The development of agriculture in recent decades has had a major impact on bird populations associated with agricultural environments in both North America and Europe. Today, most groups of birds species have declined in numbers, but insectivorous species such as the Tree Swallow (Tachycineta bicolor) have suffered the greatest drops. However, the biological mechanisms behind this decline are still relatively unknown. The aim of my thesis is to study environmental effects (type of crop, congener density) on the determinants (mass, wing size, plumage colour, etc.) of sexual partner's choice as well as parental expenditure, in particular in terms of sex ratio allocation to the offspring in an agricultural context. Among these determinants, I am particularly interested in plumage colouring in Tree Swallows. This species has the peculiarity of having an elaborate plumage, with blue-green metallized, referred to as iridescent, on the back, and white on the belly. The latter also has the peculiarity of emitting within the ultraviolet (UV) spectrum which is invisible to human eye but visible birds. Each of these signals (iridescence and UV) are known to influence independently individuals' reproduction. However, the combined effects of these two signals in the same bird species are still unknown. Thus my work will provide a better understanding of the information transmitted by plumage and on a larger scale of communication in birds.