accordingly we expect to continue these studies for several more years.

Dr. Charles T. Collins is a professor of biology at California State University, Long Beach. An active ornithologist in Southern California, he has worked on burrowing owl relocation and the tern islands at Bolsa Chica. This article was reprinted from the Santa Monica Bay Audubon Society newsletter, "AUDUBON imprint."