Birds tend to commence migration in large numbers only when they have a favourable tail wind. In North America the winds north in spring and south in autumn are ideal to assist seasonal migrations. Once started however only very bad weather will stop them. Many birds fly high when migrating because of prevailing winds at higher altitudes and also because the cold at these altitudes helps them disperse all the heat being generated by their flight muscles.Many species of wildfowl fly at 6000m and some have been observed flying at 8000m, 86mph in temp = -48 C

Not all birds from a summer breeding site overwinter at the same area. What happens, come autumn, if a male bird meets a female bird in the breeding grounds who has a different overwintering site? Whose site do they go to now they are a pair? In many species the pair bond breaks up at the end of the breeding season, but some like swans mate for life. In the case of the Bewick's Swan the male decides where to fly to for the winter and the female follows him. However, the female decides when it is time to travel back to the tundra for another year's breeding.

The reverse scenario is when birds with different breeding sites overwinter in the same area; if pairing commences on the overwintering ground, whose breeding ground to they return to. The answer may be different for different species. The only example I know of involved Mallards in the USA and in this case the male followed the female.

Timing of migration is a mix of internal stimulus which results in a feeding binge to put on fat to survive the journey and then the tendency to aggregate into flocks. Once the pre-migration flock is gathered, the feeding continues while the birds wait for suitable weather conditions. Thus while the birds' internal clock probably releases the hormonal triggers at a fairly accurate date each year, the availability of food and the presiding weather conditions decide when the migration starts and hence when we see the first spring migrants arrive and the last autumn ones leave.

A 12-year study of Common Terns (Sterna hirundo) at Cape Cod showed that an average 75% of birds, and as much as 83%, returned to the same area to nest in successive years. Eighty percent nested within 25 feet of the original nest site. Another study of Layson Albatrosses showed that in the following year a nest was on average only 13 inches away from the previous nest.

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Part of the information on this page was contributed by EarthLife.