Birds of Portugal - An Annotated Checklist has been written with two objectives in mind. First, to provide an update to the existing species lists and second, to make this information available in a practical and accessible format that can be easily understood and used by anyone with an interest in birds, including those that belong to the non-specialized public. The format chosen - an annotated list - is more than just an inventory: for each species mentioned, a brief description is provided about its migratory status, abundance and distribution in mainland Portugal. For the species that are only irregularly recorded - often referred to as 'rarities' - a brief summary of the existing records is provided. This list is an update of the previous edition, which was published in 2017. The contents have been totally reviewed. Additionally, several species, which have been recorded in mainland Portugal for the first time during the last three years, have been added to the list. Several texts were also updated, in order to provide information about recent sightings. Furthermore, the taxonomy was updated according to the most recent international standards.