Migratory songbirds breeding in Eurasia's temperate forests depend on a summer flush of insects, particularly caterpillars, to feed themselves and their offspring. In some places, these caterpillars are emerging earlier in responses to rising global temperatures. In theory, the songbirds could simply push up their departure from their winter quarters to catch the earlier flush of insect prey. If, however, the birds rely on a fixed cue such as increasing day length to begin flying north, they may be unable to adjust the timing of their migration. Precisely this disruption in the emergence of insects relative to the timing of songbird migration has been identified as the cause of a significant decline in populations of pied flycatchers in the Netherlands.