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Global warming is one of the primary drivers of habitat loss and population decline in numerous species, including birds, amphibians and marine life. Avian embryos exhibit ectothermic phenotypes during most of their incubation period and are also vulnerable to rising temperatures when parents cannot cool the nests. This vulnerability stems from their unique respiratory mechanisms, which utilize eggshell pores to exchange respiratory gases. The number of pores is fixed at oviposition, and embryos may experience hypoxia during later developmental stages, especially when exposed to elevated ambient/incubation temperatures. Our preliminary study on zebra finch (Taeniopygia guttata castanotis) embryos, where we covered 30% of the shell surface with beeswax and incubated at high (38.9°C) temperature, revealed that half of the individuals that failed to hatch had developed oedema in the hind neck region. This study shows that such physical anomalies occur during incubation prior to death. We found that embryos with oedema had a higher head-to-body ratio, independent of their relative brain mass. Furthermore, oedema formation was correlated with darker-coloured hearts, suggesting reduced blood oxygenation in these embryos. These results highlight the physiological challenges embryos face under suboptimal incubation conditions. This article is part of the theme issue ‘The biology of the avian respiratory system’. 

The ability for traits to recover after exposure to stress varies depending on the magnitude, duration, or type of stressor. One such stressor is circadian rhythm disruption stemming from exposure to light at night. Circadian rhythm disruption may lead to long-term physiological consequences; however, the capacity in which individuals recover and display stress resilience is not known. Here, we exposed zebra finches (Taeniopygia castanotis) to constant light (24L:0D) or a regular light/dark cycle (14L:10D) for 23 days, followed by a recovery period for 12 days. We measured body mass, corticosterone, and glucose levels at multiple timepoints, and relative protein expression of glucocorticoid receptors at euthanasia. Body mass significantly increased over time in light-exposed birds compared to controls, but a 12-day recovery period reversed this increase. Baseline levels of circulating glucose decreased in light-exposed birds compared to controls, but returned to pretreatment levels after the 12-day recovery period. In contrast, the glucose stress response did not show a similar recovery trend, suggesting longer recovery is needed or that this is a persistent effect in light-exposed birds. Surprisingly, we did not detect any differences in baseline corticosterone or reactivity of the hypothalamic-pituitiary-adrenal (HPA) axis between groups throughout the experiment. Moreover, we did not detect differences between relative protein expression of glucocorticoid receptors or a relationship with HPA axis reactivity. Yet, we found a positive relationship between glucocorticoid receptors and the glucose stress response, but only in the light group. Our results indicate that physiological and morphological traits differ in their ability to recover in response to constant light and warrants further investigation on the mechanisms driving stress resilience under a disrupted circadian rhythm.