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Comparative study of four populations of Yellow-legged Gulls in Western Europe.
by Anne TEYSSEDRE
L'Oiseau et R.F.O., V. 53, 1983, n° 1.
As the taxonomy of the Herring Gull group was and still remains very controversial, the authors are unanimous since DWIGHT (1925) to divide multiple sub-species into three groups:
- The argentatus group - Herring Gulls with pink legs,
- The fuscus group – Lesser Black-backed Gulls,
- The cachinnans group - Herring Gulls with yellow legs.
The populations of the first two groups, reproductively isolated in northern Europe, belong to two valid species: Larus argentatus and L. fuscus. As for Yellow-legged Gulls, they are classified according to the authors either in the first case (VAURIE 1965, BARTH 1968) or the second (VOOUS 1961), or constitute a third species (DWIGHT op cit., DEVILLERS 1977).
The dramatic expansion of the group argentatus-cachinnans-fuscus in western Europe has recently caused sympatric breeding of Yellow-legged Gull and Lesser Black-backed Gull (L. fuscus graellsii) on the coast of Cantabria, Spain (1975 BARCENA), and another contact zone of Yellow-legged Gull and Herring Gull (L. argentatus argenteus) on the island of Oléron (NICOLAU-GUILLAUMET 1977).
This allows the study of the reproductive behaviour of Yellow-legged Gulls, which still has an uncertain systematic status, vis-à-vis Larus argentatus and L. fuscus. It gives a new momentum to the study of speciation in the Herring Gull group.
As a first step, it is necessary to identify which population of Yellow-legged Gulls occur on the island of Oléron and which occur on the Cantabrian coast, named difeferntly according to different authors: michahellis or atlantis? (NICOLAU GUILLAUMET op. cit., CAMBERLEIN & FROLE 1979).
These two populations are compared both morphological and etho-ecological, between them and between another colony of Yellow-legged Gulls michahellis in southern France. To a lesser extent, this comparative study of populations also includes the subspecies atlantis (Azores Islands, Madeira, Canaries) and focuses on morphological characters.
MATERIALS AND METHODS
Field studied were carried out in three successive breeding seasons (spring 1979, 1980, 1981) and focused on three populations of Yellow-legged Gulls:
- The developing population (four couples) at Isle Oléron, which was monitored regularly from 1979 to 1981 (pop. 1);
- The colony of Port-Saint-Louis (Bouches-du-Rhone), studied from April 20 to May 15, 1981 (pop. 2);
- The colony of Fuenterrabia (Basque Country), studied from 20 to 30 June 1981 (pop. 3). The winter plumage of adults from this colony has also been observed, in October 1981.
We also examined 23 preserved specimens of the subspecies atlantis from the Azores, in the collections of the National Museum of Natural History, and we observed several groups of gulls of the same subspecies along the coast of Madeira and the Canary Islands (Gran Canaria and Tenerife) in February 1982. (Unfortunately, poor recording conditions - persistent noise of engines in ports – forced us to exclude the acoustic study of these populations.)
1) Ethological study.
We have consistently observed the behaviour of nesting (pop. 1, 2 and 3) and recorded vocalizations from an hide placed near territories. From the wide variety of sounds in the colonies, we have studied statistically four vocalizations. It is the staccato alarm call ("staccato call" of TINBERGEN 1959) and three vocalizations expressed with high energy:
- The basic call ("call note"), a signal of varying intensity and meanings (call, alarm, attack);
- The "trumpeting" or "complete long call", a territorial call;
- The "mew call", a signal of recognition in the family (partner or parent).
Thirty vocalizations of each type delivered by thirty individuals in each of the two colonies, and eight by birds nesting of the island of Oléron were recorded and transcribed into sonograms.
For each sonogram, factor duration and frequency were measured (units in brackets):
- Total length (Dt), (10-2 sec.)
- Unit length (Du) for repetitive calls (10-2 sec.)
- Rate (C) for calls,
- Number of units (K) for the staccato calls,
- Duration of upward glissando (Da) and descending (Dd) ((10-2 sec.)
- Maximum fundamental frequency (FFmax) (Hz),
- Fundamental frequency of the level (FFP) for mew call, (Hz),
- Fundamental frequency at the glissando of the slide down (FFB), (Hz),
- Number of harmonics (nH).
Averages (x) and standard deviation (Sx) of these factors were calculated for each sample and estimated for the corresponding populations (Y, Sp). Averages were then compared by the Student t test to 95% confidence interval.
Please click image to enter Xeno-Canto page for all recordings on michahellis calls
2) Study of reproductive periods.
