Knowing when to go
Birds become increasingly restless as the time for migration approaches. Some birds like Swallows gather in flocks and make short 'practice' flights together before the final departure. There is usually a lot of activity, noise and a sense of excitement and expectation.
Their readiness for migration is controlled by their glands, which in turn are affected by environmental changes - chiefly the length of daylight. The bird's brain can register how long the day is, and whether it is increasing (spring) or decreasing (autumn). Temperature may also be a factor.
The trigger for actual departure may be a spell of fine settled weather, with clear skies and little wind. These are provided by high atmospheric pressure or 'anticyclonic' conditions, a feature of late summer and autumn weather in Ireland. The opposite to this, low atmospheric pressure or 'depression' conditions, produces cloud, wind and rain. This weather is least helpful to migration.
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