JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. The relationship between mistletoes and birds has been studied from the perspectives of mutualism and seed dispersal. Here, we emphasize the role that avian dispersers play as agents of mistletoe seed transmission to plant hosts. We describe the patterns of transmission of the seeds of Tristerix aphyllus, an endophytic Chilean mistletoe, on two of its columnar cacti hosts (Eulychnia acida and Echinopsis skottsbergii) by the Chilean Mockingbird Mimus thenca. In north-central Chile, these cacti grow in relatively discrete subpopulations on north-facing slopes. We measured variation in seed transmission within 10 subpopulations varying in species composition, host density, parasite density, parasite prevalence (defined as the percentage of hosts infested in a given population), and disperser abundance. Seed transmission was independent of species, but was strongly dependent on prior parasitism. Parasitized individuals received seeds much more frequently than expected from their relative abundance. We found no correlation between the density of hosts and seed transmission. We found strong positive correlations, however, between parasite prevalence and seed transmission to both parasitized and nonparasitized hosts. Seed transmission of T. aphyllus seeds by M. thenca appeared to be frequency-rather than densitydependent. Seed transmission was also tightly and positively correlated with the abundance of seed-dispersing birds at each site. Because bird abundance and parasite prevalence were correlated, we conducted path analysis to disentangle their relative effect on seed transmission. A model including only the direct effect of bird abundance and the indirect effect of parasite prevalence through bird abundance explained roughly the same variance as a full model including both the direct and indirect effects of bird abundance and prevalence on seed transmission. Apparently, variation in bird abundance was the main determinant of variation in transmission. We suggest that mistletoes, host plants, and the birds that disperse mistletoe seeds are systems well suited for studies of the ecological and evolutionary dynamics of disease transmission.

Tristerix aphyllus (Loranthaceae) is a bird‐dispersed mistletoe that infects several cactus species in central and north‐central Chile. At Las Chinchillas National Reserve it parasitizes two species of columnar cacti (Echinopsis chilensis and Eulychnia acida) and its seeds are dispersed exclusively by the Chilean mockingbird Mimus thenca. The prevalence of T. aphyllus on cacti depended on host species and size. Echinopsis chilensis was parasitized at a higher, and E. acida at a lower, frequency than expected from their relative abundances. In both species, the incidence of T. aphyllus parasitism increased with cactus height. Birds perched and deposited seeds primarily on parasitized cacti, which resulted in an aggregated distribution of seeds. By preventing bird perching and thus reducing seed deposition, the long spines of E. chilensis and E. acida may help protect cacti against T. aphyllus infestation. However, the seeds of T. aphyllus produced a slow‐growing radicle that reached up to 91 mm long in 43 days. Radicles of this length are unique among mistletoe species and may have evolved to overcome the barrier posed by cactus spines.