An investigation of a novel anomalous pink feather colouration in the Mute Swan Cygnus olor in Britain and Ireland

Abstract

Pink feather colouration in normally white adult Mute Swans Cygnus olor is described
here for the first time. Symmetrical salmon-pink colour was first evident on primary
feather tips after moult (July–August), spread to secondary and tertiary remiges as the
year progressed, darkened as winter approached, and sometimes developed to a brown
colour. Affected feathers tended to become brittle, fragmented and lose their ability to
repel water. Surveys made at nine sites in Britain and Ireland between May 2003 and
January 2009 found 12–85% of swans with pink coloration. Highest prevalence
occurred amongst flocks dependent on artificial food on eutrophic water bodies.
Feather samples (white and pink), bill swabs and swabs of uropygial oil collected from
swans in the field, and also pink fungus isolated from a bread sample, were cultured and
subjected to high performance liquid chromatography (HPLC), to identify organisms
and pigments respectively. Salmon-pink Chrysonilia sitophila fungus colonies developed
on agars inoculated with samples from swans at Cork Lough and from the bread sample,
but were absent from those inoculated with samples from swans at Lough Aderry and
Rostellan Lake, where the birds feed mainly on aquatic vegetation. HPLC revealed that
the dominant pigments in pink feathers were generally consistent with those found in
C. sitophila, indicating that C. sitophila is the most likely agent responsible for the pink
colour on swan plumage. Field experiments implied that C. sitophilawas not transferred
to the plumage through contact with water; we therefore suggest that C. sitophila is
acquired through exposure to contaminated food via the bill and is preened onto the
plumage. A layer of environmental contaminants and debris that coats the plumage of
swans inhabiting eutrophic water bodies may provide a substrate for fungal growth.