The Prothonotary Warbler is 13 centimetres (5.1 in) long and weighs 12.5 grams (0.44 oz). It has an olive back with blue-grey wings and tail, yellow underparts, a relatively long pointed bill and black legs. The adult male has a bright orange-yellow head. Females and immature birds are duller and have a yellow head. In flight from below, the short, wide tail has a distinctive two-toned pattern, white at the base and dark at the tip.
It breeds in hardwood swamps in extreme southeastern Ontario and eastern United States. It winters in the West Indies, Central America and northern South America. It is a rare vagrant to western states, most notably California.
Behavior and ecology
It is the only eastern warbler that nests in natural or artificial cavities, sometimes using old Downy Woodpecker holes. The male often builds several incomplete, unused nests in his territory; the female builds the real nest. It lays 3–7 eggs.
These birds are declining in numbers due to loss of habitat. They are also parasitized by the Brown-headed Cowbird (Molothrus ater), or outcompeted for nest sites by the House Wren (Troglodytes aedon). It is listed as Endangered in Canada.
The Prothonotary Warbler became known in the 1940s as the bird that, in front of the House Un-American Activities Committee, established a connection between Whittaker Chambers and Alger Hiss. Chambers had testified that Hiss enjoyed bird-watching, and once bragged about seeing a Prothonotary Warbler. Hiss later testified to the same incident, causing many members to become convinced of the pair's acquaintance.
This bird is mentioned in A Sand County Almanac by Aldo Leopold as the "[J]ewl of my disease-ridden woodlot", "as proof that dead trees are transmuted into living animals, and vise versa. When you doubt the wisdom of this arrangement, take a look at the prothonotary." 
- BirdLife International (2012). "Protonotaria citrea". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Curson, Jon; Quinn, David; Beadle, David (1994). New World Warblers. London: Christopher Helm. pp. 159–161. ISBN 0-7136-3932-6.
- Dunn, Pete (2006). Pete Dunne's Essential Field Guide Companion. Houghton Mifflin.
- Stiles, Gary; Skutch, Alexander (1989). A guide to the birds of Costa Rica. Cornell University Press. ISBN 0-8014-9600-4.
- Alderfer, Jonathan (2006). National Geographic Complete Birds of North America. National Geographic Society.
- Bird: The definitive visual guide. DK Publishing. 2007.
- Chambers, Whittaker (1952). Witness. Random House. pp. 362, 564, 572, 573, 580. ISBN 0-89526-571-0.
- Linder, Doug (2003). "The Trials of Alger Hiss: A Commentary". Famous Trials. University of Missouri-Kansas City School of Law.
- Miller, John J. (30 April 2007). "The Unsung Hero of the Cold War". National Review. Archived from the original on 10 October 2012.
- Aldo Leopold (1996). A Sand County Almanac. The Random House Publishing Group.
|Wikimedia Commons has media related to Protonotaria citrea.|
- Prothonotary Warbler media on the Internet Bird Collection
- Prothonotary Warbler species account on Cornell Lab of Ornithology's All About Birds
- Prothonotary Warbler species page – USGS Patuxent Bird Identification InfoCenter
- Bird of the Month: Prothonotary Warbler "The Swamp Songster" by Lisa Petit (January 1997) – Smithsonian Migratory Bird Center
- Prothonotary Warbler recording (WAV)