Age determination of the invasive monk parakeet Myiopsitta monachus has been hampered by its juvenile plumage showing only subtle differences from subsequent plumages, and little is known about its moult. Here we examined three specific traits (grey patch on forehead, scalloped breast, and notch on primary P9) and three general juvenile traits (bill cornification, pale margins on wing feathers, and shape of flight feathers) and tested their potential as age determination criteria using 709 specimens of this parakeet collected from the metropolitan area of Barcelona in 2023 and 2024. We also studied five moult components: phenology, duration, extent, sequence, and intensity. Bill cornification, forehead patch, and scalloped breast (with continuous maturation) were exclusively associated with juvenile birds, while pale margins, shape of flight feathers (especially those of rectrices), and notch on P9 (all with discrete maturation) allowed us to age birds throughout the entire first annual moult cycle. The moult season stretched from March to November, although the mean moult duration was 168 days. Post-juvenile moult included the body and an average of 16 wing and tail feathers, while postnuptial moult was characterised by retention of several primary coverts, and frequently also of outer secondary coverts and inner primary coverts. The primary moult sequence was clearly divergent across tracts, with alula, tertials, and secondaries being less clearly so, while it tended to be convergent in rectrices. Moult intensity described a parabola for both primary and body feathers. We integrated this information in an age-determination protocol that may help increase our understanding of the population dynamics of monk parakeets

This work was supported by the research projects CGL-2020 PID2020-114907GB-C21 and PID2023-150807NA-I00 awarded to JC Senar, and L Cardador, respectively from the Ministry of Science and Innovation, Spain. This work acknowledges the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

16 pages, 18 figures, 2 tables

Peer reviewed