Common Gull (Larus canus) 1cy June - September
Is it possible to age subadult Common Gulls Larus canus?
Kenneth Bengtsson & Lennarth Blomquist
publication in: Anser 2-42(2003): p 73-92.
"we" in the text below refers to the original authors. If any errors occur in this text, please let me know and mail to marsmuusse at gmaildotcom.
Link to FULL COLOUR PDF (140 MB!, in Swedish).
This study focuses on birds of known age. Over a period of many years we have trapped and ringed Common Gulls in Pildammsparken in Malmö. The majority of these birds were ringed during their first year and are therefore of known age. In total, we have ringed 9.421 Common Gulls in Malmö during the period 1980 to 2002. We have also ringed 126 birds at Spillepeng, which is close to Malmö and another 329 chicks in Malmö harbour.
Our strategy, when we started this study, was to retrap ringed birds and to take photos of these birds in known age. We have managed to recatch 92 of our own birds, which were ringed in their first year. When retrapped, these birds varied in age between 2 and 12 years old (plumage year). We have caught also 20 birds which were ringed as chicks in other countries. Furthermore, our material consists of 25 of our own birds which were controlled, but not photographed; 101 unringed second-year birds; and a reference-group of 100 anonymous adult birds (caught and ringed as 3-years or older). In the data set we also have 374 photographed one-year old birds.
What is our purpose?
We have focused on the upperside of the wings and on the tail. One factor that brings problems to our study is the occurence of both races (canus and heinei) during winter-period, which is the main catching-period. We have not been able to determine the exact race of all birds. Therefore we use the material as one unit, but with comments about race and sex when we have data on this.
We have worked with, and are talking about, wing- and tail-patterns. These flight feathers are carried for one year. Therefore we can talk about one-year old birds, two-year old birds and so on. “Age” changes on 01 July in our study. First-year birds are birds during the first year, second-year birds are birds in second year and so on. Primaries are numbered P1-P10 with P10 being the outermost feather.
Statistic information is shown in tables 1 to 6. Information on measurements (wing and/or head is in mm. e.g. 349/95,4); information on origin, assumed sex and race is captured in the photo-text, and you also find here ring number and ringing data. The photos are taken under different light-conditions, and it is therefore not possible to correctly determine grey tone on most of them. However, the contrast between grey and white and darkness of grey is obvious in most photos.
About race and sex-differences
Heinei is a race which occurs east of Arkhangelsk (40E) south to Moscow (37E). Observations over the last decades indicate that heinei has become a more common winter visitor in southern Sweden, Denmark and north Germany (Hein & Martens, 2002). Our recoveries from Malmö show also an increased occurence over the last years.
Heinei is slightly larger, longer winged and darker than canus. In some individuals this is very obvious in first and second plumage (years), but many birds show plumages that are intermediate between both races. In first year canus, the centre of the wings is paler and, together with greater coverts, often shows an obvious pale panel on the wings. Heinei often has the wings obviously darker than canus. Vestiges of black in the tail, together with extensive black markings on the wings, are supposed to be a good indication for two-year old heinei. Older birds of the race heinei show a slightly darker grey tone on upperparts and more obvious contrast between white and grey on the primaries. We believe that canus generally shows more extensively white in plumage (all ages) than heinei.
A strict classification regarding taxon for each individual is difficult, but we are convinced that dark one-year old birds are heinei and also birds showing extensive black in second-year should be called heinei, particularly large birds. When upperparts are dark grey, combined with obvious contrast between white and grey on primaries, such birds are also labelled heinei. But we encountered many birds showing intermediate characteristics in all ages. In our data set we have documented several canus, ringed as pullus in Denmark, Finland, Russia and southern Sweden, but we have no recoveries of heinei which were ringed as pullus.
In gulls, males are larger than females, with limited overlap in size, but unfortunately small males and large females exist. Kalev Rattiste has documented that males show more white in the wings than females, but it remains to be seen how to use this in classification of individual birds.
