Mass loss, timing and duration of catastrophic moult in little penguins.

Feather regeneration is vital for birds' thermoregulation, courtship, breeding, camouflage, and locomotion, with strategies reflecting life history. Little penguins (Eudyptula minor) undergo catastrophic moult, replacing all feathers within a short timeframe while on land and not foraging. This study examined the 2015 and 2016 moult seasons on Phillip Island to explore factors influencing moult timing, duration, and mass. Moult started 9.6 days earlier in 2016 (∼Feb. 15) than in 2015 (∼Feb. 24), and year was found to be the only significant predictor of this moult start date. Moult duration was similar between years (medians: 18.0 days in 2015, 17.5 in 2016) and only slightly reduced with later start dates (-0.04 days per day delay; ∼58 min). Average daily mass loss during moult were best explained by moult duration and starting mass, with longer moult and greater starting mass leading to greater mass loss. The timing and duration of little penguins' moult, along with the need for significant pre-moult mass gain, are likely influenced by external factors like local prey availability. Moult plasticity likely benefits little penguin survival.