Baseline immune activity is associated with date rather than with moult stage in the Arctic-breeding barnacle goose (Branta leucopsis)

Variation in immune defence in birds is often explained either by external factors such as food availability and disease pressure or by internal factors such as moult and reproductive effort. We explored these factors together in one sampling design by measuring immune activity over the time frame of the moulting period of Arctic-breeding barnacle geese (Branta leucopsis). We assessed baseline innate immunity by measuring levels of complement-mediated lysis and natural antibody-mediated agglutination together with total and differential leukocyte counts. Variation in immune activity during moult was strongly associated with calendar date and to a smaller degree with the growth stage of wing feathers. We suggest that the association with calendar date reflected temporal changes in the external environment. This environmental factor was further explored by comparing the immune activity of geese in the Arctic population with conspecifics in the temperate climate zone at comparable moult stages. In the Arctic environment, which has a lower expected disease load, geese exhibited significantly lower values of complement-mediated lysis, their blood contained fewer leukocytes, and levels of phagocytic cells and reactive leukocytes were relatively low. This suggests that lower baseline immune activity could be associated with lower disease pressure. We conclude that in our study species, external factors such as food availability and disease pressure have a greater effect on temporal variation of baseline immune activity than internal factors such as moult stage.

鸟类免疫防御能力的变异通常可通过两类因素解释：一类是食物可获得性、疾病压力等外部因素，另一类是换羽与繁殖投入等内部因素。本研究通过在北极繁殖的白颊黑雁（Branta leucopsis）的换羽周期内测定其免疫活性，在同一样本设计中同步探究了上述两类因素。本研究通过测定补体介导裂解、天然抗体介导凝集的水平，以及白细胞总数及分类计数，评估了个体的基础天然免疫（innate immunity）水平。换羽期的免疫活性变异与公历日期显著相关，而与翼羽生长阶段的关联程度相对较弱。本研究认为，这种与公历日期的关联反映了外部环境的时间动态变化。为进一步探究该环境因素的影响，本研究比较了北极种群白颊黑雁与温带气候区同换羽阶段同种个体的免疫活性。在预期疾病负荷更低的北极环境中，白颊黑雁的补体介导裂解水平显著更低，血液中白细胞数量更少，吞噬细胞与反应性白细胞的水平也相对偏低。这表明较低的基础免疫活性可能与更低的疾病压力相关。本研究得出结论：相较于换羽阶段等内部因素，食物可获得性、疾病压力等外部因素对研究物种基础免疫活性的时间变异影响更大。