when
it was seen by many people, including those on field trips to the area
during our International Migratory Bird Week celebration. It associated
with a flock of other yellow-rumps, but those birds were somewhat antagonistic
towards it. It often foraged close to the ground.
This is what would commonly be called an albino, but only an individual completely lacking in pigment (including the eyes and soft parts) can accurately be called an albino. Partial albinos are termed "leucistic."
The wings and tail of this bird were entirely white. Soft yellow was visible where most yellow-rumps have bold yellow on the flanks. The back and head were pale gray, and some pale gray streaking replaced the prominent streaking of normal-plumaged yellow-rumps. This bird appeared to be a male.
While relatively common in some families of birds, abnormally pigmented individuals are rare in the Parulidae (wood warblers). Most interestingly, a xanthochroistic (excessive yellow pigmentation) Yellow-rumped Warbler was seen in New Brunswick, Canada this spring at the same time our bird was present.
That either of these birds survived their first year to migrate north is quite amazing. Aggression from normal birds, as noted in the UM-D bird, is common. Dark pigments add strength to feathers, so modification of the structural integrity of the feathers has been noted in reduced-pigmented birds. This can affect flight and foraging (perhaps this is why our bird spent so much time on the ground).
Another leucisitic Yellow-rumped Warbler was found on 23 January 1981 in Hancock Co., Mississippi. White or washed out plumage was on the head, nape, back, and underparts. An account was published in the Mississippi Kite (11:51) in 1981.
On 17 January 1999, Brush Freeman of the Texas Bird Records Committee also saw an leucistic Yellow-rumped Warbler in Bastrop Co., Texas. He provided me with a description of his sighting. Earlier, in Decemver 1998, Truman Powell also saw an albino warbler, probably a Yellow-rumped "Audubon's" Warbler, in Trinidad, Texas. His description is with Brush's.
A full report of the Dearborn sighting was published in Michigan Birds and Natural History in the October-December 1997 issue.