Moerdijk Field Research 2005-2006

(last update: 14 August 2010)

Discussion

We found much phenotypic variation in the Herring and Lesser Black-backed Gulls breeding in one single Dutch colony. For each species, there was variation in each of the 11 plumage and moult characters that we studied. The most striking result of our study is the range of upper-parts grey tones. We measured grey tone in the field, and afterwards we checked measurements from photographs showing both the grey-scale together with scapulars.

In Herring Gull, the range for upper-parts grey tone was from 3 to 7. Despite his small sample size of 20 Dutch birds, Barth (1968, 1975) already showed a large variation in Herring Gulls from the Netherlands. Barth used Munsell values in his measurements, later converted to the Kodak Grey Scale by Jonsson (1998). We used an Agfa scale, which is identical to Jonsson’s Kodak scale. The darkest Dutch birds in Barth’s sample measured Munsell 5.3, which is only slightly paler than the average grey tone for nominate argentatus from East Finnmark. We have birds a little darker in our Moerdijk sample; two birds measured Kodak 7 (equal to Barth’s Munsell 5.0), which is slightly darker than average nominate argentatus from East Finnmark.

Barth defined the ‘British form’ as birds which measure Munsell 5.7 and higher (in our analysis Kodak 5.5 and lower). The pale Moerdijk birds in our analysis match Barth’s L. a. argenteus from the U.K. From the Moerdijk Herring Gulls, 95% falls in the Kodak range 4.0-6.5 (equal to ... Barth’s Munsell uit 1966) and are therefore categorised to belong to the ‘British form’.

The Moerdijk Lesser Black-backed Gulls grey tone ranged from 8 to15. Plate 5 illustrate dark Lesser Black-backed Gull oLP with grey tone 15, together with an individual showing the most frequent grey tone of birds breeding in the colony (grey tone 11).

The modal grey tone among the Moerdijk Lesser Black-backed Gulls is close to the population mean of 11.7 for intermedius from the Swedish SW coast given by (Barth (1975). Upper-part grey tone is the prime characteristic for separating graellsii, intermedius and fuscus Lesser Black-backed Gulls. With Moerdijk modal grey tone very close to SW Swedish birds, one may ask if there is much migration between birds from SW Netherlands and SW Scandinavia. According to Barth’s and Jonsson’s studies, nominate fuscus from the heartland of its range (SW Finland & NE Sweden) shows grey tones of 13-17 (mean value 14.4). In our Dutch sample, six birds out of 888 showed grey tone 14.5 or 15.0, matching average nominate fuscus in the darkness of the upper-parts.

Despite intense ringing programmes in S Norway and SW Sweden, genuine intermedius was rarely found breeding in the SW Netherlands between 1990-2010 (pers. Med. Norman van Swelm). Contrary with what would be expected; ring recoveries show it is the paler British graellsii that can be found in low numbers, breeding in mixed pairs in the SW Netherlands. Therefore, it seems that the dark birds are simply part of the Dutch intergrade population, without (recent) influences of mixed breeding intermedius.

Herring Gulls in Finnmark belong to the darkest populations for Europe (Barth research).

Plate 5. Breeding Lesser Black-backed Gulls, Moerdijk, the Netherlands: grey tone 11 in o31 (average) and 15 in oLP (dark).

Our survey reveals that about 20% of individuals of both breeding taxa have obvious iris speckling (category 2-5: speckling more than 1%, obvious visible in the field). In some cases it can be argued that the eye is injured, but we found many birds with both eyes heavily speckled. When sub-adults are excluded, the picture remains the same for both species. We conclude from our data that iris speckling is part of the Dutch population variation. This suggests that caution is needed when the presence of iris specking is used in identification of western and eastern taxa (i.e. argentatus v cachinnans and fuscus v heuglini).

Black bill markings are common in fully mature Lesser Black-backed Gulls (57% showed black on both mandibles). We also found 25% of fully mature breeding Herring Gulls (defined in our study as those lacking black on the primary coverts) showing black on both mandibles. Due to differences in methodology and scoring, it is difficult to compare our data with those of others. Nonetheless, we can conclude from our data that black bill markings are not necessarily a feature of non-breeding and/or sub-adult birds.

Plate 6. Bill markings in individual (coded) Herring Gulls. oHL (11cy) & oKL (10cy) with vestigial black markings. m6T (5cy) with extensive black. oHJ represents the small group of females in which the red gonydeal spot continues on the upper mandible.

Plate 7. Primary patterns of individual (coded) Lesser Black-backed Gulls. Mirro on P8 in oNL (adult), o65 (12cy) with black on P2-P10, and breeding o32 (4cy) lacking mirrors on P9 and P10.

Compared to the taxon heuglini, the average number of primaries with black is lower in graellsii (Buzun BB). About 11.5% (n = 982) of breeding Lesser Black-backed Gulls (sub-adults excluded) have eight or nine primaries with sub-terminal black markings. The mirror on P9 and even P10 may be absent in breeding birds, although the latter was only found in sub-adults, showing other immature features such as dark wing-coverts and tail markings (LBBG o32 Plate 7). The size and number of mirrors and the number of primaries with black pigmentation have at various times been cited as possible field marks that might be useful in separating the various Lesser-black-backed Gull taxa, including heuglini. The variability shown by our Dutch-breeding birds suggests the need for caution when using these elements of primary pattern in field identification.

Plate 8. Thayeri patterns in Dutch Herring Gulls oHP, mNH, m68, m1V, NLA 5.363.004

Interesting primary patterns in Herring Gulls include birds showing thayeri patterns on P9 and on P10. This thayeri pattern on P9 is quite common in nominate argentatus from Finnmark (over 20% of the birds, according to Barth (1975). Goethe (1961) scored 160 Herring Gulls from N Germany and found 6 birds with a thayeri pattern on P9 or P10. Adriaens and Mactavish (2004) scored 133 Belgian birds and found one individual with a thayeri pattern. We found five such birds in our sample. Normally, thayeri patterns are associated with limited sub-terminal black markings in the wing tip (e.g. Northern birds with no sub-terminal black marks on P10 or lacking black in P5 or even P6). However, our data show that this is not necessarily the case: e.g. bird mNH has a thayeri pattern on P10 but also shows a near complete black sub-terminal band on this feather (Plate 8). Herring Gull NLA 5.363.004 combines thayeri patterns with a red orbital ring (the only such Herring Gull in our sample), as is commonly found in SE Scandinavia. The origin of NLA 5.363.004 is not known (it was not ringed as pullus), but it has been breeding at Moerdijk over the last five years.

Plate 9. Arrested primary moult in Dutch breeding Lesser Black-backed Gulls. See for more examples THIS page.

Some Lesser Black-backed Gulls in our sample showed arrested moult, as described by Stewart (2006) for British birds. We found 13 birds with P1 renewed (e.g. LBBG oCJ; Plate 9) and four birds with P1-P2 renewed (bird oCH). These individuals illustrate that Dutch birds occasionally moult inner primaries on the wintering grounds. Another group of birds showed inner primaries that are obviously older than the outer primaries. In these birds moult was apparently arrested (e.g. to migrate south in the autumn) and continued much later, leaving a distinct difference in feather wear visible in the wing. We found five birds with P1 obviously older than P2-P10 (LBBG oET, Plate 9) and seven birds with P1-P2 much older than P3-P10 (bird oJX). These 12 birds match the moult strategy described for adult nominate fuscus (Jonsson, 1998b). Further studies are needed to determine whether moult pattern is an individual, population or sub-species trait and, hence, the extent to which it can be used in identification.

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