If food and habitat do not differ much between populations of the same subspecies and the possibilities offered by the environment are similar, the reproductive periods mainly depend on latitude. We know the time of nesting in Yellow-legged Gulls in the Camargue (ISENMANN 1976) and those on the Azores (CHAVIGNY & MAYAUD 1932); and it was determined by observations of the first eggs laid in the small colony on Oléron in 1979, 1980 and 1981, and by the age of the chicks in the colony Fuenterrabia in 1981.
ISENMANN (1973) conducted a complete biometric study on Yellow-legged Gulls in the Camargue. In our study, birds of the Oléron and Basque Country populations were not captured. They were observed with binoculars from a hide and their feathers were collected from nesting sites. We also studied their winter plumage.
We collected thirty primaries near Fuenterrabia at the time of moulting (October 1980) to analyze characteristics. These feathers are all large (x = 33.2 cm; Sx = 0.9 cm). Their lengths correspond to the first (P10) and second primaries (P9), compared to specimens of the Herring Gull group that we measured. However, an examination of the wings of gulls of this group stored at M.N.H.N., together with diagrams and photographs of the five feathers by DWIGHT (1925), GOETHE (1961) and BARTH (1968) showed us that the dividing line between black and grey pigmentations differs in P9 and P10 by:
- At P10, the division follows a regular upward concave curve between the spine and the inner edge, leaving the spine at a distance of 21 to 25 cm from the apical end of the pen;
- Secondly, this curve, first rising, reaches its maximum before descending to the inner edge. The departure from the spine is between 13 and 18 cm from the apical end.
This allowed us to classify 19 feathers in the first category (R1) and 10 in the second (R2). Only one of these birds could not be classified with certainty among the first feathers because of the relatively small extent of its black pigmentation along the spine (19 cm).
The first two feathers found on two dead gulls on the Cantabrian coast in October 1981 have been added to this material for the comparative study of wing patterns. Finally, we completed the study of the morphological atlantis from:
- An examination of part of the 23 specimens originally from the Azores, placed at the Laboratory of Ornithology MNHN; and descriptions of 20 other specimens from the Azores studied by DWIGHT (op. cit. ) and STEGEMANN (1934);
- Secondly, observations with binoculars from gulls in Madeira and the Canaries in February 1982, mainly concentrated at fishing ports (Funchal, Las Palmas, Santa Cruz, etc.).
1) Ethological comparison.
There appeared to be no difference in behaviour between the three breeding populations. However, acoustic analysis brings new information (Tables I-II and Tables III-IV and Figure 1 and Figure 2):
- When comparing Oléron and Camargue populations, there is no significant difference (t <2.0) in averages for all four vocalizations (24 factors analyzed).
- On the opposite, all four types of vocalizations significantly differ between all populations of Oléron and the Basque Country (as well as those of Camargue vs. Basque Country).
Especially, three calls with high-volume output – basic call, mew call and trumpeting - differ highly significantly regarding most of the analyzed factors (13 out of 18 factors).
The staccato calls are less dissimilar among the six factors compared here, only frequency (FFmax) differs significantly between these populations.
2) Comparison of reproductive ages.
Most michahellis in the Camargue have egg-laying dates in the first half of April (ISENMANN 1976, pers. Obs. 1981).
Cantabrian gulls, nesting at the same latitude, lay eggs a month later, in the first half of May, Hence, when we observed the nests in the colony (end June 1981), almost all of them were still occupied by one or two non-feathered chicks, aged one to three weeks, and only very few juveniles could already fly.
On the Isle of Oléron (about 400 km north) eggs were laid in the period late April - early May in 1979, 1980 and 1981, which is about twenty days later than Camargue.
This corresponds with the period of egg-laying in atlantis on the Azores (latitude similar as Lisbon, 37°N) according CHAVIGNY & MAYAUD (op. cit.).
3) Morphological comparison.
The main morphological characteristics of all four populations (Oléron, Camargue, Basque Country and Atlantic islands) are as follows:
a) Yellow legs, steel grey upperparts and red orbital rings are common in all four populations.
b) Head and neck of Yellow-legged Gulls on Oléron - sedentary population- remain white in winter (G. BURNELEAU obs), which is like Mediterranean michahellis.
On the contrary, the winter plumage is mottled in Cantabrian Gulls in October. JOIRIS (1978) describes Portuguese Yellow-legged Gulls ("lusitanius"), observed on 27 January 1978, as smaller and paler than michahellis and having the head and neck white.
From these two observations, we may conclude the distinctiveness of Biscay and Portuguese populations in winter plumage: adults have already completed their prenuptial moult by October and the end of January.
Among the 23 specimens from the Azores (latitude as Portugal) examined in the laboratory, two individuals captured in February appeared to be unspotted, in January only slight traces of grey appeared on the front, while all seven individuals captured late August and September appeared (more or less) spotted.
