Bird migration is defined by its regularity, the outward and return journeys occurring at the same time each year, and the travellers following similar routes and using the same areas at each journey's end. The regularity of these movements reflects the predictability of the resources (such as food) on which the birds depend.
In the absence of a guaranteed larder, movements become more opportunistic and less resemble true migration – yet the birds involved are still thought of as migrants. Perhaps the best-studied are the ‘irruptive migrants’ – species that include the common crossbill, waxwing and brambling.
All make an obvious irruptive movement away from their breeding areas under certain conditions – typically when their numbers are high but the abundance of favoured food is low. These movements, made in search of feeding opportunities, may bring the birds to our shores in large numbers. Of course, they do ‘migrate’ back to their breeding grounds later in the year, though these return journeys are much less conspicuous.
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Main image: Species such as waxwings are known as 'irruptive migrants'. © Gary Chalker/Getty