For the Birds Radio Program: Cooperative Breeding in the Florida Scrub-Jay
The Florida Scrub-Jay is a poster child for cooperative breeding. The main reason ornithologists split this isolated group from Western Scrub-Jays is its family structure.
The basic unit of society in Florida Scrub-Jays is the mated pair, which virtually always remains together for life. The “divorce rate” among them is only 4 percent, and these cases virtually always arise when one bird is incapacitated or unable to breed. There’s about a 21 percent mortality rate among adult Florida Scrub-Jays. Many individuals have survived longer than a decade, and the oldest known bird was over 15 years old when it was last trapped and released. Their pairings can last over 8 years, and both observational data and DNA indicate that there is absolutely no infidelity.
In a given year, about half of all Florida Scrub-Jay territories are occupied by just the pair. The other half are occupied by both the pair and from one to six “helpers,”—usually comprised of offspring from previous years. The longer a pair is together, the more helpers they have. And helpers in this case really do improve the number of young raised each year, so the more successful a pair of jays is, the more successful they will remain. The number of eggs laid in a nest is completely independent of the number of helpers, but the number of hatchlings that survive to fledge and become independent increases significantly. Only the breeding female incubates the eggs and broods and shades the nestlings. Her mate provides much of her food during this time. When a pair has helpers, the breeding male is relieved of some of the responsibility of finding food for the young, and can spend more time acting as a sentinel. Because predation is the main cause of death of scrub jays, and because sentinels can often detect snakes, hawks, and other dangers in time to warn the group, having helpers gives a pair a much greater likelihood of raising young and staying alive themselves. Helpers care for the nestlings, search for food, and act as sentinels. Research suggests that the main reason for higher chick survival when helpers are present is that predation is lower. Sentinels also often alert the group to an incoming supply of peanuts in areas where people feed scrub-jays.
When a breeding adult Florida Scrub-Jay dies, a new mate is selected not from related helpers on a territory, but from either an unrelated helper than has joined the group or from a bird attracted from outside the group. The many related helpers on a territory usually get a start on their own territory by taking over a nearby territory, often aided by their parents, or by “budding off” a small territory adjacent to their parents.’
Florida Scrub-Jays raise young only from mid-March until mid-June—the rest of the year, the pair and helpers remain on the territory alerting one another to danger and finding and caching acorns. They feed on acorns throughout the year. Even though scrub-jays are omnivores, acorns supply nearly half of their food calories in some seasons.
Intense research on Florida Scrub-Jays was begun by the late Glenn Woofenden and John Fitzpatrick, now director of the Cornell Lab of Ornithology, in 1969. Dr. Woolfenden continued his work on the species until his recent death, and John Fitzpatrick still heads to Florida every spring to census the birds and track the individuals he’s come to know personally. I find it lovely that a bird that so epitomizes fidelity has inspired so much fidelity in its human followers.