Prothonotary Warbler

Description

The Prothonotary Warbler is one of the most dazzling of North American birds. Males and females look alike, but males are somewhat more colourful. They have golden yellow heads and breasts, yellow-green backs, and slate-blue wings and tails. Prothonotaries don't have wing bars, but white tail spots are quite prominent.

The Prothonotary Warbler may be confused with the Yellow Warbler, which is almost all yellow, with rusty streaks on its breast. The Blue-winged Warbler also looks similar, but it has a black streak through its eye and whitish wing bars.

Prothonotary Warbler
Yellow Warbler	Blue-winged Warbler

 

"Prothonotary" is a big name for such a little bird. Human prothonotaries are religious and legal clerks who sometimes wear a golden hood and a blue cape. The Prothonotary Warbler is also known as the "golden swamp warbler" in some regions.

For a warbler, the Prothonotary Warbler has quite a long bill - one of the diagnostic features that places it in its very own scientific genus (Protonotaria). The average Prothonotary weighs about 14 grams (about ½ oz), and measures about 14 cm long (5.5 inches).

Its territorial song is a very loud, memorable, ringing "Tsweeet-tsweet-tsweet-tsweet," uttered in groups of four to six. Thus, together with its yellow colour, the Prothonotary is "Tweety Bird" come to life!

Breeding Range

The Prothonotary is the quintessential "Carolinian" species, breeding throughout much of the eastern U.S, and north to extreme southwestern Ontario. It is most abundant in the southeastern U.S. and up the Mississippi River valley.

 

Being at the fringe of its range in Canada, the Prothonotary Warbler is almost entirely restricted to a few areas in southwestern area, mostly adjacent to the Lake Erie shoreline (Holiday Beach, Pelee Island, Point Pelee, Wheatley, Rondeau, Long Point, and Point Abino). It also regularly occurs in Hamilton, and occasionally nests at Pinery Provincial Park on Lake Huron. The "core" populations in Canada reside in Rondeau Provincial Park and the Long Point region.

Known breeding occurrences of Prothonotary Warblers in Ontario from 1981-2001 and historically.

Prothonotaries in the Spring

In spring, the Prothonotary begins to return to southern Ontario in the first week of May. Males generally precede females by about 2 weeks, and older birds of both age groups precede younger ones. The entire adult population is usually back on its nesting grounds by the first week of June, but some younger females dilly-dally and may not arrive until the end of June. By the time the females are back, the males have usually already established their territories and begun to select potential nest sites for the females to inspect.

The Prothonotary is the only warbler in eastern North America that builds its nests in tree cavities. Since it cannot excavate its own, it uses naturally formed tree hollows and cavities excavated in previous years by chickadees and Downy Woodpeckers.

Small, shallow cavities that are fairly low (usually 1-3 metres) are greatly favoured, especially if they are over open pools of water. Other than that, Prothonotaries are not terribly fussy about the cavity. In fact, Prothonotaries have been known to use some pretty strange nesting sites, including a tool box, the pocket of an old coat, a paper sack, a coffee can, a tin pail, a mail box, a box on a moving ferry, a Chinese lantern, an old hornet's nest, a glass jar, and a tea cup. Like many other cavity-nesters, Prothonotaries also readily accept bird boxes, both traditional wooden ones, and others that are made of more unusual materials. For example, one nest box program in Michigan used wax cardboard milk containers with great success, while another program in Ohio used plastic Metamucil bottles (donated by a local senior citizen's centre)!

Whatever the cavity, Prothonotaries fill it almost to the brim with chunks of shredded green moss, often mixed with a few dead leaves. The nest is lined with fine grasses and rootlets. It is thought that the green moss may act as a natural fumigant to suppress lice infestations and/or that it also helps keep the nest insulated against temperature extremes. In any case, the Prothonotary Warbler will not inhabit an area unless it has an ample supply of moss. Shady, swamp forests are great spots for the proliferation of mosses.

Indeed, the Prothonotary is very much dependent on deciduous swamp forests. Prothonotary swamps almost always have large expanses of relatively deep, open standing water. Pools that measure at least 0.5 hectare are greatly favoured, especially if they retain water through at least the end of June. Ideally, water depth ranges from about 0.5 to 1.5 metres. 

In Canada, swamps favoured by Prothonotaries are typically dominated by water-tolerant trees like silver maple, black ash, and black gum. Prothonotaries also occur in oxbow floodplains and along the margins of slow-moving, warm-water creeks and rivers that are often lined with large willows. Regardless, the shrubby understorey is usually sparse, usually consisting of scattered buttonbush shrubs and tree saplings. Because of relatively closed canopy conditions and relatively deep water, there usually isn't much in the way of emergent marshy vegetation. While Prothonotaries can tolerate some marshy elements, pools of water that become choked by shrubs, cattails, and Phragmites are generally avoided.  

