Do all mi­grants fly twice a year?

BBC Wildlife Magazine - - Q&a - Mike Toms

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.

Con­ti­nen­tal cross­bills boost UK pop­u­la­tions in ‘ir­rup­tive’ years, feed­ing on conifer seeds.

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