Hematological parameters vary with life history stage in the pale-breasted thrush (Turdus leucomelas)

The avian life cycle is composed by a progressive sequence of life history stages (LHS). Changes in energy expenditure and exposure to stressors at different LHS require corresponding changes in behavior, physiology, and morphology. Variation in hematological parameters, such hematocrit (Hct), hemoglobin (Hb), and heterophil to lymphocyte ratio (H/L ratio), can have permissive, stimulatory, and preparative actions to help maintain homeostasis through different LHS. Few studies have examined differences in these parameters among different LHS in free-living birds, with most of them restricted to temperate zones. We collected blood samples and measured hematological parameters every week for over a year from a population of a common resident bird species in southeastern Brazil, the pale–breasted thrush (Turdus leucomelas). Hematocrit and hemoglobin concentration were highest during the onset of the reproduction and lowest during molt. Furthermore, H/L ratios were higher at the end of the reproduction, indicating that the breeding season could be the most stressful period of the year for this population of thrushes. There was no difference between sexes for any hematological parameter at any LHS. These results show that there is a permissive physiological effect for Hct and Hb to facilitate LHS transitions and that reproduction could be the most stressful event for this species. Lastly, these results mirror those from temperate species despite distinct environmental differences between these regions.

鸟类的生命周期由一系列连续递进的生活史阶段（LHS）构成。不同生活史阶段的能量消耗变化与应激源暴露差异，要求行为、生理与形态特征产生相应的适应性改变。血液学参数的变异——如血细胞比容（Hct）、血红蛋白（Hb）以及异嗜性粒细胞与淋巴细胞比值（H/L比值）——可通过许可性、刺激性与预备性作用，帮助机体在不同生活史阶段维持内稳态。
目前针对野生鸟类不同生活史阶段间上述血液学参数差异的研究较为匮乏，且多数研究仅局限于温带区域。本研究针对巴西东南部一种常见留鸟——白胸鸫（Turdus leucomelas）的种群，在超过一年的周期内每周采集血液样本并测定其血液学参数。
研究结果显示，血细胞比容与血红蛋白浓度在繁殖启动阶段达到峰值，而在换羽期降至最低；此外，H/L比值在繁殖末期升高，表明繁殖季可能是该鸫类种群全年应激水平最高的时段。所有血液学参数在任意生活史阶段下均未表现出性别差异。
本研究结果表明，血细胞比容与血红蛋白可发挥许可性生理效应，以促进不同生活史阶段的转换；同时繁殖活动或许是该物种面临的最高强度应激事件。最后，尽管本研究区域与温带区域存在显著环境差异，但本研究结果与温带鸟类的相关研究结论具有一致性。