VAN DEN BREMER L, VAN TURNHOUT C, SCHEKKERMAN H, DEUZEMAN S, VAN DER JEUGD H & FOPPEN R (2019) Can differential population trends of Common Chiffchaff Phylloscopus collybita and Willow Warbler Phylloscopus trochilus be explained by changes in survival and reproduction?. LIMOSA 92 (1): 36-44.

Common Chiffchaff and Willow Warbler are common breeding birds in the Netherlands. The population indexes derived from the Common Bird Census in The Netherlands over the period 1984-2017 show a moderate increase for Common Chiffchaff, with large fluctuations, and a steady decrease for Willow Warbler (Fig. 1), with notable differentiations between habitats (Fig. 2). Annual population changes over 1996- 2015 were best explained by variation in adult (both species) and first-year (Common Chiffchaff) apparent survival as estimated from the Dutch Constant Effort Site mistnetting scheme (Fig. 4), not by the breeding productivity index derived from the same scheme. Firstyear survival of Willow Warblers was related to the weather in June and July: the drier and warmer these months, the higher the first-year survival. Conditions in winter and during migration seem to have some influence on the survival of adult Willow Warblers: in dry years in the African Sahel zone the survival decreased. We found no relation between the survival of Common Chiffchaff and weather in Dutch breeding areas (temperature and precipitation) and in wintering areas in Southwestern Europe (NAO-index). Causes of annual fluctuations in numbers and demographic parameters are not necessarily decisive for long-term population developments. We discuss possible underlying factors driving the contrasting trends of both species. It is plausible that changes in the landscape in the breeding areas play a significant role in this. Succession of forests and a decrease of forest plantations due to changes in forest management have a negative impact on Willow Warbler breeding habitat. However, the conditions experienced in the non-breeding season may also be important. Identifying the key drivers of these population changes is complex, and currently hampered by the lack of information on breeding data (in particular nesting success). The strong correlation with the European indexes for both species (Fig. 1) suggests a strong underlying mechanism that is active on a large geographical scale.

[pdf only for members] [dutch summary]