CAMPHUYSEN C J & A GRONERT (2010) Breeding biology of Herring gull Larus argentatus and Lesser Black-backed Gull Larus fuscus on Texel (The Netherlands) 2006-2010. LIMOSA 83 (4): 145-159.

The breeding populations of both Herring Gull (HG) and Lesser Black-backed Gull (LBBG) have increased markedly in the Netherlands in the 20th century. In the 1990s and early 21st century, however, contrasting trends were recorded: a population decline in HG and a continued increase in LBBG. To try and understand these populations trends, a study of the species' breeding biology and foraging ecology commenced in 2006, focusing on a large mixed colony on the southern tip of the island of Texel (western Wadden Sea; Fig. 1). This paper reports on the timing of egg laying, clutch size, egg volumes, hatching and fledging success during the first five years of study (2006-2010). Comparisons are made with historical data for HGs on Schiermonnikoog (Eastern Dutch Wadden Sea, 1963-65), Terschelling (Central Dutch Wadden Sea, 1967-69, 1983-84), and Schouwen (Dutch Delta area, 1991-1994), and of LBBGs on Terschelling in the 1990s.
      The most striking result was that, contrary to expectation, the reproductive success of LBBG was very low, while that of HG was reasonably high, certainly higher than in the 1980s, just prior to the population crash. From these data, it is suggested that the population of LBBG is at the brink of a collapse, whereas that of HG is stabilising, perhaps even recovering.
      The Kelderhuispolder colony is part of more extensive colonies on the southern tip of Texel, totalling 11,500 pairs of LBBG and 5060 pairs of HG during the most recent survey. Transect counts within the 8.3 ha study colony revealed that in 2009 and 2010 approximately 1985 nests of LBBG were built (240 Apparently Occupied Nests /ha) and 1025 nests of HG (123 AON/ha, totalling 363 pairs/ha).
      Basic breeding biology parameters are summarized in Tables 2 (LBBG) and 3 (HG). The timing of egg-laying in HG was much advanced in comparison with previous studies, and even now, there was a tendency to nest earlier every year on Texel. LBBG commenced laying in early May (median first egg date 10 May), with no sign of advance over time, so that the difference with HG was increasing (9 days in 2010). Remarkably, there was no firm evidence that early-nesting HG performed better than late-nesting birds (with the exception of the latest pairs; our studies were mostly confined to the central 95% of the laying date distribution; Table 5). Reproductive success of LBBG, however, declined through the season (the earliest, central two and latest quartiles of pairs pairs fledged 0.69, 0.57 and 0.21 young/pair, respectively). Differences in breeding success of LBBG between nesting areas were mostly caused by different levels of chick predation (cannibalism); in HG no such differences were observed (Table 6). Chick predation overall was particularly high in LBBG (>60% in 2006-2009), but much reduced in 2010, leading to a higher reproductive success (Table2).
      Clutch size and egg volumes were compared with historical data and with colonies in the neighbourhood. In HG, a tendency towards smaller egg volumes over time was found, while LBBG egg volumes were similar to those in the 1990s. Since egg volume and breeding success were strongly correlated, it is concluded that a more extensive analysis is required.
      Subtle differences in habitat choice between the species suggested that the competition for nesting space may be limited for the moment. The high predation pressure may have been caused by a shortage of suitable resources, but the study of the foraging ecology should shed more light in these issues. Future work will reveal whether the foraging grounds are shared and foraging competition is an issue. All fledged chicks were colour-ringed to monitor first-year and later survival as well as recruitment and colony fidelity in years to come.

[pdf only for members] [dutch summary]