Jennifer Berens, Corina J Logan , Melissa Folsom, August Sevchik, Luisa Bergeron, Kelsey McCune

Morphological and physiological variation among individuals has the potential to influence multiple life history characteristics such as dispersal, migration, reproductive success, and survival. Individuals that are in better "condition" can disperse or migrate further or more successfully, have greater reproductive success, and survive longer, particularly in years where environmental conditions are harsh. Condition is defined in various ways, but is most often measured using an individuals energetic state. These traits are difficult to measure directly, therefore a variety of morphological proxies to quantify energetic condition are used instead, including fat score, weight, ratio of weight to tarsus length, and a scaled mass index. However, there is mixed support regarding whether these energetic condition indices relate to life history characteristics, and whether the relationship is linear. Additionally, although some investigations use multiple morphological proxies for energetic condition, rarely have there been direct comparisons among proxies to validate that they measure the same trait. In this investigation, we define condition as an energetic state and we attempt to measure it by comparing two morphological indices (fat score and the scaled mass index) to validate whether they measure the same trait and whether they correlate with measures of reproductive success in our study system, the great-tailed grackle (*Quiscalus mexicanus*). We found that the morphological proxies did not correlate with each other, indicating that they do not measure the same trait. Further, neither proxy significantly correlated with reproductive success in males, measured as whether a male held a territory containing nests or not. We found that females with a high scaled mass index had a significantly lower probability that their nest would survive on any given day. However, there was no relationship between female fat score and nest survival. These results indicate that morphological measures of energetic condition should be validated before relying on their use as a condition proxy in grackles and birds in general. Future research should investigate behavioral mechanisms underlying our result that higher scaled mass index correlated with lower nest survival to better understand the importance of energetic condition for reproductive success - a necessary component for selection to act.