Okay, I meant to write an entertaining post. I honestly did. But I have been sick with different kinds and forms of cold since the beginning of December with no interruption, and so has my entire family. And I am sick of winter, too: first there was no winter at all, with scarcely a night below freezing all through December and January and only rain, rain, rain. Sickening! And now that we had all started to accept skipping winter altogether this year, we are suddenly hit by a Siberian cold front with day temperatures around -12°C (10°F). That is very cold for the south of Germany.
So, to cut things short, there is sickness all around me, and I have thus decided to have my February post whistle to that tune as well and give an update to the mysterious die-off amongst our Eurasian Blackbirds that occurred around my home turf in Heidelberg last summer – blogged about here.
A short summary of events described in this former blog post of mine:
- We are talking about the thrush called Eurasian Blackbird Turdus merula, not the American Icterids.
- This thrush is extremely common in urban and suburban Germany and one of the most conspicuous bird species here, comparable to the American Robin in North America.
- Last summer, birders suddenly started to notice that the Blackbirds were gone from their yards around Heidelberg. Because this was such a common species, we lacked quantitative data to be certain that this lack was an actual “situation” and not just the ordinary seasonal variation. Birders in other regions of Germany – when asked – confirmed however that they were seeing their usual large numbers of Blackbirds, suggesting the “situation” around Heidelberg was real.
- We were starting to find many dead Blackbirds, and those very few Blackbirds we saw alive behaved and looked very strange, with their plumage in bad shape, their heads often bald, limited ability to fly and significantly reduced flight distance. A few dead birds were collected by authorities and sent to a laboratory for investigation.
We now know a lot more, and the following information is largely based on the excellent work of our local bird data compiler Armin Konrad, who was amongst the first to notice the die-off in the first place and was also instrumental in coordinating the surveys and research that finally led to the identification of the culprit!
The Blackbird die-off that was observed in north-west Baden-Württemberg, neighbouring parts of Rheinland-Pfalz and southern Hessen in 2011 was caused by the Usutu Virus.
The Usutu Virus’ origins are in Africa south of the Sahara. Its host range includes mosquitoes of the genus Culex, birds, and mammals, with the mosquitoes acting as the virus’ principal vector. It appears as if some bird species are more susceptible to an infection than others, but many species are affected. In Heidelberg, we soon noticed a significant decrease not only in Blackbirds but also Song Thrushes, Serins, and Greenfinches. Species that were apparently less affected were Corvids, pigeons/doves, the Paridae, sparrows, warblers, and Black Redstarts. These observations suggest that the probability of being stung by a mosquito at the nest is one of the main factors: small non-migratory birds in open nests are affected the most, while larger bird species, species that migrate to Africa (immune?), and species breeding in cavities are less affected.
The Usutu Virus has spread considerably in central Europe over the last 10 years. The first detection outside Africa occurred in Vienna/Austria between 2001 and 2006. It subsequently spread to Budapest/ Hungary (2005, 2006), Zürich/ Switzerland (2006), and north-east Italy (2008/2009). The virus was also found in Spanish mosquitoes in 2006, but an outbreak in Spain’s avifauna has not occurred (yet – or wasn’t noticed).
It is sadly safe to presume that the virus will spread further in Europe, and birders in countries as yet unaffected might want to monitor their common songbirds in urban and suburban environments more closely in the following years to document the outbreak’s progress. These are the main symptoms to look for:
- Obviously, a significant population decrease particularly in Eurasian Blackbirds, and dead birds on the lawn.
- The plumage particularly at the neck and head is in very bad shape, with huge gaps exposing the skin. Some birds can be completely bald, resembling vultures.
- Significantly reduced flight distance. While it is not unusual to approach an urban Blackbird to within 2 yards, 2 inches are not the norm and strongly indicative that something is rotten in the state of Denmark.
- Impaired motoric movements, e. g. inability to fly, staggering, falling over. I frequently observed Blackbirds walking on the ground like rodents, even ducking under low tufts of grass.
American birders, defend your robins – shoot European vagrants on sight!
To end this post on a positive note: it appears as though most (all?) affected populations eventually develop an immunity and rebound to their former levels. “Hope is the thing with feathers at neck and head”, and as we can’t do anything to stop this outbreak anyway, hope and patience are options we’ll have to be comfortable with.