It seems that there is no strict correlation between the time of moulting and latitude among various populations of atlantis: all adults on Madeira appeared white-headed in early February 1982 - as the type specimens from the Azores – but it is not the same in the Canary Islands (about 400 km south) as two weeks later they were still spotted on the head (This heterogeneity of populations of Yellow-legged Gulls on Atlantic islands is perhaps not restricted to moult timing: it seems that the vocalizations of birds on the Canaries is much closer to Madeira birds. However, technical difficulties and the shortness of our stay on the two islands did not allow statistical analysis of vocalizations to verify this).
c) According ISENMANN (1973), the second primary in michahellis (P9) usually has a white mirror of variable size. In atlantis this mirror is rare (3 cases out of 23 specimens in MNHN), and if present, they were small.
Six out of eight Yellow-legged Gulls on Oléron had a mirror on P9. All twelve P9 primaries of Cantabrian gulls showed a mirror, often quite large (see photographs, click image below).
4) Interpretation of results.
Apart from the 20-day difference in egg-laying period, which can be explained by the difference in climate and nesting conditions (isolated couples / colonies), the Yellow-legged Gulls of Camargue and those breeding on island Oléron are very similar. The comparative study on morphology and ecological ethology of these two populations did not reveal any significant differences between them. Therefore they belong to the same subspecies L. (cachinnans?) michahellis.
On the opposite, French Yellow-legged Gulls differ completely from Cantabrian birds: by vocalizations, by reproductive period (difference of one month while at the same latitude) and winter plumage. Furthermore, the Cantabrian form can not be assimilated with atlantis because it differs in the wing-tip patterns. It therefore is a subspecies not yet described.
We showed here that populations of Yellow-legged Gulls from France and the Spanish Cantabrian coast are two very distinct forms: now, what can be the origin of the Cantabrian form?
There is a chain of Yellow-legged Gull populations on the coast of the Iberian Peninsula forming a geographical link between the Mediterranean and Cantabrian populations. This allows to assume a relationship between the two forms. It is not very likely that the Cantabrian form is the northern limit of clinal variation of Yellow-legged Gulls around the Iberian Peninsula in acquiring pigmentation of head-feathers. In this case, it would be most likely the intermediate form between michahellis and a neighbouring subspecies with mottled winter plumage head (atlantis from the Azores?).
The Yellow-legged Gulls on the island of Oléron do not form mixed couples with Herring Gulls, nor does there exist mixed couples of Lesser Black-backed Gulls and Yellow-legged Gulls on the Cantabrian coast (pers. obs. between 1979 and 1981). Currently, sympatric populations are relatively small and reproductive isolation has not fully crystallised.
Following the hypothesis of a relationship between Mediterranean and Cantabrian Yellow-legged Gulls, the reproductive isolation vis-à-vis L. argentatus argenteus and L. fuscus graellsii respectively is important, and could confirm the evolution of the three classical groups into three species; a classification proposed by DEVILLERS (l977).
But it is equally possible that Cantabrian Yellow-legged Gulls are genetically closer to L. argentatus argenteus than they are to cachinnans michahellis. Indeed, the classical division of Herring Gull sub-species into three groups (argentatus, fuscus and cachinnans) has always been based on morphological comparison of specimens in museum collections, at the expense of other types of studies.
Now we have to research the heterogeneity of Yellow-legged Gull populations from multiple perspectives: morphologically and eco-ethologically. Doing so, will we still be able to construct a third group opposite Herring Gull and Lesser Black-backed Gull?
In the determination of species, comparative studies on populations must be completed by the factors of reproductive isolation. The role of morphological characteristics in the sexual recognition among gulls has not been tested or limited tested (SMITH 1966) by ethological experiments. For instance, the one-month difference in egg-laying date between Cantabrian Yellow-legged Gulls and those nesting in the Camargue (breeding at the same latitude) could be a significant single factor alone to isolate these two forms.
The classic systematic basis, to group gulls mainly on morphology, is questioned. Its validity can be tested by enlargement of comparative etho-ecological studies among different forms in a group and by investigating the role of morphological and acoustic characteristics when birds form a couple.
I like to thank C. ERARD who allowed me to study the collections of the Museum National d'Histoire Naturelle, L. HOFFMANN who invited me in his research center in the Camargue (Tour du Valat), P. NICOLAU-GUILLAUMET of the Centre for Research on Population Biology Bird for his many opinions and advice, and F. SENOSIAN for his valuable contribution to the study of the Cantabrian gulls.
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|Yellow-legged Gull lusitanius 256.G 01 November 2010, Guipuzcoa, Basque. Picture Ernesto Villodas.