Many pairs of closely related species show so-called chain migration, in which the different species maintain the same latitudinal sequence in the non-breeding season as in the breeding season but at lower latitudes: the species that breeds furthest north also winters furthest north, and the species that breeds furthest south also winters furthest south (Chapter 22). An example is provided by the Lapland Longspur Calcarius lapponicus and Chestnut-collared Longspur C. ornatus in North America; both are migratory, but the latter both breeds and winters south of the former. In some cases the northern species replaces the southern one as it vacates its breeding range in autumn to move even further south.
In other pairs of closely allied species, the member that breeds furthest north in the northern hemisphere winters furthest south, to give 'leapfrog migration', reversing their latitudinal sequence between summer and winter (Chapter 22). Some species pairs among leapfrog migrants differ markedly in body size, and in each case the smaller species breeds furthest north, in the coldest areas, and on average migrates to winter furthest south in the warmest area. Together they follow 'Bergman's Rule', in that higher latitude forms are larger, but in such leapfrog migrants the pattern holds only for their winter and not their summer distributions. Examples include the finches Carpodacus rosea and C. erythrina, the knots Calidris tenuirostris and C. canutus, the godwits Limosa limosa and L. lapponica, and the curlews Numenius arquata and N. phaeopus, but with some geographical overlap between the species in each pair (Salomonsen 1955).
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