Klaus Hein & Sönke Martens (2002), in an extensive study in north Germany, found that it is possible to determine race and sex in many of the trapped Common Gulls. Local canus could be classified by bill and wing: head + bill < 88,5 mm and wing < 357 mm are females; head + bill > 91 mm and wing > 364 mm are males.
For heinei the authors described: head + bill < 88,5 mm and wing < 366 mm are females; head + bill > 91,5 mm and wing > 390 mm are males.
More information on measurements and colour development and saturation on legs and bill can be found in the article of Hein & Martens.
First year plumage (1CY-2CY)
We have not much to say about the juvenile plumage. Normally this plumage has been replaced by first winter-plumage when we start catching in winter. However, we have some pictures of juvenile individuals that hatched near Malmö, like the one in fig 1. In late autumn we sometimes catch birds with retained juvenile plumage. Full juvenile plumage can be found as late as 08 November, illustrated by the bird in fig 2, from 1996. A moderate number of birds still show juvenile feathers in neck, mantle and scapulars throughout the whole winter (see fig 3 and fig 6).
Plumages in first winter birds vary from bright to dark individuals (fig 4 and fig 6). Most birds show a “normal plumage”, like illustrated in fig 5. Pale birds (like in fig 4) are believed to be canus, while darkest individuals are assumed to be heinei (fig 6). A pale central panel on the wings is believed to indicate canus (like the one ringed in Helsinki, fig 5) and birds with dark greater coverts and inner-primaries are assumed heinei (fig 6). However, our data indicate that some canus may show quite dark wings (see fig 7, bird hatched in W Finland).
One year old Common Gulls normally have no signs of mirrors in P8-10, but some birds shows faint white in P10 (see fig 8). We carefully checked 374 one year old birds and found this white P10 patch in only 13 birds (3,5%).
The size and pattern of the black tail-band varies and pattern on upper-tail coverts show variation in intensity. Some individuals have the upper-tail coverts completely white, while others have more or less obvious spots. Our data indicate both canus and heinei can have white or spotted upper-tail coverts, and both races also vary a lot in tail-band pattern (see canus with white and spotted coverts in fig 1, fig 5 and fig 7). Still, we suspect that really heavy spotting on upper-tail coverts in an indication for heinei (see fig 2).
We did not document moult timing, because of difficulties in catching gulls in the summer period. The small number of birds that we managed to trap over the years include two birds (one pale bird and one dark: fig 9, fig 10), which moult to second generation plumage. The pale bird (fig 9) was ringed as pullus in Malmö and is a certain canus. The dark bird (fig 10) might be a summering heinei. The dramatic differences between first and second year plumages are obvious, in particular considering the worn juvenile feathers.
Second year plumage (2CY-3CY)
There are few problems to age birds in this plumage, as it shows typical characteristics in most individuals. Average birds have adult-like plumage, including a grey mantle and white tail. The primaries show more blackish than in adult birds, while wing-coverts and the alula have black markings to a various extent. Normally, seven primaries show black and the white tip on P7 and particulary on P8-P9 are missing on most birds. Mirrors are normally present but smaller than in adult birds, particularly the mirror on P9. The primary coverts, especially the outer lesser coverts, have black markings to a variable extent, but we also found a lot of two-year old Common Gulls with these coverts adult-like grey.
Ageing of first and second plumage birds is often straight forward, but we have one bird which is difficult to age (see fig 11). It probably is a retarded second-year bird, which have retained some of the first-year feathers. Another slow developing bird is shown in fig 12, but normally the differences between first and second year plumages are obvious.
Ageing may be more problematic in advanced second-year birds, which can be difficult to separate from retarded three-year old birds (more about this later). Second-year plumage is showing huge variation, from adult-like birds with limited dark markings on wing coverts and all white tail, to birds with quite a lot of dark markings on both wing and tail.
