Long after the event, or four things we (should) know about bird invasions

The most significant single event in the study of alien bird invasions occurred in 1981, with the publication of John L. Long's seminal book ``Introduced birds of the world'' (full title: Introduced birds of the world: The worldwide history, distribution and influence of birds introduced to new environments''; David & Charles Ltd., Newton Abbot, UK). The significance of this book derives not just from its content, but also from its timing, coincident with the 1982 Scientific Committee on Problems of the Environment (SCOPE) programme on the ecology of biological invasions. It was not long before studies started to appear that exploited the data in Long's book to address the questions posed by SCOPE regarding alien invasions. As a result, we arguably have a more complete understanding of the invasion process for birds than for any other taxon. Nevertheless, there are still some key issues in the study of bird invasions where understanding is not all it should be. The aim of this review is to highlight four of these issues by arguing that (1) we do not know half so much about bird invasions as we think; (2) propagule pressure promotes invasions; (3) colonization pressure matters; (4) there is no evidence that escape from parasites promotes alien (bird) invasions. We expect some of the views expressed to be controversial, and others less so, but either way we hope this paper will stimulate others to provide better evidence for-or against-our propositions.