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Simple Summary: This study explored how blood parasites, known as haemosporidians, impact the oxidative status of Rufous-collared Sparrows, a common bird species in Chile. Oxidative status measures the balance between harmful molecules called reactive oxygen species (ROS) and the bird's ability to neutralize them with antioxidants. The study found that birds infected with haemosporidians had higher levels of ROS, indicating more oxidative stress than uninfected birds. While the overall antioxidant levels did not differ between infected and uninfected birds, the infected birds showed a greater imbalance in ROS/TAC, suggesting they suffered more oxidative damage. This was especially true for birds in their reproductive phase. These findings suggest that haemosporidian infection, combined with the demands of reproduction, may impose significant health costs on these birds. Understanding this could help us learn more about how infections affect wildlife, and the strategies parasites use to survive in their hosts. Haemosporidians are common blood parasites in wild bird populations, yet their effects on oxidative status remain understudied. Here, we measured the levels of thiobarbituric acid-reactive substances (TBARS) as an indicator of reactive oxygen species (ROS), total antioxidant capacity (TAC) as an indicator of non-enzymatic molecular antioxidants, and TBARS/TAC ratio as an indicator of oxidative status. We also used parasite genus-specific primers and PCR techniques to detect haemosporidians in 117 adults of Rufous-collared Sparrow (Zonotrichia capensis) from four locations in south–central Chile. Mixed-effect models were employed to compare oxidative indicators between infected and uninfected birds. Infected birds showed significantly higher TBARS levels, but no significant differences in TAC, leading to a higher TBARS/TAC ratio, especially in reproductive individuals. This suggests increased oxidative damage in infected birds, irrespective of sex or body condition. A positive relationship between TBARS and TAC was observed in both groups, but the antioxidant response was weaker in infected birds, indicating differential oxidative stress responses based on infection status. Body condition did not differ significantly between infected and uninfected individuals. These results demonstrate that haemosporidian infections impose oxidative costs on birds, potentially compounding the oxidative costs associated with reproduction. [ABSTRACT FROM AUTHOR]

Most animals must cope with seasonal fluctuations in environmental conditions, including variations in food availability and composition. Accordingly, it is expected that most species should exhibit reversible seasonal phenotypic adjustments in their physiology. Here, we assessed seasonal variation in the activity of three digestive enzymes (sucrase, maltase, and aminopeptidase-N) in one omniviorous bird species (Rufous-collared Sparrow (Zonotrichia capensis (P. L. Statius Muller, 1776))), three granivorous bird species (Black-chinned Siskin (Carduelis barbata (Molina, 1782)), Common Diuca Finch (Diuca diuca (Molina, 1782)), and Mourning Sierra Finch (Phrygilus fruticeti (Kittlitz, 1833))), and one insectivorous bird species (Plain-mantled Tit-Spinetail (Leptasthenura aegithaloides (Kittlitz, 1830))). Based on the adaptive modulation hypothesis, we predicted that the omnivorous species should exhibit the largest seasonal variation in the activity of its digestive enzymes in relation to granivorous and insectivorous species. We found that Z. capensis adjusts total activities of disaccharidases, total sucrase activity varied between seasons in C. barbata, and total activity of aminopeptidase-N only changed seasonally in L. aegithaloides. Moreover, this last species modified the tissue-specific activity of both disaccharidases as well as the wet mass of its intestine. Taken together, our results suggest that seasonal dietary changes occur in most of the species, regardless of the trophic categories in which they belong. Consequently, a better knowledge of the diet and its seasonal variation is necessary to better account for the results recorded in this study. Key words: passerine, diet, digestive enzymes, birds, seasonal changes, Zonotrichia capensis, Carduelis barbata, Diuca diuca, Phrygilus fruticeti, Leptasthenura aegithaloides, Rufous-collared Sparrow, Black-chinned Siskin, Common Diuca Finch, Mourning Sierra Finch, Plain-mantled Tit-Spinetail. La plupart des animaux doivent composer avec les fluctuations saisonnières des conditions ambiantes, dont les variations de la disponibilité et de la composition de la nourriture. Il est donc à prévoir que la physiologie de la plupart des espèces devrait présenter des ajustements phénotypiques saisonniers réversibles. Nous avons évalué les variations saisonnières de l'activité de trois enzymes digestives (sucrase, maltase et aminopeptidase-N) chez une espèce omnivore (bruant chingolo (Zonotrichia capensis (P. L. Statius Müller, 1776))), trois espèces granivores (chardonneret à menton noir (Carduelis barbata (Molina, 1782)), diuca gris (Diuca diuca (Molina, 1782)) et phrygile petit-deuil (Phrygilus fruticeti (Kittlitz, 1833))) et une espèce insectivore (synallaxe mésange (Leptasthenura aegithaloides (Kittlitz, 1830))) d'oiseaux. Sur la base de l'hypothèse de la modulation adaptative, nous avions prédit que l'espèce omnivore présenterait de plus grandes variations saisonnières de l'activité des enzymes digestives que les espèces granivores et insectivore. Nous avons constaté que Z. capensis module les activités totales des disacchari-dases, que l'activité totale de la sucrase varie en fonction de la saison chez C. barbata et que l'activité totale de l'aminopeptidase-N ne varie avec les saisons que chez L. aegithaloides. En outre, les individus de cette dernière espèce modulent l'activité dans les tissus des deux disaccharidases, ainsi que la masse humide de leur intestin. Collectivement, nos résultats indiqueraient que des modifications saisonnières du régime alimentaire ont lieu chez la plupart des espèces, peu importe leur catégorie trophique. Aussi, une meilleure connaissance des régimes alimentaires et de leurs variations saisonnières est nécessaire pour mieux expliquer les résultats obtenus dans le cadre de l'étude. Mots-clés : passereau, régime alimentaire, enzymes digestives, modifications saisonnières, Zonotrichia capensis, Carduelis barbata, Diuca diuca, Phrygilus fruticeti, Leptasthenura aegithaloides, bruant chingolo, chardonneret à menton noir, diuca gris, phrygile petit-deuil, synallaxe mésange., Introduction Most animals must face seasonal fluctuations in environmental conditions, including variations in food availability and composition. In this sense, it is expected that animals that do not migrate should [...]