INTRODUCTION

In contrast to most other gull populations in northern Europe, the Lesser Black-backed Gull Larus fuscus is a long-distance migrant that may winter as far south as central Africa (Cramp & Simmons 1983, Kilpi & Saurola 1984, Bakken et al. 2003, Bonlokke et al. 2006). Traditionally, Norway has been home to two subspecies of the Lesser Black-backed Gull; the black-mantled nominate L. f. fuscus in mid and northern Norway and the greyish-mantled L. f. intermedius in the south (Cramp & Simmons 1983). L. f. fuscus populations have declined during the last decades and the subspecies is now endangered all over its distribution range in the Baltic Sea and northern Norway (Bevanger & Thingstad 1990, Strann & Vader 1992, Hario et al. 1998, 2004). In contrast, the Norwegian intermedius population increased steeply during the 1980s and early 1990s (Lorentsen 2007), and since the late 1980s greyish-mantled Lesser Black-backed Gulls with a L. f. intermedius appearance have been found in colonies in northern Norway which were previously occupied solely by the nominate fuscus (Strann & Vader 1992, Bustnes et al. 2006). It is not known whether these new birds originate from expanding intermedius colonies in southern Norway, or from colonies of a third subspecies, L. f. graellsii, which has increased in the North Sea area and in Ireland (Creme et al. 1997, Garthe et al. 1999), as these two subspecies are difficult to distinguish.

A large number of ring recoveries of Norwegian intermedius have consistently shown that they predominantly winter around the Iberian Peninsula, in the western Mediterranean, and down to Morocco and Mauritania (Bakken et al. 2003). The migration pattern and wintering areas of the Norwegian fuscus is, however, virtually unknown with only two recoveries from the Black Sea area being published. Based on these recoveries, it has been assumed that the Norwegian fuscus has a southeast migration (Bakken et al. 2003) in line with the Finnish and Danish fuscus populations for which a large number of recoveries have documented an eastern flyway through the Black Sea area and then south through the eastern Mediterranean, down the Rift Valley to freshwater systems in Ethiopia, Uganda and Kenya (Cramp & Simmons 1983, Kilpi & Saurola 1984, Kube et al. 2000, Hario et al. 2004, Bønløkke et al. 2006).

The aim of this study was to unravel the migration pattern of adult and juvenile Lesser Black-backed Gulls from northern Norway, and also to document whether L. f. fuscus and greyish-mantled birds show different migrations. To achieve these aims, a ringing program has been carried out in 12 colonies along the coast of northern Norway (Table 1).

STUDY AREA AND METHODS

We defined northern Norway as composed of the three northernmost Norwegian counties: Nordland, Troms and Finnmark (Fig. 1). In this region we marked birds in 12 colonies varying in size from two to 400 pairs (Fig. 1, Table 1). Nine of these colonies had a mixture of fuscus and intermedius/graellsii, while in three colonies only fuscus was breeding (Fig. 1, Table 1). In the mixed colonies there were about equal proportions of the black and grey-mantled subspecies (Table 1). The ringing program started in 2000 for an indefinite time. This study presents resightings from 2000 through early May 2008.

Table 1.

Location of Lesser Black-backed Gull study colonies in northern Norway (see Fig. 1) with information on colony size and composition and marking effort.

Adult birds were trapped during the incubation period in late June by using a walk-in nest trap. When caught, the birds were weighed, measured and equipped with alphanumeric colour rings and a metal ring. Colonies were visited a second time in late July to mark fledglings (>∼300 g) with colour rings and a metal ring.

In colonies where both fuscus and greyish-mantled birds were found (Fig. 1), adults were determined to subspecies visually when birds were in the hand. Most birds were easily defined as black-mantled fuscus or greyish-mantled intermedius/graellsii. Juveniles could not be determined to subspecies. All resightings were opportunistic observations by different observers in many countries. Resightings in the breeding colonies are not covered in this study.

Migration routes were classified as eastern or western, with Denmark as a delineation point; birds observed east of Denmark were classified as eastern migrants while birds seen west of Denmark was classified as western migrants. When birds had been observed in more than one year or more than once in one non-breeding season, we selected the observation from the first year following ringing to construct Fig. 2. To classify winter areas we defined the overwinter period as 1 November through 31 March.

Figure 1.

Study colonies of Lesser Black-backed Gulls in northern Norway. Open dots indicate colonies of mixed subspecies while black dots indicate pure fuscus colonies (see Table 1 for additional details).

RESULTS

A total of 199 adults and 1284 juveniles were marked with colour-rings between 2000 and 2007. Sixteen (8%) of the adults and 83 (6.5%) of the juveniles were resighted outside the breeding colonies until early May 2008.

Wintering areas

Twenty-nine juveniles from mixed colonies and 13 from pure fuscus colonies were resighted between 1 November and 31 March. The winter areas comprised 12 countries, with 83% of the birds resighted along the western migration route. There was no difference in routes between colony types; 23 (79.3%) juveniles from mixed colonies were found in the area between England/France and Mauritania, two in Italy, two in Israel and one in Cameroon (Fig. 2); from pure fuscus colonies, 11 (84.6%) were found between England and Senegal, while one was observed in Israel and one in Kenya (Fig. 2).

