Figure 1. Schematic representation of well-timed (a) and mis-timed (b) breeding in birds, taking Great Tits and caterpillars as example. The distribution of first egg dates is indicated by the black curve. After about 3 weeks the young hatch (blue curve) and their food demands are highest when they are about 10 days old (red curve). The green curve indicates abundance of caterpillars, the main food source for the nestlings. In (a) the birds lay their eggs early enough to match the period of highest food demand with the period of maximal caterpillar abundance. In (b) caterpillar phenology has advanced but the birds were not able to advance at the same rate and hence the young hatch too late in relation to caterpillar abundance.

Figure 2. Schematic representation of the relationship between spring temperature and phenology of Great Tits and caterpillars. Caterpillars hatch earlier and grow faster under warm temperatures and hence their abundance (green line) peaks earlier in warmer springs. Since caterpillars are the main food source for the nestlings, the optimal breeding time reaction norm of the birds (black dotted line) should follow the same relationship with temperature. The birds however have to start egg laying (black line) about 1 month before the caterpillar abundance, which is the necessary time from laying to the first egg to having 10 days old nestlings. In (a) the birds follow more or less the optimal reaction norm and thus meet the optimal breeding period. In (b) the optimal reaction norm has changed since caterpillars hatch earlier but the birds are still following the ‘old’ reaction norm. This situation leads to a mismatch between food abundance and demands by the birds as depicted in Figure 1b.