The study of communuty structure assumes, in general, that bird populations are more or less static in time even though it is implicit from the behaviour of most birds that this unrealistic. Few studies consider the possible impact of migrant birds on resident communities. In areas where the probability of large flows of migrants is high, such impact may be highgly significant in terms of numbers of migrants, species diversity and biomass (Finlayson 1992). Another factor to take into account is the unpredictability of the daily impact during the migration period.

A project has been proposed for Doñana in which these questions are to be addressed over a series of years during which the annual cycle in marsh, scrub and pine woodland communities will be examined. The project already examines these questions in other geographically related areas of south-western Iberia and the extension to Doñana is fundamental in the development of this research programme.