The “average” bird (see e.g. fig 13) has white tail-feathers, 7 to 8 primaries showing black, one large and one small mirror, black markings on the primary coverts, black on the alula and black tip on P7-P9 (particularly P8-P9) without white tips. The absence of a white tip on P8 is believed to be an important character to separate well-developed second-year birds from retarded third-year old birds. On 91 second-year birds we only found three birds showing a white tip on P8, while 16 out of 18 three-year old birds had a white tip on P8. The white tip on P9 is more frequently lacking in second-year birds (only one out of 91 birds had this white tip), but unfortunately the white tip is also often missing in older birds.
Out of 138 second-year birds, 24 (17 %) had vestiges of black on the tail. Probably this is more usual among heinei than on canus. All 151 second-year birds had a mirror on P10, while 129 (85 %) birds had also a mirror on P9. Hence, only 22 birds (15 %) had only one mirror; none had no or three mirrors. Normally second-year birds have 7 primaries showing black, but we found a large range, spanning from six to all ten primaries black-marked. Statistical information is presented in tables 1 to 6.
So one can describe an “average” second-year bird, but there are plenty exceptions and strange looking individuals. Fig 13 shows a normal bird with a small spot on P3, two mirrors, absence of white tips on P7-P9, black marks on primary coverts and an all white tail. A very standard second-year Common gull! Fig 14 depicts a strongly patterned bird, probable heinei. It only has a mirror on P10; it has black on all ten primaries; several wing-coverts show strong black markings and it has black in the tail. In contrast the bird in fig 15 shows two mirrors, black on only six primaries, adult-like coverts and an all white tail. Fig 16 shows another well-developed second-year bird with a tendency to white tips on P7-P9 (small, but present). This bird also has black on the tertials, which is seldom seen.
The pattern of P8 may serve as a good character to separate advanced two-year old birds from retarded three-year old birds, but advanced birds may look like the one in fig 17. This well-developed bird has obvious white tips on P7, P8 and P9 (it proved to be the only bird with a white tip on P9, out of 91 controlled second-year birds). In general plumage this bird very much resembles a retarded three-year old bird and it would not have been aged correctly if it wasn’t ringed in its first year. It even shows a mirror on P10 that merges with the white tip (a character only starting to become common in seven years old birds, according to our material). However, the bird has seven primaries with black and the mirror on P9 is not fully developed.
Third year plumage (3CY-4CY)
In their third year Common gulls are almost impossible to distinguish from four years old and older birds. The average three-year birds have mirrors on P9 and P10, black on six primaries (range 5-7) and white tips on P7 and P8 (about 50% also have a white tip on P9, see fig 18). Without exception the tail is all white in three-year old birds.
Retarded three-year old birds may be separated from advanced second-year mainly on the white tip on P8 and normally the mirror on P9 is fully developed. However, some second-year birds (3,3%) have a white tip on P8 (see fig 17) and some third-years do not show a white tip or have a very small one (two birds out of 18 in our data set). The third-year bird in fig 19 lacks a white tip on P8, has a dark alula and small dark markings on wing-coverts. The same bird has been documented as four-year old (fig 24 ) and even in this plumage it still lacks the white tip on P8 and shows the dark alula.
Most three-year old birds have two mirrors. But we found one bird (out of 24 proven three-year birds) already showing a small mirror on P8 (see fig 20). This bird could not be aged correctly as three-year old without ring-information. When adult-like Common Gulls have a dark alula, this may indicate third-year age. In the group of 18 proven third-year birds, we found four showing this dark alula. However, three out of 15 four-year old birds were still showing a dark alula and even one out of twelve five-year old birds showed this feature. The combination of a dark alula together with small dark markings on wing-coverts is supposed to indicate third-year birds (see fig 21), especially if birds also have a white tip on P8 (to separate it from advanced second-year birds). An interesting example bird from the overlap zone between canus and heinei is shown in fig 22. This probable female was ringed as pullus near Arkhangelsk on 40E and shows the typical grey tone for canus. The photo is taken with flash, but we remember the bird in the field to be comparable with the tones in this image.
