How much do we know about the breeding biology of bird species in the world?

Knowledge on species’ breeding biology is the building blocks of avian life history theory. A review for the current status of the knowledge at a global scale is needed to highlight the priority for future research. We collected all available information on three critical nesting parameters (clutch size, incubation period and nestling period) for the close to 10 000 bird species in the world and identified taxonomic, geographic and habitat gaps in the distribution of knowledge on avian breeding biology. The results show that only one third of all extant species are well known regarding the three nesting parameters analyzed, while the rest are partly or poorly known. Most data deficient taxonomic groups are tropical forest nesters, particularly from the Amazon basin, southeast Asia, Equatorial Africa and Madagascar – the places that harbor the world's highest bird diversity. These knowledge gaps could be hampering our understanding of avian life histories. Ornithologists are encouraged to pay more efforts to explore the breeding biology of those poorly‐known species.