Hematological and biochemical reference intervals for populations of seabird species from an Archipelago in Rio de Janeiro, Brazil

Introduction Establishing reliable hematological and biochemical reference intervals for seabirds is essential for health assessment, rehabilitation, and conservation programs. However, baseline physiological data for these animals remain scarce. This study aimed to determine hematological and biochemical reference intervals for two free-ranging seabird's species commonly found along the coast of Rio de Janeiro, Brazil: the brown booby, Sula leucogaster (Boddaert, 1783), and the magnificent frigatebird, Fregata magnificens (Linnaeus, 1758). Methods Between March 2024 and May 2025, blood samples were collected from apparently healthy individuals in the Natural Monument of the Cagarras Islands Archipelago (MoNa Cagarras), the main breeding areas for these species in the State of Rio de Janeiro. Hematological and biochemical parameters were analyzed using standard laboratory procedures. Reference intervals were established following the guidelines of the American Society for Veterinary Clinical Pathology. Results Overall, hematologic parameters for both species were similar to those previously reported for individuals maintained in rehabilitation centers. The most notable differences were observed in total thrombocytes and white blood cell counts, which presented higher values. Regarding biochemical parameters, uric acid, and creatine kinase levels were higher in brown boobies. Conversely, alanine aminotransferase, aspartate aminotransferase, and albumin levels were lower when compared to other reports. In frigatebirds, alanine aminotransferase levels were also elevated, but other parameters showed little variation. Discussion For both brown boobies and magnificent frigatebirds, this study presented higher values in total thrombocytes and white blood cell counts, as well as uric acid and creatine kinase levels. These findings possibly reflect capture-related physiological responses from wild individuals. The reference intervals described here represent the first in situ values established for these two seabird species from the MoNa Cagarras in Brazil, providing essential baseline data for health and clinical evaluation, rehabilitation monitoring, and the conservation of tropical seabirds, while highlighting the importance of assessing the health of wild animals within their natural habitat.