The two events are commonly known as La Nina and El Nino.

VOA: special.2009.09.29

But recent research has shown that some wild birds' patterns of flights and stopovers during migrations, or moulting times, differ between El Nino and La Nina years.

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The best hope lies in trying to identify the broader changes under way - for example with the retreat of the Arctic sea-ice or the cycle of warming in the Pacific Ocean with El Nino and La Nina.

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