BERGKAMP PY & BOELE A (2005) Chiffchaffs Phylloscopus collybita in winter 2000/01: response to cold spells and comparison with other winters. LIMOSA 78 (4): 125-138.

Unusually high numbers of Chiffchaff (and other insectivorous passerines) were seen in The Netherlands during the winter of 2000 2001. An estimated number of at least 1000-1500 Chiffchaffs tried to winter that year. It is suspected that high temperatures in autumn determine the number of wintering birds. The majority of observations came from the milder western part of the country. A shift of Chiffchaffs from marshes to urban areas was evident during the first cold spell in mid- December. During a second period of frost in mid-January, the number of birds in marshes and other natural habitats decreased again while the numbers in other habitats remained stable.
      Data from a winter bird monitoring program and other data series were analysed to answer the question which factors determine the number of wintering Chiffchaffs. There was no relationship between the size of the breeding population and its offspring and the number of birds in December. This number correlated positively with average minimum temperatures in the preceding autumn (September- November), and negatively with the severity of the preceding winter. However, after a severe winter, a positive effect of autumn temperatures on the number of birds in the next winter was absent. This could indicate that many Chiffchaffs that tried to winter in the Netherlands did not survive the preceding cold winter. Indeed, the ratio between Chiffchaff numbers in the Netherlands in February and November correlated significantly with the severity of the winter, suggesting that the colder the winter, the larger the proportion of Chiffchaffs that does not survive. Common Chiffchaff Phylloscopus collybita

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