swallows once led naturalists to conclude they were related, but it is now established that they are unrelated, and that the same lifestyle has led to the same adaptations. Ecological implications In temperate climates, the availability of flying insects as a food source is seasonal, and this is probably why many birds that rely on this food source during the breeding season migrate in winter. Migration is timed to the availability of the birds' preferred food. For instance, it has been observed in Great Britain that migrating swallows arrive earlier in the spring than swifts, this correlates with the later profusion of small insects that swifts feed on. Weather also affects the availability of flying insects. Swallows, for example, are obliged to go where the insects are, and depending on the weather they may adjust their choice of prey or be forced to seek out prey in different locations. The preference for certain kinds of aerial insect as a food source seems to correlate with gregarious or colonial behavior versus territoriality. For birds that take advantage of swarming insects, which are by nature found in local concentrations, colonial breeding can be a successful strategy. An example is the cliff swallow of western North America. Its relative the barn swallow hunts larger, non-swarming insects, and is more solitary. Certain neotropical tyrant flycatchers will join mixed-species foraging flocks, as will some Asian drongos. Such flocks stir up flying insects, which can then be picked off in quick sallies. References Bird behavior Bird feeding