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Body changes
Hormones tell this swallow when to migrate
Natural signs, including the gradually changing day length (In the UK days get shorter in late summer and longer in early spring), cause this to happen automatically. These hormones make birds behave differently. They start flying around restlessly and begin to gather into flocks.
In spring, hormones also start preparing a bird for breeding. Its sexual organs, which have been hidden away during the winter, enlarge in preparation for mating. This change is another signal to leave for summer quarters. Some birds may pair up during the journey, but they wait until reaching their destination before looking for a place to nest.
Last modified: 11 January 2005