Why did Lapwings <i>Vanellus vanellus</i> in managed habitat advance egg laying during a period without warming early springs?

From 1950, spring temperatures in The Netherlands increased. Previousresearch suggested that advances in first egg dates of Lapwings Vanellusvanellus best correlate with climatic factors rather than with changes in farmingpractices. In an area constantly and uniformly managed especially for breedingmeadow birds (the reserve Giethoorn-Wanneperveen), nesting phenology ofLapwings was monitored over almost three decades (1988–2014). During thisperiod local average air temperatures across early spring (1 February – 31March) showed no change. Although first laying dates of the initial clutches(varying between 7 and 30 March) did not change either, the median laying dateof 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 wereassociated with temperatures, in that egg laying usually followed an increase intemperature in the previous weeks. As a consequence, whereas first layingdates of initial clutches correlated with temperatures in the 21 February – 31March interval, they did not with temperatures in the previous 1–20 Februaryinterval. Likewise, median laying dates of the first egg of all the clutches did notcorrelate with the temperatures in February and early March, but the two variableswere strongly correlated in the overlapping 11 March – 10 April interval.We found no associations with precipitation. That median laying dates (but notfirst laying dates) advanced without changes in the overall average springtemperature nor in habitat management, can only partly be explained by thefinding that hatching success steadily increased during the study (note that themore frequent replacement clutches would have delayed the measured medianlaying date in the earlier years). As hatching success of earlier clutches is higherthan that of later clutches, there may now be selection for earlier laying