Age and sex determination of Mallards Anas platyrhynchos in autumn

Stina Andersson,Jonas Waldenström,Magnus Hellström,Daniel Bengtsson

doi:10.34080/os.v26.22530

Abstract

Male and female mallards Anas platyrhynchos are easily told apart, whereas ageing is problematic, due to individual timing of moult and lack of easily defined age criteria. From examination and photographic documentation of mallards caught at Ottenby Bird Observatory (56°12′N, 16°24′E), we describe nine characters of plumage and bare parts to be used for ageing in autumn. The reliability of these characters was tested by letting experienced bird ringers determine putative age of birds from photos. Age determination from any single character proved to be uncertain, as correctly assigned mallard photos of each character was in the range of 51–85% for males and 48–89% for females. For both sexes, the lowest figure represented post-humerals and the highest represented tertials. Rectrices, tertial coverts, and greater coverts had high scores (71–85%). Using all characters, 91% of the males and 95% of the females were correctly aged. As young mallards, with the progress of pre-breeding moult (completed from October onwards), acquire tail and tertials identical to adults, untypical individuals are better not assigned to an age category.

Highlights

The mallard Anas platyrhynchos is the most numerous and widespread dabbling duck in the world with a Holarctic distribution covering most of the northern hemisphere, including Alaska and southern Greenland
Females have an orange bill with a variable blackish pattern, often covering a large part of the upper mandible
Males in eclipse plumage may appear supericially similar to females, but differentiate from females by blacker crown and more greyish face and neck

Summary

Introduction

The mallard Anas platyrhynchos is the most numerous and widespread dabbling duck in the world with a Holarctic distribution covering most of the northern hemisphere, including Alaska and southern Greenland. It is only absent in desert areas, the extreme north, and tropical regions. Assigning birds to age and sex categories are pivotal for research purposes; for example, age can have strong effects on infection susceptibility (Costa et al 2010, van Dijk et al 2014) and population dynamics (Hauser et al 2007). Can be far more challenging, partly because of the protracted breeding season, where the difference between early and late clutches can differ with almost ive months (Cramp & Simmons 1977), and the individual temporal variation in performance of moult

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