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

abstract: 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). Research has shown that individuals that are in better "condition" can disperse or migrate further or more successfully, have greater reproductive success, and survive for longer (Wilder et al., 2016; Heidinger et al., 2010; Liao et al., 2011), particularly in years where environmental conditions are harsh (Milenkaya et al., 2014). An individual's body condition can be defined in various ways, but is most often considered an individual's energetic or immune state (Milenkaya et al., 2014). Since these traits are hard to measure directly, researchers have instead used a variety of morphological proxy variables to quantify condition such as 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 (Wilder et al., 2016; Labocha et al., 2014), and whether the relationship is linear (Mcnamara et al., 2005; Milenkaya et al., 2014). Additionally, although some researchers use multiple morphological proxy variables 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 in our study system, the great-tailed grackle (Quiscalus mexicanus). We found that the morphological proxy variables did not correlate with each other, indicating that they were not measuring the same trait. Further, the proxy variables did not correlate with reproductive success, measured as whether a female had a fledgling and whether a male held a territory containing nests. These results improve our understanding of measures of condition in grackles, and birds in general, and the importance of condition for reproductive success - a necessary component for selection to act