Many raptor species exhibit plumage polymorphism. Through the expression of a particular phenotype, polymorphism allows individuals to exploit alternative spatial or temporal environments and food resources most successfully. Barn owls display variation in their ventral plumage, ranging from reddish to whitish and from heavily spotted to no spots at all. In heterogeneous landscapes in Switzerland and Israel, reddish barn owls (Tyto alba) inhabit territories with proportionally more arable fields and consume proportionally more voles than their whiter counterparts. This is consistent with the habitat-matching choice hypothesis, but whether this also occurs in other regions and with the American barn owl has not yet been tested. This study sought to examine the relationships among prey composition, landscape composition, and degree of reddishness in barn owls throughout Napa Valley, CA. Pellet analysis was used to determine the proportion of mice (Peromyscus, Reithrodontomys, and Mus), voles (Microtus), and gophers (Thomomys) in the owls’ diet. The results show clear evidence for the existence of polymorphism in the heterogeneous landscapes within and surrounding vineyards, though the role of habitat heterogeneity remains incompletely resolved. These data also further reveal insights into predator-prey relationships and potentially provide vineyard producers with information about pest removal services.

Laura Echávez graduated from Cornell University with a bachelor’s in Animal Science. Prior to arriving at Humboldt State University, she spent several years working seasonal jobs across the Western US, mostly with birds of prey. Laura’s current research focus is on plumage variation in American barn owls, the relationship of plumage with habitat, and the effects on predator-prey interactions in Napa Valley vineyards. Her research goal is to further our understanding of the effect of polymorphism on rodent pest removal, which will hopefully assist farmers’ decisions regarding nest box placement relative to and the management of cultivated and uncultivated areas.