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.
In a specific case, observations in Great Britain have shown that migratory swallows typically arrive at their destinations before swifts do, coinciding with the later surge in small insect populations that swifts rely on for food. Weather conditions also impact the abundance of flying insects.