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). 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 proxies 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 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 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 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 improve our understanding of measures of condition in grackles and birds in general. Future research should further investigate our unexpected result that higher scaled mass index led to lower nest survival to better understand the importance of energetic condition for reproductive success - a necessary component for selection to act.

个体间的形态变异可对多种生活史特征产生潜在影响，例如扩散、迁徙、繁殖成功率与存活情况（Wilder等，2016）。研究证实，身体状态更佳的个体，其扩散或迁徙的距离更远、成功率更高，繁殖成功率更优，存活寿命也更长（Wilder等，2016；Heidinger等，2010；Liao等，2011），在环境条件严苛的年份尤为明显（Milenkaya等，2014）。个体的身体状态可通过多种方式界定，但最常用的定义是将其视为个体的能量或免疫状态（Milenkaya等，2014）。由于此类性状难以直接测量，研究者转而采用多种形态替代指标来量化身体状态，例如脂肪评分（fat score）、体重、体重与跗跖长度的比值（Labocha等，2014）、标度质量指数（scaled mass index，Peig与Green，2009），以及用于评估免疫系统功能的血液学指标（hematological indices，Fleskes等，2017；Kraft等，2019）。然而，针对这些身体状态指标是否与生活史特征相关（Wilder等，2016；Labocha等，2014），以及二者的关联是否呈线性（Mcnamara等，2005；Milenkaya等，2014），现有研究结果并不一致。此外，尽管部分研究者会使用多种形态替代指标来评估身体状态（如Warnock与Bishop，1998），但极少有研究直接对比不同替代指标，以验证它们是否可测量同一性状。在本研究中，我们将身体状态界定为能量状态，并尝试通过对比脂肪评分与标度质量指数这两种指标，验证它们在我们的研究类群——大尾拟八哥（Quiscalus mexicanus）中是否测量了同一性状。我们发现，各形态替代指标之间并无相关性，表明它们并未测量同一性状。进一步而言，两种指标均与雄性的繁殖成功率无关——此处的繁殖成功率以雄性是否占据带有巢区的领地为衡量标准。我们还发现，标度质量指数较高的雌性个体，其巢穴在任意单日的存活概率显著更低。然而，雌性的脂肪评分与巢穴存活率之间并无关联。上述结果增进了我们对拟八哥乃至鸟类身体状态衡量方式的认知。未来研究应进一步探究这一意外发现：更高的标度质量指数会导致更低的巢穴存活率，以更好地理解能量状态对繁殖成功率的重要性——而繁殖成功率是自然选择发挥作用的必要组成部分。