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This paper compares the patterns of migration of two congeneric warblers, moving from Britain to Africa, and aims to see how these might be influenced by the distribution of food supplies. Most Sedge Warblers fattened in southern England or northern France and overflew Iberia, while Reed Warblers paused and fattened in Portugal. The occurrence, duration of stay and rate of weight gain of Sedge Warblers depended on the abundance of the plum-reed aphid whose seasonality and distribution was broadly sufficient to predict the migration pattern. Reed Warblers showed no similar restrictions of diet, did not respond to aphid abundance and were able to achieve a similar rate of weight gain any time in September or October in Portugal. Migration strategies are discussed in evolutionary terms and shown to be appropriate responses to the pattern of food availability for birds with different feeding adaptations.
Bibby et al. (Sun,) studied this question.