Bijlsma R.G. 2013. Arrival of Wood Warblers Phylloscopus sibilatrix on the breeding grounds. Drentse Vogels 27: . In an earlier paper (Bijlsma 2012) it was shown that spring phenology of Wood Warblers on Dutch breeding grounds had advanced in the past decades. These data had been collected in woodlands on the Veluwe (central Netherlands, 1968-2012) and in Drenthe (northern Netherlands, 1981-2012). Phenology was based on standardised studies, in which mean arrival date of the first three singing males was used as a proxy of arrival date. The present paper is about spring arrival of the entire study population in mixed woodland in western Drenthe, for males ánd females, in 2011-13 (in May-August, the number of hours spent in the field in the study plot was resp. 937, 856 and 949 in 2011-13, with daily visits). In this area, the number of singing males varied between 4 and 164 (in 1990-2013), on average 25 territories/annum. In 2011-13, the number of singing males was resp. 12, 34 and 23, of which resp. 58%, 35% and 70% were paired. Mean arrival date of males varied between 17 and 26 April, median arrival being 2-6 days later (Table 1). Arrival of males in 2011 was highly synchronized, but in 2012-13 newly occupied territories were found as late as mid-June, likely males that had resettled after failure of a first brood (Fig. 1). A similar arrival schedule was found in females, though with an average delay of 2-17 days compared to males. Males and females arrived very early in 2011, with median arrival dates of resp. 19 and 21 April. However, the interval between mean female arrival and first laying amounted to 24 days in 2011, compared to 7 and 13 days in 2012 and 2013 respectively (when females on average arrived 17 days later than males, and arrival was late anyway; Table 1). For the study plots Veluwe (1974-2013) and Drenthe (1981-2013), spring arrival date of males positively correlated with mean April temperature (Fig. 2). Start of laying positively correlated with female arrival (Fig. 3), but sample size is small (N=16, 2011-13, Drenthe only). The present study is not yet of sufficient detail and duration to calculate fitness consequences of arrival dates, except that early breeding coincides with larger clutch size (Bijlsma 2012).