Figure 8.14 Egg laying dates in tree swallows. (a) Egg laying dates collected over a 42-year period demonstrate that tree swallows bred 9 days earlier in 1991 than in 1959. (b) The variation in egg laying date is negatively correlated with the mean air temperature in May.

Data from P. O. Dunn and D. W. Winkler, Climate change has affected the breeding date of tree swallows throughout North America, Proceedings of the Royal Society of London B. 266 (1999): 2487–2490.