Effect Measure

Vagrant birds and bird flu

I have professional colleagues who are dedicated birders but it has never interested me, and their interests are mainly independent of their lives as epidemiologists, toxicologists or whatever else they do at work. But the biosphere is truly interconnected in strange ways and sometimes what seems an unrelated realm intrudes itself front and center in a different context. Bird migration is a good example. How is bird flu spread? Is it human enabled movements of infected poultry or the rare bird trade? Or is it the “natural” movements of wild, migratory birds, the natural reservoir for the avian influenza virus? This question has stimulated increased attention to bird migration, something that many of us thought was well-known and well-mapped out, but in reality seems to be only crudely known and mapped only in coarse generality with many gaps. Outside of the usual destinations are questions of “vagrant birds,” i.e., birds found far from their usual breeding and migration areas. These birds are potential vectors for introducing a virus into a new geographic area, i.e., agents of spread.

A new paper in the Journal of Ornithology (I don’t have access, so I am operating off a press report) contains some interesting information from a team of ornithologists and ecologists from the University of Marburg, the Ornithological Society in Bavaria and the Helmholtz Centre for Environmental Research (UFZ). The bottom line from this study is that the determining factor of vagrant birds intruding into areas far from their usual destination is not the distance from their usual origin but its direction:

The scientists assessed several thousand reports of Asian birds from the leaf-warbler and thrush families that had strayed to Europe. They discovered that the distance between the breeding grounds in northern Siberia and the wintering sites in southern Asia was often similar to the distance between the breeding grounds and Europe. The more similar the distances and the more numerous a particular species, the higher the probability of this species of bird straying to Europe.

The birds’ body size is not a factor. For a long time, people suspected that the vagrants had been blown off course by the weather. The new findings, however, support the hypothesis that the vagrant birds end up in the wrong wintering areas as a result of an error in their migratory program. (redorbit)

My friends the amateur birders are the source of the data. Many of these folks are obsessive recorders of what they see, with of course a premium placed on the unusual. This provided a long record of sightings of “out of place” species going back over 150 years:

[The research team's] source was the list of confirmed sightings in the Handbuch der Vögel Mitteleuropas (handbook of birds of Central Europe) from the start of ornithological records to the early 1990s. Eight species from the leaf-warbler family and six from the thrush family caught the scientists’ attention as vagrants. One species that was spotted particularly often was the Yellow-browed Warbler (Phylloscopus inornatus), which was reported by voluntary ornithologists in Central Europe around a thousand times between 1836 and 1991. This species breeds in the Siberian taiga south of the Arctic Circle and overwinters in the subtropics and tropics of South-East Asia. The other Asian leaf-warbler species were observed much less frequently, if at all, in Central Europe. By contrast, five thrush species were reported nearly 100 times.

Birds of the same size and wingspread appeared as vagrants in proportions to their numbers but not necessarily concurrently. If weather events were the determining factor it would affect different same-sized species equally. Nor were smaller birds disproportionately affected. The conclusion was that a vagrant bird still flew the usual from the breeding ground but in the wrong direction. Instead of heading further into Asia they go the wrong way and wind up in Central Europe. The researchers posit a rare defect in the migration programming of individual birds.

So what’s the implication for bird flu spread? The news article says that vagrant birds are not likely to be a significant factor. I don’t know if that is the conclusion of the reporter or the scientists (probably the latter) as it is unattributed, but I take the views of ornithologists on this issue at face value. They have a stake in bird flu spread being from human, not natural, mechanisms. The question of the role of vagrant birds in bird flu spread might depend upon whether infection itself alters the direction programming (this would be an adaptive response of the virus). More generally, we need to know more about what birds fly where, how much variation there is in these migrations, and which birds are more or less likely to carry the virus over the usual duration of their migrations.

I don’t usually read the Journal of Ornithology. I probably should. So much to know. So little time.