Acorn Dispersal by California Scrub-Jays in Urban Sacramento, California

Abstract

California Scrub-Jays (Aphelocoma californica) harvest and cache acorns as a fall and winter food resource. In 2017 and 2018, I studied acorn caching by urban scrub-jays in Sacramento, California, to characterize oak and acorn resources, distances jays transport acorns, caching’s effects on the jay’s territoriality, numbers of jays using acorn sources, and numbers of acorns distributed by jays. Within four study areas, oak canopy cover was <1%, and only 19% of 126 oak trees, 92% of which were coast live oaks (Quercus agrifolia), produced acorns. Jays transported acorns for ≥117 days. Complete (28%) and partially recorded (72%) flights from acorn sources to caching sites averaged 160 m and ranged up to 670 m. Acorntransporting jays passed at tree-top level above other jays’ territories without eliciting defense. At least 20 scrub-jays used one acorn source in one 17.5-ha area, and ≥13 jays used another 4.7-ha area. Jays cached an estimated 6800 and 11,000 acorns at two study sites (mean 340 and 840 acorns per jay, respectively), a rate much lower than reported in California oak woodlands, where harvest and caching are confined within territories. The lower urban caching rate may result from a scarcity of acorns, the time required for transporting longer distances, and the availability of alternative urban foods. Oaks originating from acorns planted by jays benefit diverse wildlife and augment the urban forest. The California Scrub-Jay (Aphelocoma californica) is common in oak woodlands and residential areas in the Pacific states (Beedy and Pandolfino 2013, Curry et al. 2017). In central California, the species declined during the mid-2000s as a result of West Nile virus (Airola et al. 2007, Wheeler et al. 2009), but it appears to have recovered (Pandolfino 2017). Despite its abundance, the California Scrub-Jay has attracted fewer ecological studies than have its more threatened sister species, the Florida Scrub-Jay (A. coerulescens; Woolfenden and Fitzpatrick 1996) and Island Scrub-Jay (A. insularis; Curry and Delaney 2002, Caldwell et al. 2013). The primary ecological study of the California Scrub-Jay took place in a coastal California oak woodland (Carmen 2004). The California Scrub-Jay’s acorn-caching behavior is widely known, noted as early as the late 1800s in Mark Twain’s fanciful Jim Baker’s Bluejay Yarn (Clemens 1875). In California oak woodlands, individual scrubjays cache approximately 7000 acorns per year, which serve as a critical food resource during fall and winter (Carmen 2004). The species’ social organization there consists of territorial pairs and nonterritorial floaters. In areas with an adequate supply of acorns, Carmen (2004) reported territory sizes varying from 0.7 to 6.5 ha and averaging 2.2–2.5 ha. Territorial birds harvest and cache acorns almost entirely within the territories they occupy year round, unless the acorn crop fails, when some territories are abandoned (Carmen 2004). The importance of oaks to the various scrub-jay species, and jays’ important ecosystem roles in dispersing and caching acorns, have long been recognized (Grinnell 1936, Griffin 1971, DeGange et al. 1989, Carmen