Arsenic (As) has been ranked first place in the Substance Priority List of hazardous substances for environmental health by the Agency for Toxic Substances and Disease Registry (ATSDR) of the USA during the last 20 years. This metalloid is of major interest because of its toxic effects on humans and animals, mainly in its inorganic form. Calcium (Ca) is an essential element for organisms and its deficiency in the diet may increase the absorption and accumulation of toxic elements, including As. In this regard, Ca administration may have a protective role against As and metal toxicity in different organisms. However, studies on this topic are rare for wild birds. The main aims of this thesis are: (i) to find out the current status and knowledge gaps of As-related research in passerine birds, (ii) to explore the effects of Ca availability on biomarkers of oxidative stress (antioxidants and oxidative damage) in passerine nestlings in relation to As and metal exposure, and (iii) to investigate if environmentally relevant As levels affect physiology, growth and survival of birds. For this purpose, I first prepared a literature review providing a broad overview of As exposure and effects in passerines, pointing out that an experimental approach is needed to explore the adverse effects of this metalloid in free-living passerines at environmental levels. To fill this gap, I performed an As manipulation experiment in great tit (Parus major), where nestlings inhabiting an unpolluted area were dosed with water or sodium arsenite (control, low and high As groups), whereas those living in a metal-polluted area were dosed with water (industrial control). Nestlings accumulated As in liver, bone and feathers in a dose-dependent way. Nests in the high As group produced fewer fledglings per successful nest, whereas chicks in the low As group showed slower wing growth, which could have negative post-fledging fitness effects. I found limited effects on the blood biochemical parameters. Non-significantly increased oxidative stress biomarker values in the high As group suggest that the exposure was close to the level altering reduction-oxidation balance. The effects of Ca availability on oxidative stress in great tit nestlings in relation to As/metal exposure were evaluated during a Ca supplementation experiment, where ad libitum Ca was provided to some nests (Ca-supplemented group) or no Ca to others (control group) in a metal-polluted and a control zone. The Ca availability had very limited effects on the antioxidant status (only on catalase activity). However, blood antioxidant levels changed over the range of metal concentrations depending on the Ca levels in plasma, suggesting that higher Ca levels stimulate antioxidants and mitigate the impacts of metals. Altogether, this thesis compiles the existing pool of knowledge regarding As exposure and effects in passerines, and identifies and fills the main gaps, elucidating the physiological and developmental effects of environmentally relevant As levels, and the role of Ca as modulator of metal/As-related oxidative stress in wild passerines.