Post‐Juvenile Moult Extent is Largely Driven by Habitat Openness Not by Time Constraints Across New Zealand Native Passerines

The extent of post‐juvenile moult in passerines is influenced by both time constraints and environmental conditions. Conventional interpretations assume that shorter periods between fledging and adverse seasonal conditions should lead to less‐extensive post‐juvenile moults. However, this relationship often fails to hold across species. We propose a threshold model in which time constraints limit moult extent only up to a certain point, beyond which environmental factors become the primary evolutionary drivers of moult extent. We tested this model using the native passerines of mainland New Zealand, a largely non‐migratory assemblage with a long breeding season (September–January), allowing us to disentangle time constraints from other ecological pressures. Post‐juvenile moult extent was quantified using two metrics: mass of wing feathers replaced and occurrence of primary replacement. Phylogenetically corrected GAMs revealed no simple linear decline in extent with later fledging dates, peaking at intermediate values, particularly in species that replace primaries. Environmental predictors also played a key role: extent increased with habitat openness and decreased with latitude. The relative importance of predictors differed between metrics: habitat openness explained most variation in feather mass replaced, whereas fledging date best predicted primary replacement. These results are compatible with a threshold relationship between time constraints and post‐juvenile moult extent in New Zealand passerines, suggesting that when time constraints are moderate, environmental adaptations play an important role in shaping this moult.