From 1950, spring temperatures in The Netherlands increased. Previous research suggested that advances in first egg dates of Lapwings Vanellus vanellus best correlate with climatic factors rather than with changes in farming practices. In an area constantly and uniformly managed especially for breeding meadow birds (the reserve Giethoorn-Wanneperveen), nesting phenology of Lapwings was monitored over almost three decades (1988–2014). During this period local average air temperatures across early spring (1 February – 31 March) showed no change. Although first laying dates of the initial clutches (varying between 7 and 30 March) did not change either, the median laying date of the first egg of all the clutches (varying between 21 March and 8 April) advanced by ten days (from 4 April to 25 March). Interestingly, laying dates were associated with temperatures, in that egg laying usually followed an increase in temperature in the previous weeks. As a consequence, whereas first laying dates of initial clutches correlated with temperatures in the 21 February – 31 March interval, they did not with temperatures in the previous 1–20 February interval. Likewise, median laying dates of the first egg of all the clutches did not correlate with the temperatures in February and early March, but the two variables were strongly correlated in the overlapping 11 March – 10 April interval. We found no associations with precipitation. That median laying dates (but not first laying dates) advanced without changes in the overall average spring temperature nor in habitat management, can only partly be explained by the finding that hatching success steadily increased during the study (note that the more frequent replacement clutches would have delayed the measured median laying date in the earlier years). As hatching success of earlier clutches is higher than that of later clutches, there may now be selection for earlier laying.
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Vol. 105 • No. 1