Baseline and stress-induced blood properties of male and female Darwin’s small ground finch (Geospiza fuliginosa) of the Galapagos Islands

Birds are renowned for exhibiting marked sex-specific differences in activity levels and reproductive investment during the breeding season, potentially impacting circulating blood parameters associated with stress and energetics. Males of many passerines often do not incubate, but they experience direct exposure to intruder threat and exhibit aggressive behaviour during the nesting phase in order to defend territories against competing males and predators. Nesting females often have long bouts of inactivity during incubation, but they must remain vigilant of the risks posed by predators and conspecific intruders approaching the nest. Here, we use 33 free-living male (n = 16) and female (n = 17) Darwin's small ground finches (Geospiza fuliginosa) on Floreana Island (Galapagos Archipelago) to better understand how sex-specific roles during the reproductive period impact baseline and stress-induced levels of plasma corticosterone (CORT), blood glucose and haematocrit. Specifically, we hypothesise that males are characterised by higher baseline values given their direct and relatively frequent exposure to intruder threat, but that a standardised stress event (capture and holding) overrides any sex-specific differences. In contrast with expectations, baseline levels of all blood parameters were similar between sexes (13.4 ± 1.9 ng ml⁻¹ for CORT, 13.7 ± 0.4 mmol l⁻¹ for glucose, 58.3 ± 0.8% for haematocrit). Interestingly, females with higher body condition had lower baseline haematocrit. All blood parameters changed with time since capture (range 1.2–41.3 min) in both sexes, whereby CORT increased linearly, haematocrit decreased linearly, and glucose increased to a peak at ∼20 min post-capture and declined to baseline levels thereafter. Our results do not support the hypothesis that sex-specific roles during the reproductive period translate to differences in blood parameters associated with stress and energetics, but we found some evidence that blood oxygen transport capacity may decline as finches increase in body condition.