Local and large-scale climatic variables as predictors of the breeding numbers of endangered Lesser Black-backed Gulls on the Norwegian Coast |
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Authors: | Jan Ove Bustnes Tycho Anker-Nilssen and Svein-H?kon Lorentsen |
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Institution: | (1) The Polar Environmental Centre, Norwegian Institute for Nature Research, 9296 Troms?, Norway;(2) Norwegian Institute for Nature Research, Tungasletta 2, 7485 Trondheim, Norway |
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Abstract: | In the 1970s and 1980s, the nominate subspecies of the Lesser Black-backed Gull (Larus fuscus fuscus) showed a dramatic drop in breeding numbers on the Norwegian Coast, and in 2000, the population in some colonies was only
10–20% of the population in 1980. This decline has been attributed to the collapse in the stock of Norwegian spring spawning
herring (Clupea harengus). In this study, we examined whether local climate (sea and air temperatures), winter NAO (North Atlantic Oscilliation),
and the year-class strength and size of 0-group herring could predict the relative changes in breeding numbers between years,
mainly after the population collapse. Breeding birds were counted in 19 of the years between 1980 and 2007 in an archipelago
on the coast of Helgeland, northern Norway. The best model predicting changes in breeding numbers for the period between 1980
and 2005 (for which data on 0-group herring was available) included mean local air temperature in winter (January–March) and
winter NAO, explaining 57% of the variation between years, while the other factors had little effect. When also adding the
years 2006–2007 (no herring data), the best model included only mean air temperature in winter, explaining 41% of the variation.
In conclusion, the high positive correlation between breeding numbers and climatic factors probably resulted from a higher
availability of important fish prey after mild winters, for which 0-group herring presently may only account for a limited
proportion. However, this prey might have been of much more importance prior to the population decline. |
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