Rapid Morphological Evolution of Exotic Passerine Birds on Islands Blake Mathys* and Julie Lockwood Graduate Program in Ecology & Evolution, and Department of Ecology, Evolution, & Natural Resources, Rutgers University, New Brunswick, NJ 08901   *mathys@eden.rutgers.edu Abstract Exotic species introduced to islands can serve as powerful ‘natural experiments’. Passerine birds in particular have been preferentially introduced to islands, and because they were intentionally released, we know with relative certainty the date of introduction, location of source population, and initial propagule size. Here we test whether passerine birds native to mainlands and introduced to islands evolve in accordance with the Island Rule; that is, do they increase in size on islands. We took measurements of 38 insular populations on Bermuda, Puerto Rico, and Hawaii and on individuals from each species’ mainland source populations. We compared these measurements using univariate tests and Principal Component Analysis. Overall, body size decreased in the island populations, and in many cases, body proportions changed. This pattern runs contrary to the Island Rule, and suggests that exotic birds are evolving within the specific context of where they were introduced. This result highlights the utility of testing for large-scale evolutionary patterns using exotic species. Results Methods We measured museum specimens and live individuals from 21 species of passerine birds introduced to Bermuda, Hawaii, and Puerto Rico. We took mass, wing chord, tail length, tarsus, head length, culmen, bill depth, and bill width on field-caught and museum individuals. We corrected the museum specimen measurements for shrinkage due to preservation using known correction factors. We initially compared island and mainland measurements using t-tests and ANOVAs to identify single variable differences. We then reduced the measurement data to two variables using Principal Component Analysis (PCA) We compared overall body size (PC1) and proportions (PC2) between populations of each species using t-tests and ANOVA Si = 1.0 = Line of No Change Results and Discussion Overall, body size showed a strong pattern of size decrease (15 decreases versus 3 increases in PC1 out of 38 populations). More than 50% of the populations showed changes in body proportions (PC2). These results indicate that change in body size and proportions is widespread in exotic island passerine populations. These changes run counter to the expectations derived from the Island Rule, which predicts size increase for these populations. Further study of these patterns will lend insight into the role of propagule size, island area, competitive influences, time since population isolation and other factors on the observed morphological changes in these insular populations.