Aim: It has long been recognized that tropical bird species demonstrate a wide range of traits characteristic for slow life histories, as indicated by lower investment in reproduction, slower senescence or slower physiological pace of life. Nevertheless, evidence for latitudinal variation in avian developmental rates has, so far, been inconclusive and based on limited geographic or phylogenetic sampling. The aim of this study was to test for global associations of avian developmental rates with latitude and migratoriness.

Location: Global.

Taxon: Birds.

Methods: We compiled data on developmental (incubation and fledging) periods of 3500 passerine and non-passerine bird species distributed worldwide, which represented nearly all extant avian orders and families. Associations of developmental periods with latitude and migratoriness were analysed with phylogenetically-informed comparative models that controlled for major sources of variation in developmental rates (body mass, clutch size, and developmental mode across altricial-precocial spectrum).

Results: Our analyses provided support for a negative latitudinal gradient in the duration of fledging, but not incubation period. After controlling for latitude, we also found that migratory birds showed faster developmental rates than resident species. Finally, we found that developmental rates were non-linearly associated with clutch size (fastest developmental rates at intermediate clutch size) and that duration of fledging period had higher evolutionary lability than incubation period.

Main conclusions: Our global comparative analysis provided convincing and phylogenetically-robust evidence for a long-held hypothesis on latitudinal variation in avian developmental rates, although the pattern was apparent only at the post-embryonic stage of development (fledging period). This challenges a common belief on longer incubation periods in tropics and indicates that incubation and fledging periods may differently respond to external environmental factors, which can possibly be driven by their different evolutionary lability. The results may contribute to our understanding of biogeographic variation in species diversity.