While taxonomic and biogeographical biases are often acknowledged, those for certain biological responses and species traits are routinely overlooked, generating major gaps in knowledge and conservation of biodiversity. Biases in research on birdsan over-sampled, diverse vertebrate classmay be readily detectable, and wetlands are important species-rich ecosystems in which to identify biases and research gaps for birds. The Pantanal, one of the world's largest wetlands, is globally relevant for bird conservation. In this wetland, we determined spatial, temporal, taxonomic and biological response-related biases in ornithological studies to guide future research in this ecosystem and, ultimately, in major global wetlands. Avian research was geographically biased, with 61 studies conducted in the Brazilian Pantanal and only one in Bolivia. Most studies were concentrated near urban centres, with poorly explored areas in the central Pantanal. Research was also over-represented during the dry season when field conditions are more favourable, but such temporal bias may hamper migration studies. Considering their richness, some families were studied disproportionately more (e.g. Jacanidae) or less (e.g. Tyrannidae). Some species (e.g. Wood Stork Mycteria americana and Yellow-billed Cardinal Paroaria capitata) were included in > 25% of studies, whereas a relatively low number of threatened bird species were studied. Behaviour was the most studied response, followed by abundance and reproduction, which were considered for > 65% of species studied. We conclude that further research needs to be focused on unexplored areas and periods, less detectable species, and ecological processes (e.g. interspecific interactions). Additionally, our results can provide useful information to better address future work and bird conservation actions in other large wetlands. For example, major gaps detected here constitute a primary roadmap to guide research in under-sampled regions, such as the Canadian peatlands and Tonlé Sap Lake. Specifically, more studies on waterbirds in highly diverse wetlands from low-income countries (e.g. Okavango and Sundarban Delta) may help to disentangle the essential functional role provided for these species and to prioritize conservation actions in regions with limited research capacity.

Seasonally flooded ecosystems are comprised by heterogeneous vegetation mosaics (i.e., environments), wherein habitat availability and food resources are spatially and temporally variable. Overall, these factors determine the abundance, richness, and diversity of birds. To evaluate season-and environment-dependent bird abundance, richness, and diversity, we sampled understory birds along two seasons (dry and wet) in three forest environments, i.e., one semideciduous non-flooding forest (Cordilheira) and two evergreen flooding ones (Cambarazal and Landizal), in the northern region of the Pantanal. Birds were captured using mist nets that were located in four sites for each environment between 06:00 h and 11:00 h, during 3 months in the dry season and 3 months in the wet season. We captured 505 individuals from 21 families and 70 species of understory birds. Seasonality significantly affected bird composition, with higher abundance, richness, and diversity in the dry season. Indeed, most of the individuals were captured in the dry season (n = 365), comprising 62 species and 20 families. During the wet season, we captured 140 individuals from 38 species and 15 families. Although the Landizal, an evergreen flooding forest environment, comprised a number of individuals about 20 and 40% higher than the Cambarazal and the Cordilheira, respectively, we did not detect any significant difference in understory bird composition among environments. This study reinforces the assumption that seasonality determines understory bird abundance, richness, and diversity, which increase during the dry season, in the Pantanal. We suggest that this pattern is related to higher habitat and food resources availability during the dry season.