One of the most notable things about just about any bird population is that some of the birds are only present for some of the time.
Between 1500 and 4000 species of birds migrate, the exact answer is not really known yet and the experts differ as to their answers.
Some birds travel south in some winters but not others. This is not migration, but dispersal resulting from a population boom. These population booms can occur regularly, i.e. every 4 or 10 years perhaps, but there is no return in the spring. Destruction or loss of food resources can also cause the irregular eruptions of birds to new habitats.
Seasonal migration is a major factor in the life of many birds. In some cases this migration is very obvious and involves huge distances but at other times it is much more subtle. In the UK most people know that Swallows, Swifts and many small passerines such as Wood Warblers and Redstarts are migratory birds. They are present in the summer, but fly south along with many other species to warmer climes for winter. Similarly, many ducks and geese are only present in the UK during winter, when we are their south. They return north to the nearctic in summer to breed.
What is less well known is that a number of our more regular birds are migrants as well. Birds that we see all year such as Robins and Starlings either leave or come to Britain during the winter. For some of these species, birds move south a certain degree over their whole range so that though my local birds have left and gone south their more northern cousins have also moved south and arrived. The overall effect can be that to a cursory observation, the population is sedentary when in fact it is quite dynamic. A further complication to this picture is that in some cases only part of the population is migratory.
In other words some local birds move south, some remain and some more northern birds arrive. This means that although I see a species of bird, say Chiffchaff all year round, the birds I see come from two different populations. Anyway, whatever the true nature of the migration observed the fact of migration itself is a fascinating and awe inspiring facet of bird ecology. The following pages are a brief introduction to extent and mechanics of migration. I have tried to use examples from all across the world but as my time, space and information are limited I can only apologize if your favorite example has not been mentioned.