Migratory bird species have been found to advance northward over the northern continents in spring at average rates of 30-300 km per day, in step with improving conditions, as reflected in the northward advance of particular isotherms. In general, early-migrating species progress more slowly than later ones. Barn Swallows Hirundo rustica, which breed over a wide span of latitude, take more than three months each year to re-colonise all parts of their breeding range. The rate at which a continent is colonised each spring, from low to high latitudes, is often much slower than the migration speeds of individual birds, as birds must often wait for snow-melt or other conditions to improve.
Many birds benefit from arriving relatively early in their breeding areas, as they are able to obtain better territories and mates than later arriving ones, and often show better breeding success. However, if they arrive too early, they risk food shortage, and may sometimes die from late cold snaps. These conflicting selection pressures are likely to influence the average spring migration (and arrival) dates of different species. Nevertheless, species migrate at different dates, depending partly on when their particular foods become sufficiently available, and in general earlier in warm springs than in cold ones. The arrival dates of many species in particular localities are spread over a shorter period than their autumn departure dates. Some species spend only 2-3 months each year on their high-latitude breeding areas.
Most migratory birds vacate their breeding areas on the northern continents in autumn from north to south, in reverse order to that in which they re-colonised in spring. However, not all species follow this pattern and, in some single-brooded migrants, southern-nesting populations (having completed their breeding earlier) move out first.
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