|White Wagtail (Scilly, 21 Oct 2009). Autumn birds like this typically look|
smart, having completed their moult before migrating. Photo: Steve Young.
|Pied Wagtail (Seaforth, Lancashire, 4 Sep 2003). This rather dishevelled|
female is undergoing a longer wing and body moult. Photo: Steve Young.
Birds need to be in tip-top condition when they migrate, not only in terms of their fat reserves and general health, but also regarding their plumage. Moult does not take place during migration, as this would put a strain on the bird and would also impact on its aerodynamics, thus reducing its flight efficiency. Although some birds, such as adult waders, may suspend their moult during migration, most migrants complete their post-breeding and post-juvenile moults prior to migration. This means that most migrants are in completely fresh, neat and immaculate plumage when on their autumn migration, and this is particularly true of passerines.
Pied Wagtail is much more sedentary than White Wagtail and, like many essentially resident species, it has a rather protracted autumn moult. To confirm this, Birds of the Western Palearctic (Vol V) states that, in a non-migratory population of Pied Wagtails in southern England, the average duration of primary moult was estimated as 76 days, with an average start of 16 July and an average completion date of 30 September. Body moult more or less covered the same period. However, in migratory White Wagtails in northern Finland, the moult starts on 8 July (on average) but they take only 46-48 days to complete their moult. This means that, by the end of August, these high-latitude White Wagtails will have completed their moult and be ready to migrate south. Unfortunately, there is no data in BWP for Iceland, from where most of our migrant White Wagtails originate (at least those in western Britain). However, it is clear from my own observations in Somerset that White Wagtails in autumn (which usually appear from the first week of September) have already completed their post-breeding or post-juvenile moults, whereas the local Pieds are still in obvious moult well into September.
|White Wagtail (Aswan, Egypt, 15 Oct 2005). Moult is completed prior to|
migration, so plumage should appear immaculate. Photo: Dominic Mitchell.
|Pied Wagtail (Seaforth NR, Lancashire, 12 Sep 2003). In contrast, Pied|
takes longer to complete its moult, and appears scruffy. Photo: Steve Young.
The important point to make, therefore, is that in late August and September migrnt White Wagtails are always in newly moulted, ‘clean’, immaculate and fresh winter plumage, whereas local Pieds are still looking scruffy, ‘moth eaten’ and often dishevelled. As far as the young birds are concerned, in late August and September, you are in fact comparing first-winter Whites with Pieds that still retain a significant amount of their weak and fluffy juvenile body plumage. Such birds have black breast patches, whereas first-winter Whites have already acquired a neat and contrasting narrow crescent-shaped necklace across the lower throat/upper breast. In addition, this stands out as it contrasts strongly with the almost completely white underparts. On adult Whites, the necklace is thicker.
Pied Wagtail is a darker, sootier grey on the upperparts (adult males are largely black above), but the important feature of autumn Pied is its extensive dark sooty-grey flanks. White Wagtails have white flanks, with just some restricted pale grey confined to the sides of the breast, around the bend of the wing. The white flanks are readily apparent in flight and this allows even flying birds to be identified with some degree of confidence.
To sum up, autumn White Wagtails look clean, neat and immaculate; they are pale grey above, with white flanks and a neat and contrasting black necklace across the lower throat/upper breast. September Pieds, on the other hand, are rather scruffy and moth eaten, a result of their active moult. They retain significant amounts of black on the breast, they are a dark smoky-grey above and they have extensive smoky-grey breast sides and flanks. To put it simply, if you see a scruffy White Wagtail in autumn, then it probably isn’t one!
I would stress that the above relates to observations in western Britain, which involve migrant Icelandic White Wagtails; I have not studied autumn White Wagtails in eastern Britain.