Fig. 1: The duration of avian developmental phases.

a Schematic illustrating four distinct phases of avian ontogeny. Phase 1 and phase 2 corresponding to Hamburger–Hamilton stages (HH) 1–24 and 25–32, respectively, represent embryonic developmental stages primarily associated with embryogenesis (i.e., non-growth). In contrast, phases 3 (HH33 to hatching) and 4 (post-hatching fledging period) correspond to developmental periods consisting largely of growth. b Stacked bar chart showing time intervals associated with phases 1 to 4 for 20 bird species with available information on the timing of embryonic developmental stages, with species are ordered by total developmental duration. Coloured symbols next to species names correspond to the major taxonomic groups identified in Fig. 2. Inset graph shows the staging data and fitted curves used to estimate the time points separating phases 1–3. Inset table reports the coefficient of variation (CV) and percentage of total developmental period length (% total) accounted for by each of the four phases. Source data are provided as a Source Data file.