Fourth year plumage (4CY-5CY)
We have 15 proven four-year old birds in the data set. The majority of these birds have two or three mirrors and black on six primaries (range 5-7). The alula is dark on three of them, while two birds have small dark markings on the primary coverts. Three out of 15 have a mirror on P8. All 15 birds have completely white tails. The most mature looking four-year old bird is shown in fig 23. We have also some less developed, retarded birds in this age-class; fig 24 shows a bird with black in the alula, small markings on the greater primary coverts and lack of a white tip on P8. This black tip on P8 is normally an indication for two-year old birds and the bird in fig 24 is the only older birds in our material that shows this character. The same bird is shown as three-years in fig 19.
Fifth year plumage (5CY-6CY)
In the data set of 12 five-year old birds, five have three mirrors. The majority of the birds have black on six primaries but the range is 5-8. The only bird which shows black on P3 is depicted in fig 25. The black alula is not a safe criterion to age three-year old birds, as it still present in one five-year old bird as well (see fig 26). All 12 birds have white tails and white tips on P7-P8. A mature bird in which the mirror on P10 merges with the white tip (broken black sub-terminal band on P10) is shown in fig 27. This P10 pattern becomes more common on older birds, usually from about seven-year old plumage, but sometimes seen in younger birds (e.g. the second-year bird in fig 17).
Sixth year plumage (6CY-7CY)
We have ten six-year old birds in our material and they are surprisingly similar. For example all have only two mirrors (while 42% of five-year old birds and 37% of 7-12 year-old birds have three mirrors). This demonstrates that 10 birds still is a small sample size and that coincidence may influence the conclusions. In fig 28 we show a probable male heinei, identification based on darker grey tone of wing-coverts and mantle and the sharp contrast between white and grey on primaries. This bird has black on seven primaries, which is more than average, and supports the hypothesis that heinei has more black in the wing-tip than canus. This same individual is also documented as seven-year old bird, with an identical wing pattern (no photo added).
Seventh year plumage(7CY-8CY)
Our seven seven-year old birds show 2 or 3 mirrors, black on 5 to 7 primaries and white tip on P7-P9. Fig 29 shows a seven-year old bird which is the same individual as the four-year old bird in fig 23. There are slight differences between these plumages, especially the wing-tip pattern changed gradually throughout the years: the mirror on P10 is now merging the white tip (broken band on P10), and suddenly it has developed a black spot on P5 (these characters were not present as four-year old bird). Fig 30 shows a canus (breeding bird in Malmö, killed in traffic) on which the black sub-terminal band is strongly reduced; this is the only bird showing this amount of white on P10 in our set of 73 photo-documented 3-12 year-old birds. In the reference-group of 100 anonymous adult birds (no certainty about exact age) we found this character on six birds.
Eight to twelve year plumage (8CY-13CY)
We only have small numbers of each age class and therefore we group them together. Fig 31 shows an eight-year old bird, which was documented before as five-year old on fig 25. Now, in eight-year plumage this bird has lost the marking on P3 and shows reduced markings on P4. Otherwise both plumages remained rather similar, despite three years of age difference. Fig 32 shows a nine-year old canus female hatched in Malmö. Note the rather pale grey upperpart tone and poor defined contrast between white and grey on primaries (compare with fig 28 and fig 34). Fig 33 shows another canus, a ten-year old bird hatched on Kandalasha, SW Kola peninsula. It is very similar to the female from Malmö depicted in fig 32. Fig 34 shows a twelve-year old male, probably heinei, which was ringed as one-year old winter visitor in Belgium.
How do adults develop?
