Introduction

LBbG (L. fuscus) only started breeding in Belgium in the mid 80's, with dramatically increasing numbers in two decades, from one pair in 1985 to 2863 pairs in 2001. Colonization started at the coastal nature reserve "Het Zwin" at Knokke, but numbers never exceeded 40 pairs here. Rapid development LBbG numbers came from the reclaimed industrial harbour area of Zeebrugge from 1987 onwards with 2695 pairs in 2001. Other colonies can be found at the inner harbour of Zeebrugge and roof top breeders at Oostende, south along the Belgian coastline (only 8 pairs in 2001). 

Counting method

Prior to hatching all nests in the harbour of Zeebrugge were counted annually. From a high watching point, the proportion numbers of nesting LBbG's compared to Herring Gulls were estimated. Next step was counting all nests, without discern of species, marking them with a small match stick and discount the proportional ratio afterwards. For lacking annual values, numbers of adjacent years have been used to estimate the annual number, considering the long-term trends and development of locations within the colony concerned.

Results

LBbG only recently settled as breeding species in Belgium and all colonies were occupied after Herring Gulls settled for several years earlier. However, number of Herring Gulls have been low over the first years, sometimes zero over longer periods in the 1960's, and it's only from the mid 1990's, with the expansion of the industrial area of Zeebrugge (especially the Outer harbour of Zeebrugge) that counts exceeded 100's. Outer harbour of Zeebrugge is the only place where numbers of LBbG exceed Herring Gull numbers, now holding over 94% of the Belgian population. The explosive expansion at the Zeebrugge harbour was followed by expansion further south, at Oostende LBbG started breeding on roof tops, as Herring Gulls already had been doing from 1993 onwards, but numbers remain low. During the period of 1992-2001, LBbG numbers almost doubled with an average increase of 99,3%. An estimated 1,8% of all European LBbG's can be found in Belgium. 

Discussion

The rapid expansion in Belgian colonies must be set in an international perspective. LBbG have expanded dramatically over the last decades in NW Europe, now breeding as far south as the Ebro Delta in Spain. In the Netherlands, numbers of breeding LBbG have been growing strongly over the years 1960-2000, now breeding in vast colonies along the coast and inland  industrial complexes and on roof tops. There have been two "grow waves": by protection rules early in the century and another by the increase in food supply from the 1960's (fishery, agriculture and landfills). Now many landfills in the Dutch Delta close, there is a strong decrease in numbers of breeding Herring Gulls at e.g. Saefinghe (Westerschelde), with numbers dropping from over 8.000 to just 1.000. LBbG is an opportunistic species, exploiting these new food supplies and using new human habitats (reclaimed areas and roof tops).
Input at the Belgian colonies initially came from abroad, stimulated by immigration and good nesting habitat, especially the expansion of the Zeebrugge harbour area. The vast sandy areas became extremely suitable for nesting tern species and with vegetation, for gull species as well. While Zeebrugge expanded the industrial area, adjacent colonies in the Dutch Delta and along the British coast reached their maximum and LBbG had to search for new nesting places. In the Netherlands, numbers had been growing already for a much longer period, now resulting in severe competition for nesting sites and traditional colonies in the dunes along the Dutch coast were heavily effected by predation of Fox, forcing all gulls to start breeding at roof tops or to join existing colonies at industrial areas. Similar problems with foxes have been registered by Mike Marsh at Orfordness in Britain. 
In the Netherlands, large areas of colonies have been claimed by industry, increasing pressure on remaining colonies. Considering these trends in nearby large colonies in Britain and the Netherlands, the settlement of LBbG in Zeebrugge was hardly a surprise. 
Ring recoveries suggest LBbG originate from the colonies in the Dutch Delta area (province of Zeeland, north to the mouth of Rhine at Maasvlakte near Rotterdam) and from the British east coast of the southern North Sea. Year 2001, 96 ringed LBbG were counted at Zeebrugge, of which 27 were ringed as adult on the nest at Zeebrugge in previous years. Others were ringed abroad:
27 from Europoort (Dintelhaven), the Netherlands, see map,
35 from Maasvlakte, the Netherlands, see map,
7 from Ordfordness, Britain, across the North Sea. 
In previous years, a ringed LBbG from the inland industrial complex of Moerdijk, the Netherlands was recorded as well. However, remember ring reading may result in exaggerated figures, as not all colonies have ringing programs.
The increase in subsequent years was stimulated largely by high reproduction success of the colony at Zeebrugge itself. The average breeding success may have been over 1.0 over several years. In 2001, reproduction success of ringed LBbG was measured in 82 nests, which had one or both parents colour-ringed. The 82 nests average 1.9 fledged juvenile per pair. This is extremely high and nicely matches the doubling ratio of 99.4%.
Zeebrugge obviously is an attractive breeding place for LBbG. The LBbG chicks diet completely existing of fish (12 regurgitations), where Herring Gull also regurgitate food left overs from garbage and birds (n = 12). LBbG hunt after fish on their own, or helped by fishing trawlers, patrolling the rich fish-grounds of the southern North Sea. The high breeding success in partly based on low predation of LBbG in the colony of Zeebrugge. As said before, the individual range of nests and the distance between nests at Zeebrugge is much higher than the "completed" colony of e.g. Maasvlakte. This figure is mirrored by predation on chicks and eggs; at Maasvlakte predation on neighboring nests is a common phenomenon, one of the factors diminishing breeding success.

future

Given these good breeding results, one may expect to find an ongoing strong increase at Zeebrugge. The LBbG growth rate for 2001 was an astonishing 126%. For sure, the next years will show increasing numbers of breeding LBbG as well. Further inland dispersion and settlements on roof tops can be expected as well, as is already a trend in surrounding countries.
Compared to colonies at Maasvlakte, the individual territorial range at Zeebrugge is much higher (the Maasvlakte colony being "fully occupied"), so a further increase from abroad may be expected. However, as is nicely demonstrated at Maasvlakte, these breeding colonies at industrial development areas are highly vulnerable to tremendous loss of good nesting sites as construction works may start, even within the breeding season, resulting in loss of several 100's of eggs and recently hatched chicks.