单选题 .  ①Migratory songbirds breeding in Eurasia's temperate forests depend on a summer flush of insects, particularly caterpillars, to feed themselves and their offspring. ②In some places, these caterpillars are emerging earlier in responses to rising global temperatures. ③In theory, the songbird could simply push up their departure from their winter quarters to catch the earlier flush of insect prey. ④If, 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. ⑤Precisely 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.25.  The primary function of the highlighted sentence is to ______