Arrested or Suspended - who knows?!

 

 

This female linnet caught this morning prompted me to look at the confusing issue of arrested or suspended moult.

Although still in main moult with a primary moult score, on both wings, of 5400000000, it looks possible that it wont drop any more primary feathers, so it may have arrested or suspended its main moult. But which is it?

Moult and Ageing of European Passerines by Lukas Jenni and Raffael Winkler says –

'Following Harper (1984), the phenomenon by which a moult stops before the whole plumage has been renewed is called moult interruption. This term and the related terms suspended and arrested moult have generally been used only for primary and secondary moult and, in order to prevent confusion, we retain this usage. Two different types of primary and secondary moult interruption are recognized (King 1972, Harper 1984). In the case of suspended moult, the primary and/or secondary moult is subsequently resumed at the point of interruption. Thus, the suspension normally divides a single moult into two temporally separated phases. Those primaries and secondaries not moulted during the first phase of the moult are renewed later during the second phase. In the case of arrested moult, the next primary and secondary moult starts at the normal site of initiation or another site other than the point of interruption. Thus, an arrested moult cannot later proceed to completion and the subsequent moult is in fact a different moult entirely.'

So, unless this bird is retrapped before its next “recognised” moult cycle its impossible to say if it has arrested or suspended its moult. However the “BTO Moult code flowchart for birds age 4, 5, or 6” (and DemOn) only gives the option to record arrested moult (perhaps here used synonymously with Jenni & Winkler's moult interruption), so if it had finished its main moult it would have to have been reported as arrested moult.

A further search of the internet found one of the articles referred to above by Jenni & Winkler - Moult interruption in passerines resident in Britain by D.G.C. Harper. This contains the following statement –

'Moult interruption during breeding has also been reported in unpublished studies on Whitethroat Sylvia communis, Willow Warbler Phylloscopus trochilus, Linnet Carduelis cannabina and Yellowhammer Emberiza citrinella (R.A. Cawthorne cited in Ginn & Melville 1983; M. Lawn cited in Boddy 1983; and M. Boddy cited in Boddy 1983). Unfortunately no details have yet been published. It is however interesting that only females are mentioned in these reports; this raises the possibility that moult interruption in these species is more prevalent among females than among males. All these instances are reported as involving moult suspension; evidence supporting this interpretation would be valuable.”

I've also seen this in a female Whitethroat (see our Blog for August 2020). Interestingly, I also retrapped this female in September 2021 and it appeared to have Arrested its moult again.

Mick P