Validating morphological condition indices and their relationship with reproductive success in great-tailed grackles

Morphological variation among individuals has the potential to influence multiple life history characteristics such as dispersal, migration, reproductive success, and survival (Wilder et al. 2016). Individuals that are in better “condition” can disperse or migrate further or more successfully, have greater reproductive success, and survive longer (Heidinger et al. 2010; Liao et al. 2011; Wilder et al. 2016), particularly in years where environmental conditions are harsh (Milenkaya et al. 2015). Body condition is defined in various ways, but is most often measured using an individual’s energetic or immune state (Milenkaya et al. 2015). These traits are difficult to measure directly, therefore a variety of morphological proxies to quantify condition are used instead, including fat score (Kaiser 1993), weight, ratio of weight to tarsus length (Labocha et al. 2014), a scaled mass index (Peig and Green 2009), as well as hematological indices for immune system function (Fleskes et al. 2017; Kraft et al. 2019). However, there is mixed support regarding whether these condition indices relate to life history characteristics (Labocha et al. 2014; Wilder et al. 2016), and whether the relationship is linear (McNamara et al. 2005; Milenkaya et al. 2015). Additionally, although some investigations use multiple morphological proxies for condition (e.g. Warnock and Bishop 1998), 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 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 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 measures of condition should be validated before relying on their use as a condition proxy in grackles and birds in general. Future research should further investigate our unexpected 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.