Our material indicates that the Common Gull attains a mature plumage in its fourth year (4CY-5CY). After the fourth year, only minor changes occur in the plumage resulting in slightly more white and less black in the wing-tip; for example, the mirror on P10 may merge with the white tip in some individuals. Such slight changes in wing-tip pattern are illustrated by the bird in fig 23 and fig 29, four years old and seven years old respectively. We add a series of birds in three different adult plumages (fig 35-39). First, fig 35 (upper bird) to fig 37 show a probable female heinei, aged third, fourth and fifth plumage. When we compare the four-year old bird with the three-year old, it appears that the mirrors and white tips on the primaries are slightly larger and this bird lost the dark alula. Obvious differences between forth and fifth plumages are not present. The series 35 (lower bird) and fig 38-39 show a probable male canus in its fifth, eight and eleventh plumage. In the two older plumages the mirror on P10 is breaking through the black band and the black marking on P5 is missing on the eleventh-year old bird. Otherwise the wing remains almost comparable throughout the three age-classes.
In a very well-studied Estonian population (canus) they found that the amount of white in the wing-tip diminished in older plumages (after the sixth breeding-season). This may be explained by older birds having a weaker condition because of age, when they are compared to “middle-age” breeding birds (Kalev Rattiste and Lauri Saks, unpublished data).
Summary of characteristics
In table 1 to 6 we show the statistical background for the six main characteristics that we have measured. In these tables we also show statistics for the anonymous adult reference-group of 100 birds (birds ringed in mature plumage, no certainty about exact age).
Number of mirrors: 3,5% of first-year birds show an faint mirror on P10. All second-year (and older) birds have a mirror on P10. All three-year old (and older) birds also have a mirror on P9. 15% of second-year birds have only one mirror, while 18% of three-year or older birds have a mirror also on P8 (and in this respect match the adult reference group showing three mirrors in 20% of the birds). A study on breeding Estonian Common Gulls (canus) showed that 28% of the adult birds have three mirrors (Kalev Rattiste and Lauri Saks, unpublished data). Maybe it is the influence of heinei in our material that causes the difference?
Numbers of primaries with black markings: The average second-year birds have black on seven primaries, while three-year and older birds have black on six primaries. However, this ranges from 6 to 10 primaries in second-years and ranges from 5 to 7 in third-years and older. We found one five-year old bird with black on eight primaries. The adult reference group is dominated by birds with black on six primaries (90% of three-year and older birds). When all mature birds in the data set are grouped together (3-12 year old birds), the distribution of black is: five primaries (11,5%), six primaries (71,3%) or seven primaries (16%).
White primary tips on P7-P9: A white tip on P7 was found in 16% of the second-year birds and in all older birds. A white tip on P8 we found in only 3% of second-year birds, in 89% of third-years, in 92% of forth-years and in all older birds. The white tip on P9 was present in just one out of 91 second-year birds, in 54% of the 3-5 year old birds and in 81% of six years or older birds. The reference group shows 100% white tips on P7-P8 and 74% has a white tip on P9. It has to be mentioned that wear in feather tips may hamper measuring white tips.
Mirror merged with white tip on P10: We have only one individual among 73 birds of 3-12 year old in which the black sub-terminal band is completely missing (P10 mirror completely merged with white tip). In the reference group we find this on 6% of the birds. A broken sub-terminal band is found in 20,5% of 3-12 year old birds. In the reference group, a broken band is present in 17% of the birds. Among 91 second-year birds we found one bird with this character.
Black alula: A blackish alula was found in all second-year birds, in 28% of third-years, in 23% of fourth-years, and in one out of eleven five-year old birds. When third-year and older birds are grouped together, 12% show this character, compared to only 5% in the reference group.
Black markings on the tail: 17% of second-year birds had black marking on tail. Older birds (including all birds in the reference group) have all white tail feathers.
One-year old birds and almost all second-year birds can be aged positively. Some 3-year old birds can be aged with certainty based on multiple characters. Older birds are impossible to age accurately, they should be called simply “adults”.
Kjeld T Pedersen and Eddie Fritze for many long discussions about gulls, thanks to the Swedish Ornithological Society (Danielssons foundation) for financial support, to Sven Splittorff for practical help, to Kommunteknik in Malmö for access to their location to trap birds, to SYSAV and to the Ringing Center in Stockholm.
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S-232 32 Arlöv
John Ericssons väg 85F
S-217 72 Malmö
Common Gull (Larus canus) 1cy June - September