nominate Lesser Black-backed Gull (L. f. fuscus)

(last update: 24-1-2015 )

adult fuscus: May

Plumage development and ageing of Baltic Gull Larus fuscus fuscus

In this article we describe the plumage development and age-related plumage variation of Baltic Gull (nominate Lesser Black-Backed Gull) Larus fuscus fuscus. The material consists of gulls ringed in Finland as pullus and later photographed in the same country. We describe Baltic Gull as it is during the summer in Finland. Baltic Gull – in contrast to the other gull species of the area – does not moult very much in summer and the plumage does not develop much. We do not try to reconstruct the moult and plumage development which occurs in the winter quarters, which mostly are located in the tropics for this species.

This article concentrates on third calendar-year and older birds. During their birth year Baltic Gulls are easy to age, and the same is true for the majority of second-calendar year individuals. Information about the latter based on known-age individuals can be found in Rauste (1999) and Koskinen & Rauste (2006).

The primaries are numbered outwards. The age of bird is noted in calendar-years, which is very convenient when concentrating on birds in the summer half of the year. The bird is in its second calendar-year, 2cy, in the summer when it is about one year old.

Material & Methods

For this article we studied over 1000 photos of 366 different individuals of Larus fuscus fuscus. The photos were taken between 12 May 1999 and 2 August 2009, mostly at Tarastenjärvi dump, Tampere, in south-central Finland (61º33' / 24º00'), but some are from other parts of southern Finland. The birds were selected to be photographed because the ring was seen, no other selection was done at the time. We estimated values for 39 variables describing the appearance of birds (cf appendix 1). The photos of the same individual taken during the same day have been combined as the same record, the photos from different dates have been kept separate. There were 577 records in total. Naturally, all variables were not seen for most of the records. In the majority of our analyses, any one individual has been taken into account only once for a year: the date when the biggest amount of variables were seen– or the relevant variables

There are some caveats which should be remembered when evaluating photographs in general. The most problematic of these is that the views and lightning are variable and non-standardised. For example, all photos which show the moult limits on the wing of a bird cannot be included in the moult data – only those which are good enough so that the lack of moult limits would show in a similar photo.

Description of the age classes

During its second calendar-year, Baltic Gull is relatively easy to age. Second calendar-year birds quite rarely show a distinctly yellow base to the bill, 3cy birds always. 2cy birds have no adult-like uniformly blackish mantle and scapulars, 3cy do. The eye of a 2cy birds has not yet become pale, in a 3cy it normally has. If there is a moult limit in the outer primaries of 2cy individuals, it is much more conspiciuous than in the primaries of normal 3cy birds (the outer primaries of 2cy birds are in this case worn juvenile feathers). Second calendar-year bird often show some streaking or other dark patterning on the nape, 3cy rarely. In addition, 2cy birds have no mirror on the outermost primary (or rarely a small one), while about 80% of 3cy birds have one.

Baltic Gull is very variable in its 3cy summer aspect. The head is almost always white and unpatterned, as are the underparts. Very few show some slight streaks on the nape (2%, n=115). Most have adult-like scapulars (98%, n=112) or in sitting birds visible greater (75%, n=110), median (86%, n=113) and lesser (82%, n=113) coverts. A minority has distinctly brown greater coverts, more rarely also median and lesser ones.

Typical for the age class is a conspicuous moult contrast on the outer primaries (on P5 - P10, that is the primaries which are visible on a sitting bird), which most individuals show. Three individuals (13%, n=23) of those whose wing moult could be studied on all primaries, did not show the moult contrast at all. In one the outermost new generation primary was P1 and in the other P4, so their moult contrast was not visible on the closed wing. The rest, 78%, showed the contrast even on the closed wing. Among such birds, the outermost new generation primary was P5 (12%, n=66), P6 (33%), P7 (21%), P8 (24%)  or P9 (2%). Otherwise, the primary pattern is adult-like in most birds, but some show uniformly blackish-looking outer primaries without any terminal or subterminal markings. The mirror on the outermost primary may be lacking (20%, n=60), or may be small (33%), or adult-like large (47%). On P9 a mirror is only rarely visible, in our material only in one of 26. Cf. also table 1.

The underwing may be similar to an adult (44%, n=18), but the coverts, for example the primary coverts, often show some dark markings, but not much. The tail often shows some black, but not normally a uniform dark band (a complete or almost complete tail band in 12%, n=41). Quite often the tail is wholly white as in adult (44%).

