Barth, in his research in the 1960's and 1970's determined two "contact zones" for fuscus and intermedius. One zone on the eastern islands of Denmark in the waters of Kattegat. Birds from the island Anholt are intermedius and those from the island Saltholm (Oresund) seem to be fuscus.
The other division lines between intermedius and fuscus in Scandinavia is along the coast of central Norway near Tarva. 
Nowadays pale "intermedius" birds can be found as north as Trömsö, where they breed together with jet-black fuscus. This is illustrated by pictures from the Norwegian ringer and photographer Morten Helberg. Several example birtds can be found in this June section (images bottom of this page) and a more detailed description of the situation in N Norway can be found in the ADULT MARCH section of this site.

As often stated, pale intermedius and graellsii birds use the western "Atlantic" flyway to migrate to the wintering grounds in S Europe and N Africa. Nominate fuscus follow the eastern route into Egypt and the great lakes of the African Rift Valley. Research on N Norwegian birds, research by Morten Helberg and friends, can be found in the 1CY OCTOBER section.

Recently, birds breeding in N Netherlands, birds often refered to as "Dutch intergrades" have been applied with transmitters to follow their migration pattern and strategy. Publication of the preliminary results is copied on this webpage, below, and here is also a link to the full pdf file. This article is in Dutch, with english captions and summary, titled "Research on gulls with help of satellite transmitters", by: Bruno J. Ens, Franz Bairlein, Kees (C.J.) Camphuysen, Peter de Boer, Klaus-Michael Exo, Natalia Gallego, Raymond H.G. Klaassen, Kees Oosterbeek & Judy Shamoun-Baranes. It was published IN: SOVONVOGELONDERZOEK NEDERLAND, LIMOSA 82 (2009): 33-42.

FULL PDF

ONDERZOEK AAN MEEUWEN MET SATELLIETZENDERS

Bruno J. Ens, Franz Bairlein, Kees (C.J.) Camphuysen, Peter de Boer, Klaus-Michael Exo, Natalia Gallego, Raymond H.G. Klaassen, Kees Oosterbeek & Judy Shamoun-Baranes

As part of a European Space Agency FlySafe initiative, we studied the possibilities and limitations of commercially available solar-powered satellite transmitters with a Global Positioning System (GPS-PTT’s). This paper provides a brief description of our preliminary findings. We attached 25 transmitters to migratory Lesser Black-backed Gulls Larus fuscus and resident Herring Gulls Larus argentatus breeding in a colony on the island of Vlieland. We deployed 22 g. tags which record position and speed and 30 g. tags which also record altitude. The positions of the gulls are updated daily on the SOVON website http://www.sovon.nl/default.asp?id=408. We tested the transmitters before we attached them to the birds and found the accuracy of position and speed to be as specified by the manufacturer. Results were not completely clear regarding measurement of altitude.

Transmitters were programmed with different duty cycles, varying from six to 18 fixes per day. Solar energy, needed to charge the batteries, appeared to be a major limitation, especially when many fixes per day were programmed and under low light conditions in winter, but also during periods of bad weather in summer (Fig. 2). Many of the tagged birds, which were made visibly identifiable with engraved coloured leg rings, were subsequently observed or even photographed (Fig. 3), and appeared in good condition. Comparing data from colour-marking programs with the GPS-data revealed the same migration route andwintering area for the Lesser Black-backed Gulls (Fig. 4). However, because human observers are restricted to certain habitats, this leads to a clear observer bias. Ringed Lesser Black-backed Gulls are almost never recorded from the open sea although the GPS-PTT’s showed this to be an important foraging habitat during summer (Fig. 4).

Similarly, ringed Herring Gulls are rarely recorded from intertidal flats although they often feed there (Fig. 5). Thus, GPS-PTT’s are an excellent tool to study habitat choice, revealing marked differences between the two species, as well as large differences among individuals within a species (Figs 6, 7). GPS-PTT’s are also an excellent tool to study migratory behaviour, again revealing conspicuous differences between individuals (Fig. 8). However, this should not be regarded as a plea to abandon colour-marking programs in favour of studies with GPS-PTT’s. On the contrary, the techniques provide complementary information and colour-marking programs are much better suited to study survival, mate choice and mate fidelity. Comparison of survival and reproductive success of colour-marked gulls carrying GPS-PTT’s and control birds will also allow us to assess if and to what extent the attachment of transmitters affects individual fitness.