Birds offer special problems to paleontologists. In general, birds don't tend to fossilize as well as mammals. This is not because bird bones are hollow and mammal bones are not, as often is stated; the major limb bones of both are hollow. The walls of bird bones, however, tend to be thinner than those of comparable sized mammals and thus more prone to crushing and to the effects of weathering and scavenging. Nonetheless, birds preserve very nicely under some circumstances, and Pleistocene cave deposits often produce large numbers in reasonable condition.
A second problem that tends to make the published fossil record of birds less complete than that of mammals consists of the perching (passeriform) birds. Making up more than half of the living species of birds, most are relatively small, and few workers are able to reliably identify many taxa to the species level.
Other problems with birds include the fact that there are so many of them and that they are more mobile than most non-volant mammals. Any one area in the Southwest may have several times as many avian taxa as mammalian taxa. For example, there are a little over 380 species of birds on the El Paso area checklist; there are about 70 species of mammals.
I have hesitated to address bird taxa with questionable identifications at the generic level (that is, "cf. gen. et sp." or "cf. gen."). I finally have opted to include these, despite the danger of them being taken too seriously, since they are reported in the literature and may lead experts specializing in specific taxa to investigate further. Passeriformes (other than those of the genus Corvus) that are identified only as UTEP specimens or listed solely from a Harris citation should be accepted only with a considerable degree of skepticism; neither the degree of ornithological competency nor the availability of a comprehensive comparative collection is such as to instill as much confidence in avian identifications as for mammals.
Many species of birds migrate and those that do not are subject to being carried far from their normal habitats by storms. Birds also tend to end up where they shouldn't be by the stupidity of the young (a feature not limited to birds, by any means). Southwestern mammals, on the other hand, if they migrate at all, tend to do so by changing their elevation seasonally. With the exception of bats, mammals don't tend to be blown into distant lands by stormy weather, and when youths wander off, they seldom can go far.
Nevertheless, many birds are specific enough in their habitat requirements or preferences to be of aid in reconstructing past environments. Whereas individual birds may end up far from their normal habitat, lost or windblown, the chances of such individuals being preserved and recovered are low, and the presumption is that a fossil lived reasonably near the site of recovery unless the remainder of the fauna is obviously incompatible with that interpretation.
Biologists, of course, find interest in the makeup of ecological communities, and that in itself is ample justification for the study of Pleistocene avian faunas.
Last Update: 7 Jun 2009