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The breeding biology of the House Martin Delichon urblca (L) is described/ with particular reference to the influence of variation in the abundance of aerial insects. The selection of food items from the available resources is explained in terms of a prediction strategy which demonstrates that the Martin consistently maximises the efficiency of food collection,, The timing of the breeding season and breeding success are respectively shown to be closely associated with the development of the aerial phase of several insect populations and with food abundance. A hypothesis is presented, with some supporting evidence, which describes the potential selective forces acting on broods both smaller and larger than the most frequent natural brood sizes which would possibly inflict a disproportionately large mortality on these extreme brood sizes and also on the adults involved in rearing them. This is particularly relevant: to those species where artificially enlarged broods or the largest naturally occurring broods are more productive than the most frequent brood sizes, as is the case in this study. Differences in food and feeding station are described which ensure a degree of ecological isolation between aerial feeding birds in Britain.