Studies on the breeding of vulnerable and endangered bird species are hindered by low numbers of individuals, inaccessible location of nests, unfavourable environmental conditions, and complex behavioural patterns. In addition, intraspecific variation may emerge only following long-term, systematic observations of little-known patterns and processes. Here, data collected over 30 years were used to determine growth model of hyacinth macaw (Anodorhynchus hyacinthinus) chicks in the Pantanal biome of Brazil. During this period, the speed of growth and body mass of chicks varied widely. Four growth models were tested: logistic, Gompertz, Richards, and cubic polynomial. They were fitted using three biometric measurements: body mass, total length, and tail length. The best-fitting growth curves were identified using Akaike’s information criterion. The best models were the cubic polynomial for body mass, Richards for total length, and Gompertz for tail length. We confirmed the occurrence of dwarf individuals, whose body mass, total length, and tail length were 20%, 22%, and 70% smaller, respectively, than in the overall population. The dwarfs remain small in size after having fledged and are easily identified as adults. We discuss the importance of long-term studies to identify windows of opportunity for further research that will help in the conservation of endangered macaw species.

Natural history studies can provide information that can be used in species conservation and management. The present study provides information about the nests and nest sites of Hyacinth Macaws (Anodorhynchus hyacinthinus) in a mosaic of protected areas in Carajás and surrounding areas in the Amazon region of the state of Pará, Brazil. Data were collected during four expeditions conducted in 2007, 2008, 2013 and 2014. A total of 66 tree cavities were monitored, and 28 of these were used by Hyacinth Macaws. In addition, 38 breeding events were recorded (some nests were active in multiple seasons), and 17 eggs and 33 nestlings were observed. Environmental variables for the 24 cavities that were actively used by Hyacinth Macaws were evaluated. Most of the nest cavities were located in Sterculia sp. trees (86.6%) and in open areas (e.g., near pastures). Nesting tree and cavity measurements were variable, but the mean diameter at breast height of the trees containing Hyacinth Macaw nest cavities was larger than that reported from other regions, such as the Pantanal. The present study describes the unique ecology and life history of Hyacinth Macaws in Carajás, for which information is scarce. This information will facilitate the conservation and management of Hyacinth Macaws and can be used locally for environmental education.