IJZENDOORN ALJ VAN (1948) Mortality among Barn-Owls. LIMOSA 21 (4): 135-138.

In the months of January to April 1948 some twenty Barn-Owls (Tyto alba (Scopoli)) were stuffed by a local taxidermist. All these Owls were found dead or nearly so, in the Wieringermeerpolder or in the adjacent areas. They showed to be seriously stricken with some unknown disease. Often the birds would sit in full day light on some. Farm implement in a barn, where they could quite easily be taken or kiIled. Others were found dead in a well or in some other place. All the birds proved to be very thin and skinny. Their abdominal skins showed a greenish or blackish discoloration and their intestines were often more or less decayed, blackish, or dried, up.
Two of these Owls were sent to the Institute for ParasitiCal and Infective Diseases at Utrecht. There Coccidiida were found to be present, of the species: Isospora buteonis, Henry 1932. It is not quite certain, however, whether these Protozoa have been the cause of the mortality, as the voles (Microtus a. arvalis (Pallas) in this region have been combated with thallium-preparations. These poisoned voles may have been eaten by the Owls, thus causing their death. Hooded. Crows (Corvus cor.nix) , however, who also devour many voles in. this polder, appeared to be unaffected, so this supposition is not so well founded in my opinion.
A far greater mortality among many Strilgida occurred in the Netherlands in the wint.er 1934-1935 (see: Ten Kat e 1935)). Then at least 300 casualties were reported over a large area. The symptoms of the disease appear to be exactly the same.
Other winters that have been periods of great mortality among Owls in Holland are those of 1921/1922 and 1944/1945. In the wint.er of 1921/1922 there was a report, of one taxidermist who received about. 100 Barn-Owls. In the winter of 1944/1945 at least 36 Barn Owls. 6 Short.-eared Owls and 3 Long" eared Owls were kiIled off in a rather small area. In the winters of 1934/1935 and 1944/1945 three of the victims were banded and proved to originate from Oldenburg (Germany).
Mr W. T hönen (Bern) told me in July 1948 that there seemed to have been a similar disease among Owls in the winter of 1947-1948 in Switzerland, which seemed to have been attended with quite strange migrations. Alas, further details are lacking up to now.

[free pdf] [dutch summary]