Why do the Bird Lists start with Swans and Geese? (by Richard Swinbank)

When we go bird watching, many of us like to make up a list of all the birds we see.  It is easiest just to list the birds in the order in which we happen to see them.  The trouble is, that makes it more difficult to compare lists seen on different trips, or written by different people.  An obvious alternative is to list the species alphabetically.  That is better, but it is better still to try to group similar birds together, so that, for example a Chiffchaff is listed adjacent to a Willow Warbler.  So, the ideal solution is to agree a standard sequence in which species of the same family are listed together, and then families that are closely related are listed together, and so on. 

The lists of birds seen on our field trips are listed in a standard checklist sequence recommended by the British Ornithologists Union (BOU). (Though we don’t necessarily use the full recommended species names.)  The general idea is that the most “primitive” families of birds appear at the beginning of the list and the more “highly evolved” families towards the end of the list.  In the past, the sequence of birds in checklists (e.g. the Wetmore order or the Voous order) was based on anatomical differences.  However, in the past couple of decades, techniques based on studies of DNA (pioneered by Charles Sibley and co-workers) have told us much more about the genetic relationships between bird species.  This has led to a reassessment of the most logical sequence for listing birds in field guides and in checklists.  Since the scientific work is relatively recent, most of the field guides currently in use have not yet “caught up”, but the new sequence should be used in new field guides and is used (for example) in the 2003 Devon Bird Report.

The most primitive orders of birds are the ratites and tinamous (these include birds such as ostriches and kiwis), but there aren’t any of those birds naturally resident in the UK.  Next in line are waterfowl (Anseriformes) and game birds (Galliformes); these two orders constitute the next evolutionary split from all remaining birds.  This does not necessarily imply that the Mute Swan is our most primitive bird, though there are some remarkably primitive birds (the screamers of South America) that belong to the same order.  After the waterfowl and game birds the bird families follow pretty much the same sequence as they were in Voous, indicating that the anatomical differences are a reasonably reliable guide to the evolutionary tree. 

The final order of birds deserves a special mention: the passerines (Passeriformes) constitute around 5900 species, over half of all (about 9850) bird species.  The passerines are in turn split into sub-oscines and the oscines (also sometimes referred to as songbirds).  The sub-oscines’ stronghold is in the New World (e.g. tyrant flycatchers), and there are none resident in the UK.  Within the songbirds, several groups of families are closely related and these are listed together; for example the finches (Fringillidae) and buntings (Emberizidae) come together at the end of the checklist.  

OK, so is this checklist sequence the final word?  It is fair to say that the overall picture of the relationships between orders is now clear, but there are still uncertainties in the relationships between different families and within families.  So, there may well be some more revisions to the preferred checklist sequence in the future.