In the course of this year, about the fruit season, there appeared, in the orchards chiefly, some remarkable birds which had never before been seen in England, somewhat larger than larks, which ate the kernel of the fruit and nothing else, whereby the trees were fruitless to the loss of many. The beaks of these birds were crossed, so that by this means they opened the fruit as if with pincers or a knife. (The first documented record of a Crossbill invasion in England, Matthew Paris, 1251.)
One of the most striking features of bird migration is its regularity. Most populations of birds migrate at about the same dates, in the same directions, and for similar distances each year, with many individuals returning year after year to the same breeding and wintering localities. However, consistency in movement patterns is advantageous only in predictable environments, where birds can be sure of finding suitable conditions year after year in the same breeding or wintering areas. Some bird species exploit habitats or food supplies that are highly variable in distribution and abundance from year to year. The so-called irruptive migrants show great flexibility in their movement patterns, leaving their breeding areas in varying proportions and at variable dates from year to year, and concentrating wherever resources are plentiful at the time. The movements of irruptive migrants appear largely facultative in nature, occurring in direct response to prevailing conditions (Chapter 12). The term eruption is used for mass emigration from an area, and irruption for mass immigration.
Typical irruptive migrants of northern regions include: (1) boreal finches and others that depend on fluctuating tree-seed and fruit crops; (2) owls and others that depend on cyclically fluctuating rodent populations; and (3) waterbirds that depend on ephemeral wetlands created by irregular rainfall. This chapter is concerned with the seed-eaters, and the next chapter with the other two groups. They all provide striking evidence for the influence of prevailing food supplies on bird movements. But the movements of these species can be understood only in terms of their underlying ecology, which is therefore described here in greater detail than for other migrants.
Among the specialist seed- and fruit-eaters, most individuals stay in the north in years when food is plentiful there, wintering within, or just south of, their breeding areas, but moving further south in years when food is scarce. Their so-called invasions or irruptions, in which they appear in large numbers well beyond their usual range, follow periodic widespread crop failures. Irruptive migrations therefore occur in response to annual, as well as to seasonal, reductions in food supplies. The effect of food shortage is often accentuated because the birds themselves tend to be numerous at such times, as a result of good breeding and survival in previous years when food was plentiful (Lack 1954, Keith 1963, Berndt & Henss 1967, Koenig & Knops 2001). The greater the imbalance between the birds and their food, the greater the proportion of individuals that leaves, presumably as a result of competition (for Bohemian Waxwing Bombycilla garrulus, see Siivonen 1941, Cornwallis 1961, Cornwallis & Townsend 1968; for Great Tit Parus major see Perrins 1966, Ulfstrand 1962; for Purple Finch Carpodacus purpureus see Koenig & Knops 2001). Tyrvainen (1975) noted that Fieldfares Turdus pilaris left an area in southern Finland when their main food source (Rowan Sorbus aucuparia berries) had been reduced to an average of about two fruits per inflorescence. The date at which this occurred depended on both the initial crop size and the number of consumers.
Most irruptive seed-eaters are hard to study because they breed mainly in high-latitude regions, where human population density is low, and where the chance of obtaining ring recoveries is extremely small. Moreover, because of their eruptive behaviour, many such species are seldom in the same area long enough for detailed study. For these reasons, an understanding of their movement patterns must be pieced together from scraps of information collected over a long period, and scattered widely through the ornithological literature, although ring recoveries are slowly adding new information.
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