Scientists have long been trying to understand why the Neotropical region holds the highest diversity of birds on Earth. Recently, there has been increased interest in morphological variation between and within species, and in how climate, topography, and anthropogenic pressures may explain and affect phenotypic variation. Because morphological data are not always available for many species at the local or regional scale, we are limited in our understanding of intra‐ and interspecies spatial morphological variation. Here, we present the ATLANTIC BIRD TRAITS, a data set that includes measurements of up to 44 morphological traits in 67,197 bird records from 2,790 populations distributed throughout the Atlantic forests of South America. This data set comprises information, compiled over two centuries (1820–2018), for 711 bird species, which represent 80% of all known bird diversity in the Atlantic Forest. Among the most commonly reported traits are sex (n = 65,717), age (n = 63,852), body mass (n = 58,768), flight molt presence (n = 44,941), molt presence (n = 44,847), body molt presence (n = 44,606), tail length (n = 43,005), reproductive stage (n = 42,588), bill length (n = 37,409), body length (n = 28,394), right wing length (n = 21,950), tarsus length (n = 20,342), and wing length (n = 18,071). The most frequently recorded species are Chiroxiphia caudata (n = 1,837), Turdus albicollis (n = 1,658), Trichothraupis melanops (n = 1,468), Turdus leucomelas (n = 1,436), and Basileuterus culicivorus (n = 1,384). The species recorded in the greatest number of sampling localities are Basileuterus culicivorus (n = 243), Trichothraupis melanops (n = 242), Chiroxiphia caudata (n = 210), Platyrinchus mystaceus (n = 208), and Turdus rufiventris (n = 191). ATLANTIC BIRD TRAITS (ABT) is the most comprehensive data set on measurements of bird morphological traits found in a biodiversity hotspot; it provides data for basic and applied research at multiple scales, from individual to community, and from the local to the macroecological perspectives. No copyright or proprietary restrictions are associated with the use of this data set. Please cite this data paper when the data are used in publications or teaching and educational activities.

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Many organisms live in or on birds, with arthropods being the best studied ones. Ectosymbiotic arthropods may parasitise birds by feeding upon blood or feathers, thereby causing injury or competing for essential resources. Nevertheless, ectosymbionts sometimes cause no adverse effect to birds because they consume only excess preening oil, debris and microorganisms on feathers. The influence of these ectosymbionts on host body condition is still poorly understood, especially for species living in one of the most endangered biomes, the Atlantic Forest in South America. We tested for the relationships between ectosymbionts and host’s morphological traits: flight muscle condition, mass index and feather growth rate. We inspected the body surface of 282 individual birds of 45 understory‐dwelling species looking for ectosymbionts and estimated vane‐dwelling feather mite load in their remiges. We calculated ectosymbiont prevalence and mean infestation intensity in 193 individual birds from 11 species. In seven species (n = 106 individual birds), we tested for the relationship between ectosymbionts and morphological traits using generalised linear mixed models. We found a negative relationship between the presence of ectoparasites and the host pectoral muscle condition, supporting the parasitic nature of tissue‐feeding symbionts. Nevertheless, we found no relationship between vane‐dwelling feather mite loads and hosts’ morphological traits, which supports the hypothesis of likely commensalism between these organisms.