Seven greyish-mantled birds ringed as adults and four fuscus were resighted in winter areas. Birds were seen in western areas between Belgium and Morocco, apart from one greyish-mantled bird observed in Libya and one fuscus in Israel (Fig. 2).

Winter site fidelity seemed high in some of the adults; one of the fuscus was found at the same location in two winters following ringing, while of the six greyish-mantled birds observed in more than one year, five (83%) were seen at the same location. Two birds were observed at the same location in five consecutive years. Five juveniles from the same mixed colony were resighted in more than one winter, and all were observed at the same location; one in Italy, two in France and two in England.

Figure 2.

Resightings (one observation per bird) of Lesser Black-backed Gulls marked as adults (squares) or juveniles (circles) in the study colonies (small circles; Fig. 1) in northern Norway. Black squares refer to fuscus adults, grey squares to greyish-mantled adults. Black circles are birds marked as juveniles in pure fuscus colonies, grey circles are birds from mixed colonies (see Table 1). Black triangles denote previous recoveries of fuscus from Norway (from Bakken et al. 2003).

DISCUSSION

This study is the first to document the migration patterns and wintering areas for different segments of the Lesser Black-backed Gull population in northern Norway. Greyish-mantled adults had a ten times higher probability of being resighted on migration or in winter than fuscus adults. The most likely explanation for this striking difference is that fuscus adults move to areas with few observers or stay in habitats where they are less likely to be observed, e.g. off-shore on freshwater lakes in eastern Africa. This is in accordance with the migration behaviour of fuscus from the Baltic Sea (Cramp & Simmons 1983, Kilpi & Saurola 1984, Kube et al. 2000, Hario et al. 2004, Bønløkke et al. 2006). There is also evidence of different migration routes and winter areas between fuscus and greyish-mantled birds from contaminant profiles; i.e. the ratio between DDE (a DDT metabolite) and PCB was higher in fuscus than in greyish-mantled birds (Bustnes et al. 2006), suggesting that the former were wintering in areas where DDE was the dominating compound, as in eastern Africa (Hario et al. 2004; Bustnes et al. 2006). Our resightings were too scarce to be conclusive about the migration route and wintering areas of adult fuscus as only one bird was seen along the eastern migration route (in Israel), and several individuals along the western route.

Forty-five percent of the marked greyish-mantled adults were resighted, most of them in western areas. This corresponds well with the previously established migration routes of intermedius from southern Norway and of graellsii from the UK and the southern North Sea (Cramp & Simmons 1983, Bakken et al. 2003, Galván et al. 2003). It thus seems that these greyish-mantled birds, which probably colonized northern Norway in the 1980s (Strann & Vader 1992), are keeping their traditional migration routes, seemingly different from most adult fuscus in the area. This is surprising since fuscus and intermedius/graellsii may be found in mixed breeding pairs (M.H. & J.O.B., pers. obs.).

The larger sample size of marked juveniles allowed to distinguish a clear dichotomy in the migration pattern; 30% of the observed birds were classified as eastern migrants and 70% as western migrants. As we suspect that the resighting probability is much lower for birds migrating to the east (see above) the actual proportion of juveniles migrating east must be higher than our observations suggest. Nevertheless, it is surprising that many fuscus juveniles (74% of the juveniles originating from a pure fuscus colony) had a western migration. This is different from the fuscus population breeding in the Baltic Sea, which has nearly an exclusive eastern migration (Cramp & Simmons 1983, Kilpi & Saurola 1984, Kube et al. 2000, Hario et al. 2004, Bønløkke et al. 2006).

Unfortunately we obtained only few repeated observations of the same juveniles over the years — in fact all five birds ringed as juveniles and resighted in more than one winter were from the same mixed colony. We can therefore not clear conclusions whether juvenile fuscus change their migration pattern from west to east as they grow older. Such a scenario might well be possible if sub-adults and adults migrate separately, and birds adopting the final migration route only at later age (Baker 1980, Galván et al. 2003). For example, Galván et al. (2003) found in the inner parts of the Iberian Peninsula mainly adult Lesser Black-backed Gulls were observed in more than one year, suggesting that young birds were moving to other areas. As we recorded that some adult fuscus followed a western migration route, it might be that these areas offer some benefits to wintering gulls, for example as a consequence of the proliferation of refuse tip and reservoirs in southwestern Europe (Galván 2003).

In conclusion, the nominate Lesser Black-backed Gull from northern Norway seems to have a migration pattern that is different from the pattern of fuscus from the Baltic Sea. Most striking difference is the importance of the western migration route for the Norwegian juveniles. Migration pattern of the northern greyishmantled gulls is similar to the pattern described for intermedius breeding in southern Norway (Bakken et al. 2003) and the more southern graellsii (Cramp & Simmons 1983). More information about the migration routes of the North-Norwegian Lesser Black-backed Gull populations is needed, especially how they utilize the African continent in winter.