Identification of first cycle Larus dominicanus vetula:
The Cape Gull of Good Hope?
by Chris Gibbins
The two Cape Gulls Larus dominicanus vetula recently found in Portugal (Birding World, 26(6), July 2013), along with the previous bird in Paris (Jiguet et al., 2004), illustrate that this is a species we should be looking out for in Britain.
Gull watching in Europe is perhaps best in the Northern hemisphere winter, because this is the time that Northern breeders move away from their breeding areas and displaced birds may find themselves on our shores. For the same reason, the chances of finding Southern hemisphere taxa here may therefore be best in the months following their breeding season; i.e. in the Austral winter, our Northern summer.
All three European Cape Gulls have been adult or near adult birds. However, given that younger individuals are more likely to occur here as vagrants, anyone keen to find a Cape Gull is perhaps best advised to have a working knowledge of what first cycle birds look like during the Northern summer. The aim of this note is to showcase this age group at this time of year, and highlight one or two features that should make them stand out among our local gulls. The main argument put forward is that because of their absolute age and related absence of primary moult, along with the presence of a remarkably striking secondary skirt, in the Northern summer first cycle birds offer good prospects for out-of-range identification – they are the Cape Gulls of good hope.
Taxonomy and terminology
Before discussing identification, a few words on taxonomy and the terminology used in this note may be useful.
Cape Gull (vetula) is the African subspecies of the very widely distributed Kelp Gull Larus dominicanus. So far it seems that all European records of Kelp Gull have been vetula (Jiguet et al., 2004), so at least for the moment this seems the most meaningful taxon to discuss. All the photographs and observations upon which this note is based relate to birds observed in the SW Cape Province of South Africa between 26 June and 6 July 2013. Hence, all birds are assumed to be vetula.
Comparing age classes of Northern and Southern hemisphere gulls is made complicated by respective breeding seasons. Terms such as ‘first winter’, ‘first summer’ are confusing when comparing Southern and Northern species, because the seasons are effectively reversed. Moreover, because breeding times differ by around 6 months, at certain times of the year the use of calendar years does not work. For example, the laying period of Kelp is November-December (Jiguet et al, 2001) so by April first calendar year (1cy) birds will be a few months old, and so could conceivably turn up in Europe; however, there is no such thing as a 1cy Northern hemisphere gull in April (as laying has not even commenced and birds reared in the previous season have ‘ticked over’ into their second calendar year) so meaningful comparison using calendar years is not possible at this time.
The solution is therefore to talk about ‘cycles’ (as per Howell and Dunn, 2007). A first cycle bird is defined here as a bird that has not completed its first primary moult. For Northern taxa this moult occurs in the summer of their second calendar year (e.g. in Scotland Herring Gulls start primary moult around 1 May), so first cycle birds in the Northern summer are around a year old. In Cape Gull this primary moult commences in October-November, when birds are a little less than a year old. The main advantage of using cycles when comparing immature Cape to our Northern gulls is that birds in the same cycle are the most likely cause of confusion, even though their absolute ages differ by several months.
Moult: Why we should not throw the baby out with the bath-water
Moult is often cited as being useful for field identification of gulls. Due to reversed breeding times, Southern hemisphere gulls such as Cape have completely different moult periods to Northern ones; they are in moult when our birds are not, and vice versa. This should make them look strikingly different. However, the opposing argument is that displaced birds may adopt (or ‘correct’ to) the moult cycle of the birds in their new location. Indeed, Malling Olsen and Larsson (2003) specifically make this point in relation to Kelp/Cape Gull:
‘Note, however, that Kelp Gulls in the Northern hemisphere may adopt moult cycles similar to Northern hemisphere gulls, as has been observed in US and Mexican adults’ (p144)
The Paris Cape Gull supports this argument, as it was moulting in accordance with our Northern hemisphere gulls (Jiguet et al., 2004). So perhaps we should abandon any thought of moult being useful for picking out a Cape Gull?
The Paris bird was an adult. In theory it could therefore have been in the Northern hemisphere for several years; this is ample time both for it to need to adjust its moult to Northern seasons and be physiologically able to do this. Younger birds, especially first cycle ones, by definition can’t have been here so long. A first cycle Cape reared in the austral summer will only be a few months old by the time the Northern summer comes around; unlike our birds whose primaries are a year old, a first cycle Cape should have much fresher feathers that do not need replacing, and in any case if it has only just arrived, it may not be physiologically capable of moulting feathers rapidly enough to catch up to our first cycle birds (i.e. our 2cy, ‘first-summer’ gulls; Table 1).
So, while we should always be careful when using moult, in the Northern summer it seems likely to be more useful for first cycle Cape Gulls than adults. In addition, as birds may be more prone to be displaced or wander in their first year of life, the Northern summer is likely to be a productive time for first cycle Cape Gulls in Europe. At this time, because they will only be a few months old, their moult and extent of feather wear and bleaching should be markedly different to our birds.
Picking out a Cape Gull
The starting point for picking out a Cape is to be familiar with first cycle Northern gulls during the summer when they are moulting primaries. A safe window for picking out a Cape would be May-August inclusive, as this covers the start and mid part of the moult period of Northern taxa, but is well before first cycle Cape Gulls might be expected to drop their first primary (Table 1). Because of feather grey tones and the inner primary patterns (detailed later) first cycle Lesser Black-backed Gulls (LBBs) are most similar to Cape, but actually the key distinguishing features suggested here also apply to separation of Cape from Herring and Yellow-legged Gulls. Plates 1-3 show what a typical first cycle graellsii or intermedius LBB looks like in mid summer, while Plate 4 shows a Yellow-legged Gull. The key points to note in these images are the extreme wear on remaining first generation primaries and wing coverts and the fact that birds are in primary moult.
The Full Article
This blog post is a taster to full article fully illustrated with Chris’s fab photos and available as a pdf.
Just download the PDF below.