Metals and industrial substances used to make every-day objects such as computers, mobile phones, frying pans and waterproof clothes, make their way into the natural environment. These substances are considered to be environmental contaminants and can have a devastating impact on wildlife, in particular predators such as seabirds. However, our understanding of how and when seabirds become contaminated and the consequences of this contamination is limited. While we know that seabirds can ingest these contaminants by eating their prey, it is unclear why some species and some individuals within species have higher contaminant levels than others and what the consequences of this might be. Investigating these questions is critical to understand toxic effects and potential wildlife population declines. Mercury (a heavy metal) and perfluorinated compounds are particularly important to study because they have long residence time in the environment, and in animal and human bodies, with known negative impacts on them. Animals have physiological mechanisms to eliminate contaminants form their bodies. However, these mechanisms are not always effective and can be very costly in terms of energy and nutrients. Contaminants can also increase the susceptibility of animals to disease, or/and make disease more dangerous for them. Yet, the interaction between contaminants and disease is poorly known in seabirds, and so are the consequences of it on energy use, reproduction and mortality of seabirds. The aim of the ECOSEA project was to study the levels and effects of natural (metals) and man-made (perfluorinated substances) contaminants in a declining British seabird: the European shag (hereafter “shag”). The study site was the Isle of May, Firth of Forth, Scotland, an environment which has received a plethora of contaminants over 200 years of intense industrial and urban activities. Shags have been monitored here for more than 30 years, so that we know the age, sex and reproductive history of many of them. Specific objectives of ECOSEA were the following: Aim 1) describe what prey the shags feed on, and where precisely they capture them, in order to understand if different prey, as well as other bodily characteristics such as sex, size and age, explain contaminant level differences between birds; Aim 2) understand if contaminants constitute an energetic costs for the shags; Aim 3) determine if there is a relationship between contaminant levels and the number of parasites present in the stomach of the birds (nematode worms, Contracaecum rudolphii); and 4) measure the combined effects of contaminants and parasites on reproduction.