A duck is a duck is a duck, or is it? – Some notes on how to age Mallards in autumn.

Female head

2cy+ female, October. Adult females often show distinct blackish spots and a bright yellow-orange colour of the bill. [90A86379]

There are a lot of ducks on this blog – too many for some readers, perhaps – but it is not that surprising given that Mallards are our main model system for exploring disease ecology questions. Given their abundance, importance as game species, relationship with domestic ducks, synanthropic (yes, google that) behaviors, and propensity to carry interesting viruses and bacteria, it is not strange that ducks have rendered considerable research interest here and elsewhere. Ducks are also very beautiful animals, which helps.

You may think that a duck is a duck is a duck. But really this is not true; there is considerable individual variation in all measured traits in Mallards. The size differs, the plumage varies, as do migration patterns and immunocompetence, and so forth. As scientists, the variation is often what we are most interested in. The topic of today’s post is variation in plumage and the uncertainties of correctly ageing Mallards in the hand.

During our long-term studies of Mallards at Ottenby Bird Observatory in Sweden we have built up a large database of captures and recaptures of birds, which has allowed us to identify birds with known ages. For examples, if a bird was ringed in 2012 as juvenile and recaptured in 2016 we can be confident that this bird is adult at the second capture. Recently we published a study investigating different proposed plumage criteria for ageing Mallards by careful assessment of photos gathered over several years.

Generally, in birds, there is a difference between juvenile feathers (i.e. those attained directly after hatching) and feathers of older generations. In some bird species, this may include color differences, or special markings – such as spots or vermiculation; while in others differences are subtler, as in shape or wear. The key to age determination is the knowledge of in which sequence feathers are replaced (generally termed moult), and when in the year it takes place. In many passerines, birds have one complete moult of flight feathers per year and one or two partial moults of body feathers. Thus, knowing when, where and how birds moult you can search for the presence or absence of juvenile feathers. In non-passerines it becomes a bit messier, for instance with raptors where it may take several years to complete the moult of flight feathers. You could also look at other characters, such as the coloration of soft parts (bill, orbital ring, feet) and eye color, but generally those are less well known.

And what about Mallards? Let’s cite the paper (slightly edited):

“At the age of 2–4 months, juvenile mallards perform a partial moult of some feathers on the head, neck, mantle, scapulars, breast, and flanks in late summer (mainly July–September, correlated to hatching date). Both young and adults undergo pre-breeding moult from August–December, but the process may be prolonged during winter. This moult includes most body feathers, scapulars, tertial coverts, tertials, and tail, but very few lesser, middle, and greater coverts, and no primaries and secondaries. The retained feather groups allow determination of juveniles until replaced during the next year. Young females also retain at least some juvenile tertials and tertial coverts, whereas most males have moulted all tertials by late November. Males are thought to perform more extensive tertial moult due to sexual selection, and perhaps is sexual selection also responsible for the “extra” moult of mainly head and neck feathers noted in males in February–March.

In January–May, females perform a moult (of similar extent as the preceeding one) to acquire an even more cryptic plumage during nesting, brood-rearing, and moult of flight feathers, while males moult into a female-like eclipse plumage in May–July. Both sexes are flightless for about one month when all remiges (primaries and secondaries) are lost simultaneously during the summer (mainly late June–August), latest in successfully breeding females.”

For you non-birders, this may sound awfully complicated, but it is actually not that hard once you know your way around the anatomy of a bird and become familiar with the lingo of ornithology. The take-home message is that during autumn, birds will have a plumage comprised of feathers of different ages and knowing what you look for you should theoretically be able to correctly separate juvenile birds from adult birds.

1 cy Male tail

1cy male, November. Easily recognized as a young bird due to remaining juvenile RR being narrower (and shorter), worn, frayed, and with brown and buff colours. The outermost pair of feathers is often the last to be moulted, but sometimes the central pair is still retained when all the others are post-juvenile.

1 cy male tertials

1cy male, November. This bird has moulted T2 to T4, but still has juvenile T1. The four outer TC are worn and dull brownish, but note that even juvenile two outermost TC occasionally show warm brown tips. [90A88713]

However, in practice this is hard – even for experienced duckologists (see for example the figure just above). Using the photographs, we tested the validity of different criteria, either one by one, or combined, by presenting them to a panel of (mostly) volunteers experienced in age and sex determination of birds. Evaluating their scores compared to the known ages of the birds we found that no single criterion was conclusive (range 48-89% correct), but that when given access to photographs of all plumage tracts, 91 and 95% of male and female Mallards were correctly assigned, respectively. The latter setup is more similar to what is experienced in the field, but nevertheless we urge caution with ageing, especially in late autumn where many juvenile birds may have moulted tail and tertials in a pre-breeding moult and hence are very adult-like in appearance.

I am happy to send a pdf of the paper to those interested, just google my name at Linnaeus University. You could also visit the online Ringers’ DigiGuide at Ottenby Bird Observatory where we have provided quality photos and texts to use for the purpose of ageing and sexing of Mallards, as well as for other species.

The reference for the article is:

Andersson, S., Bengtsson, D., Hellström, M. & Waldenström, J. 2016. Age and sex determination of Mallards Anas platyrhynchos in autumn. Ornis Svecica 26: 61-81.

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