Dutch Birding volume 34 (2012) no 4
Eurasian Tree Sparrows with aberrant plumage at Makkum in 2009/10
Around Christmas 2009, four to six aberrant Eurasian Tree Sparrows Passer montanus were discovered in a group of normal Tree Sparrows and House Sparrows P domesticus at Makkum, Friesland, the Netherlands. Two or three of these birds stayed there during the winter and following months. Until the end of the next summer, there was no change in the appearance of these birds. Two of them bred, producing two successful clutches. Also some of the juveniles showed blackish on the breast. After the summer of 2010, the aberrant birds have not been observed anymore.
All aberrant birds had a variable amount of blackish feathers on breast and belly. Initially, they were identified as 'possible' Spanish Sparrow P hispaniolensis, but after publication of photographs on an internet forum, it was suggested that they were aberrant Eurasian Tree Sparrows, mainly based on the black cheek spot.
Photographs of the birds were sent to several experts, who concluded that the most probable explanation for the black feathers was extreme wear or damage of these feathers, revealing the blackish basal parts of the feathers. Other explanations, such as feather parasites, pollution, hybridisation, melanism and atavism are shortly discussed but set aside and considered less likely.
Rien van Wijk, Felsental 7, 6006 Luzern, Zwitserland
Enno B Ebels, Joseph Haydnlaan 4, 3533 AE Utrecht, Nederland