This is the publisher's version, also available electronically from http://www.jstor.org/stable/4088548?seq=1#page_scan_tab_contents.We studied a cooperative-breeding population of Scrub Jays (Aphelocoma coerulescens) in Oaxaca, Mexico. The jays occupied territories 1.5 ha in average size located in a wide range of habitats and were found in temporally stable groups of two to six adult individuals. No yearling was observed breeding. The jays nested at least from early April to late July. Clutch size was three, with subsequent brood reduction to two fledglings typical. Only breeding females incubated eggs. Breeding males fed nestlings more frequently than did breeding females. Ectoparasites were uncommon on adults, but nestlings commonly carried botfly larvae. Extra individuals commonly "helped" in predator defense, territorial defense, and feeding fledglings. Helpers attempted to assist in feeding nestlings, but usually were driven away by breeding adults. Singular breeding seems to be the rule, but plural breeding also may occur. Certain factors associated with the maintenance of cooperative breeding in the Florida Scrub Jay do not appear important in this Oaxacan population: usable habitats do not appear to be saturated with territories, and other jay species do not present an aggressive barrier to Scrub Jay habitat use. Received 3 October 1991, accepted 6 September 1992
