Species of the Month: Prothonotary Warbler
The Prothonotary Warbler (Protonotaria citrea) is an endangered migratory songbird restricted to Canada’s southernmost areas. Male Prothonotary Warblers are bright yellow, with a blue-gray wing. Female Prothonotary Warblers have similar plumage, but may also appear olive-green. These birds have a very limited distribution, breeding almost entirely in areas south of London. Most of the Canadian population breeds in Rondeau Provincial Park (south of Chatham), with much of the remainder in Norfolk County. The remainder of the Canadian population is found in the Skunk’s Misery area (Southwestern Middlesex County), Pelee Island, and Point Pelee National Park. This species is mostly observed within deciduous swamps, which are often dominated by Silver Maple (Acer saccharinum), Ashes (Fraxinus sp.), Black Tupelo (Nyssa sylvatica), Cottonwood (Populus deltoides) and American Elm (Ulmus americana).
Prothonotaries were bright yellow robes traditionally worn by papal clerks in the Vatican, which is how this warbler acquired its name. The species is additionally known as the “golden swamp warbler” to naturalists, due to its habitat preference for swamps, and bright yellow plumage.
Prothonotary Warblers are the only native warbler species breeding in tree cavities. These tree cavities are excavated by Downy Woodpeckers (Picoides pubescens), Norfolk County’s smallest woodpecker species (making Prothonotary Warblers a secondary cavity nester). The endangerment of Prothonotary Warblers in Canada is largely attributable to deforestation, improper forestry practices, and loss of natural tree cavities. Many conservation agencies across southwestern Ontario have utilized nest boxes to help provide habitat for Prothonotary Warblers in areas of swampland where natural tree cavities are scarce. In some protected areas, Prothonotary Warbler populations have increased as a result of conservation efforts, including a few sites in Norfolk County.
Of all of the North American warbler species, the Prothonotary Warbler is most closely related to the Swainson’s Warbler of the southern United States (Limnothylpis swainsonii; an uncommon vagrant to Norfolk County, although recorded as recently as 2022). Both species are sister to the more common Black-and-white Warbler (Mniotilta varia), a species that the Prothonotary Warbler shares many of its feeding habits with. Prothonotary Warblers can climb up and down trees, scavenging for insects in a method similar to the Black-and-white Warbler. Males of both warbler species sing a series of high-pitched tweets, which each last around 3 seconds in duration. The birdsong is most frequently heard in the early morning, although many species of warblers can be heard singing throughout the day from May to July.
Prothonotary Warblers spend the winter months (from September to April) in mangrove swamps and tropical rainforests of Central America, with some individuals being observed as far south as Colombia. The Cordilleras of Costa Rica and Panama, as well as the Yucatan Peninsula in Mexico and Belize are particularly important for Prothonotary Warblers. They are one of the earlier warblers to return to Ontario, typically arriving in the Lake Erie region by the last week of April. The males will typically arrive at breeding sites before the females, and will scout out many potential nest sites, but female Prothonotary Warblers will ultimately choose the nest site for her eggs. Nests are typically located in the understory of swamps within 10 meters from ground level. Females will do most of the nest building, with males gathering the nesting materials. Like other warblers, Prothonotary Warblers are monogamous throughout a single breeding season. They will sometimes select the same mate across multiple breeding seasons, owing to their high nest-site fidelity (especially if breeding was successful in previous years). Prothonotary warblers typically lay clutches of 3-7 eggs, with up to 3 broods per year. The incubation period is approximately 15 days, with a nestling period around 11 days. The peak of the breeding season is in June, where many bug populations (including mosquitos) are at their highest. By mid-August, declining insect numbers make conditions become more unfavorable for Prothonotary Warblers, which will subsequently begin preparing for their autumn migration. When the autumnal equinox arrives in late September, many Prothonotary warblers have departed the Lake Erie region for their tropical wintering areas.
Written By:
Matthew Palarchio, Honour’s BSc in Environmental Science candidate, Western University