Territories are well-defended, usually encompassing about 1 hectares (e.g. 100 metres by 100 metres). This means that very small pools of open water swamp are insufficient; the open water area required for each pair is almost always at least 0.5 hectare in size.

Why Build Dummy Nests?

More often than not, the male will build one or more incomplete or "dummy" nests early in the nesting season, adding just a shallow layer of moss to one or more cavities scattered within its territory. These partial nests appear to serve several functions. First, the male may use them to demonstrate to a prospective mate that he has chosen a good territory with lots of nesting opportunities. He may also be trying to fool potential predators into thinking that cavities with nests do not necessarily mean a free lunch. As well, he is informing other nest competitors (House Wrens, Tree Swallows and other male Prothonotaries) that his territory is "full." There is also some evidence that the male may also sometimes use the "dummy" nest as a night-time roosting site. Finally, and perhaps most importantly, a "dummy" nest is quite often adopted by the female upon her arrival, who then takes over completing the functional nest. Because the male has already spent a day or two laying the foundation for the functional nest, it saves her precious time in nest building, and thus gives her a jump-start on the all-too-short nesting season.

Nesting is what it's all About

By late May and early June, many of Canada's Prothonotaries have established territories, completed their nest building, and layed their eggs. For a songbird, the Prothonotary lays an unusually large number of eggs, one day at a time. The normal clutch is 5 or 6 eggs, but 7 and even 8-egg clutches are not unheard of. The female does all of the incubating. During this time, males will often tend their mates by bringing them tasty green caterpillars to munch on, when they aren't too busy chasing away avian intruders or incessantly singing.

The eggs hatch after about 12 days and the parents are kept busy for about another 10-12 days feeding the insatiable young. Again, green caterpillars are a particular favourite, while spiders aren't too far down the menu. On this protein-rich diet, the young grow very quickly, and the nest cavity quickly fills up with baby Prothonotaries. Because the cavity is usually very small and shallow, and because the Prothonotary family is often a big one, there isn't room for anybody to exercise their growing wings, let alone stretch!

Owing to the crowded conditions, the young leave the nest as soon as they're able to. This is a very difficult first flight. Not only are the youngsters incapable of sustained flight (the best they can muster at this age is a few metres), but their first flight is almost always directly over open water. Prothonotaries aren't built for swimming more than a few feet, so the first flight needs to land them on the nearest branch, or else they will drown. Also, because of the crowded nest, the young haven't been able to exercise their wings at all, so they are making their inaugural flight fueled by sheer guts and determination.

When one of the youngsters finally gathers his or her courage to fledge, the rest quickly follow the leader. The nest can be completely vacated within about 5 minutes. Fledglings invariably try to land in a nearby low-hanging branch. Up until this time, the young have all been relatively quiet. However, as soon as they leave the nest, they begin to call incessantly to their parents. With food in their bills as a reward, the parents often respond by landing on a branch a few feet above, seemingly trying to coax the young to work their way up the tree, branch by branch, to the highest parts of the tree canopy. Within a half hour or so, the entire family is often high up, and nearly impossible to see from the ground. For the next month or so, they become tree-top birds, perhaps because this gets them out of the way of marauding mammalian predators.

If this all happens before mid June, there is a chance that the parents will attempt a second nesting. In Canada, however, the Prothonotary Warbler usually has only enough time to bring off one brood of nestlings.

By mid August, nearly all of Ontario's Prothonotary Warblers are beginning to migrate southward to their wintering grounds in Latin America. All told, our family of Prothonotaries has only been in Canada for about 3 months. It is no wonder that "our" birds are considered to be "their" birds by the people living in Latin America, where Prothonotaries actually spend the majority of their lives.

The Winter

Prothonotaries migrate south to the U.S. Gulf Coast and then make a dangerous non-stop hop across the Gulf of Mexico to land in central America and northern South America. The bulk of the population winters in the coastal lowlands of Cost Rica, Panama, northern Venezuela and northern Colombia, where it is concentrated in mangrove forests. Hence, a very large breeding population is effectively compressed into a relatively small geographic area during the winter. It is easy to see how the Prothonotary Warbler's population is particularly vulnerable on the wintering grounds, whether it arises from natural disasters such as hurricanes, or from human-caused changes in the coastal environment. In fact, mangrove ecosystems are among the most threatened in the world.