The legs are most often distinctly yellow (78%, n=119), either bright yellow or slightly more dull. The rest have legs which are reddish-tinged or almost without any noticeable tone. In our material, the bill was always yellow-based, and it always showed red on the gonys and often some black on the upper mandible (46%, n=105), less often a black band or tip (26%), and quite often it was totally without dark markings. The eye was often adult-like (or slightly duller) yellow (68%, n=93), quite often variably dark-tinged (28%) and only rarely almost black.

Baltic Gulls in their fourth calendar year always have an adult-like white and unpatterned head (n=92). They do not show any subadult type brown on the coverts or scapulars. Especially in late summer there is quite often a brownish tinge on some of the coverts or scapulars, which the individuals of the older age classes may also show.

All primaries are normally of a similar colour tinge, without moult contrasts. Some have an active or suspended moult in the inner primaries during the summer. When all the primaries could be studied, four birds out of nine did not show any moult contrast. In the remaining five birds, the outermost primary of the new generation was P1, P2 or P3 (among these were also two birds from May, which indicate that the moult has commenced before the birds arrived in Finland). In addition to these, one bird with an incomplete set of primaries studied, had a clear moult contrast between P6 and P7, which would be very typical for a bird one year younger. The mirror on the outermost primary is normally quite large and almost reached both edges of the feather (92%, n=61), but can be smallish (8%). Cf. also table 1.

The underwing-coverts are normally wholly white, one bird showed some black on the primary coverts and one other slightly more extensive markings on the underwing (n=12). The tail is almost always white (87%, n=23), or it may show some black markings (13%).

The legs are normally white (89%, n=93), but the yellow tinge may be lacking. The bill is yellow, but many have some black on the upper mandible (30%, n=89) and some have a greenish tinge in the yellow, especially in autumn.

There were birds from every age class until 16cy in the material. All older than 4cy were in adult plumage for the following details: gonys with red, bill base bright yellow, legs yellow (possibly dull yellow in older age classes too, however), underwing-coverts, head and tail wholly white, tertials, upperwing-coverts and scapulars adult-like, although coverts possibly with a brown tinge even in old age classes. Some black on the bill occurred in many age classes until 13cy, one 5cy and one 11cy showed dark spotting in the eye. However, Baltic Gull may show black markings on the tail even in 6cy (a photographed and ringed bird which was not included in our material, J. Haapala in litt.) In addition, one photographed 12cy which was not included in our material, had subadult type dark areas on the under primary coverts (M.Kangasniemi in litt.)

Adult birds may show a moult contrast in the inner primaries because of the winter moult, and during autumn they often moult some inner primaries before the migration. A 3cy-type moult contrast visible on the outer primaries occurred in two 5cy birds (P5 and P6 were the outermost new primaries) and one 6cy bird (P6). All older than 3cy birds showed a mirror on P10, but it was quite small in, for example, one 8cy and one 11cy bird. There are more variation in the other patterns of the wing, cf. table 1. It can be seen that after 4cy there is hardly any further change in the pattern. Birds older than 2cy have almost always black on seven to nine primaries, only one individual with black restricted to six primaries and one with black on all ten. In all age classes, the most usual is that seven primaries show black markings.

Table 1. Primary markings of Baltic Gulls. Innermost black refers to the innermost primary which still shows some black area subterminally. Innermost band refers to the innermost primary with a uniform black subterminal band reaching from edge to edge of the feather. Among 3cy birds, black markings were not at all visible in 27% of birds, which do not had wingtip markings developed to adult-like.

Tables

Acknowledgements

Markku Kangasniemi, Hannu Koskinen and Visa Rauste suggested improvements to the presentation of the study. Markku Kangasniemi sent us many photos for the analysis. Jukka Haapala, Seppo Niiranen and other stuff at the Finnish Ringing Centre provided us with ringing data. Without work of Risto Juvaste in the ringing project of Lesser Black-backed Gulls there would be no the material we used. Other gull ringers have also done much important work.

Literature

Koskinen H & Rauste V 2006: Primary moult of Baltic Gull during the first 15 months. Dutch Birding 28 (3) 158-161.
Rauste V 1999: Kennzeichen und Mauser von "Baltischen Herringmöwen" L. (f) fuscus und "Tundramöwen" L. (f) heuglini. Limicola 13 (4) 153-188.