The population index for Wren derived from the Common Bird Census in The Netherlands over the period 1984-2010 shows marked annual fluctuations without a clear long-term trend. (Series of ) winters with severe cold spells in the mid-1980s and mid-1990s, and again in 2010, caused crashes from which the population recovered in four to five years, to fluctuate around a stable level thereafter (Fig. 1). Annual population change was negatively related to both winter severity and population size in the previous year (Fig. 2), with Pollard et al.’s test indicating negative density dependence. Annual population changes over 1995-2010 were best explained by variation in (adult) apparent survival as estimated from the Dutch Constant Effort Site mistnetting scheme; the breeding productivity index derived from the same scheme did not significantly contribute to this. Both adult and first-year survival were related to winter severity (Fig. 3), accumulated subzero temperatures having a greater influence than the number of days with snowlay. No density effects were detected on survival rates. Breeding productivity showed negative effects of current population size and of severity of the previous winter, but these were relatively weak (Fig. 4).
Schekkerman H. & Kampichler C.
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