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A ardour for Prothonotary Warblers


Most weeks during the spring and summer, Jackson Audubon Society’s PROW team is a group on a mission. PROW is ornithological shorthand for Prothonotary Warbler, the drop-dead gorgeous wood warbler that is even more stunning in person than in photos. The PROW team is the adopted name of our volunteers who brave the biting insects, alligators, and muggy Mississippi weather to document the success of our long-term nest box project.

This bird is known by some as the “golden swamp warbler” or “swamp canary.” Most birders and ornithologists prefer Prothonotary Warbler (Protonotaria citrea). The name is thought to originate from prothonotaries, clerks in the Catholic Church who wore yellow vestments.

The location of our project is LeFleur’s Bluff State Park, a 305-acre urban green space that borders the Pearl River near Jackson, Mississippi. The park is a National Audubon Society Important Bird Area and provides habitat for Swainson’s and Hooded Warblers, Yellow-billed Cuckoos, and other Neotropical migrants besides Prothonotaries. The Cornell Lab of Ornithology states that the warblers avoid patches of forest less than 250 acres. This is one reason for their strong presence in the park, along with the habitat that it provides.

Since 2000, JAS has been maintaining nest boxes in the park for Prothonotaries. It is safe to say that our Audubon chapter has a passion for these birds. In 2019, we decided to discover just how much the nest boxes were contributing to their reproductive success. We began collecting data on nest building, eggs, chicks, and fledging.

Why is the Prothonotary Warbler the focus of our organization’s signature conservation project? The bird is a species of conservation concern and has many conservation challenges, as do many other Neotropical migrants. They are habitat specialists that prefer swampy forest, wet bottomland hardwood forest, or some mix of forest and water. This type of habitat is routinely at risk almost everywhere. They breed almost entirely in the eastern United States, especially concentrated in the South, with a few in Canada, where they are considered an endangered species. Their migration to wintering habitat takes them into Latin America. This means that they are an international species that require international protections and international conservation solutions.

The current population is approximately 1.6 million birds. Some may say that this isn’t so bad. After all, many endangered species number less than 100 individuals. The problem is that the warbler has experienced an alarming 40 percent drop in its population since the 1960s. Moreover, after the widespread logging of cypress swamps and bottomland hardwood forests in the late 1800s and early 1900s, the number left today is a small fraction of what once was. The best time to work with birds that need our help is when their numbers are still sufficient to have plenty to work with.

Cavity nesters

A female Prothonotary tends to chicks in a nest within a natural tree cavity at LeFleur’s Bluff State Park in Mississippi. Nest-building begins in late April in southern states and in mid-May in
northern breeding areas, and the first chicks of the year fledge in early to mid-June. Photo by Bill Stripling

These warblers are cavity nesters that readily take to nest boxes provided for their use. This is another way in which the birds are ideal subjects for a conservation project. Suitable tree cavities can always be a limiting factor for obligate cavity nesters, and so nest boxes can have an immediate positive impact on the population. They are secondary cavity nesters, using tree cavities excavated by woodpeckers. Prothonotary Warbler is the only eastern wood warbler that nests in cavities. The only other cavity-nesting wood warbler in the U.S. is the western Lucy’s Warbler.

Eastern Bluebirds, a species that has benefitted handsomely by having their own nest box projects, will take over a nest box from Prothonotaries at the park if they can. A 1.25-inch nest hole excludes bluebirds while allowing the warblers to enter. Brown-headed Cowbirds would enter the boxes if they could and parasitize the nest by laying their eggs with the warbler eggs. The host bird would then raise the cowbird chicks as if her own. The 1.25-inch nest hole stops the cowbirds as well but not the tiny Carolina Wrens. We have had two instances of wrens taking over a box.

During our project, we discovered that the warblers gained tremendously by having nearby nest boxes that provide additional breeding opportunities. We monitored 15 nest boxes over the last three years, 13 of which were used in 2021. So far, we have kept the boxes in the same locations for the sake of consistency. Ten other “conservation project” nest boxes are provided that are not monitored weekly.

We know from direct observation that the Prothonotaries also use natural tree cavities in the park, as they have since long before humans arrived on the scene. Nest boxes are mounted on poles in shallow water about 6 feet above the water and away from branches that could allow swamp-dwelling snakes access to eggs and chicks. Incubation is by females for 12 to 14 days, and the male defends the territory and does participate in feeding the chicks. The birds tend to return to the same nesting location for the next breeding season, a behavior ornithologists call “site fidelity.” Breeding starts in April and most often abruptly ends at the end of July. In rare cases when nesting activity of some kind occurs after July, it doesn’t produce viable results.

We calculated that 57 chicks were fledged (successfully left the nest) from our boxes in 2021. It is nearly impossible to witness the moment chicks leave the nest by direct observation. Constant surveillance by camera could, perhaps, capture images of that. We know the situation in each nest as eggs hatch and chicks develop, and then when the chicks are gone with no signs of mortality, we assume successful fledging.

Many Prothonotaries use the island of green that is LeFleur’s Bluff State Park. Over two decades of consistently implementing the project is sure to have enhanced population numbers. Site fidelity will ensure that many of the birds will return over their lifespan of up to eight years, although most don’t survive nearly that long. Our nesting numbers compare very favorably with Louisiana, a well-recognized epicenter of the Prothonotary Warbler population.

Avoiding ‘forced fledging’

Our priority is always the birds’ wellbeing. When we believe that the chicks are close to the end of the 10-day fledging period, we stop checking that box until we estimate that the chicks are gone, and it’s safe to resume. This is to avoid “forced fledging” young birds, a term used to describe frightening the nestlings into leaving the nest prematurely. The chicks and eggs are never touched by our team. We want to make sure nothing of any conceivable nature can be transferred to them. We perform nest checks quickly to minimize the time that the female is kept away from incubating eggs or that the pair is kept away from feeding chicks.

What’s next in the life of our newly fledged and growing young birds? First, the parents continue to feed the young for up to 35 days. Soon, they are developed and strong enough for the next big step in their life cycle, a 5,000-mile migration to their distant wintering grounds. The more westerly breeding Prothonotary Warblers, such as those in Mississippi and Louisiana, make an epic nocturnal flight across the Gulf of Mexico. It is an amazing journey for such a tiny bird to fly at night, nonstop, across an expanse of ocean as the first part of their migration. There are no landmarks over open water to take advantage of and quite obviously no sun to orient with. Flying at night likely means that they are using the Earth’s magnetic field and possibly stars to navigate in the dark.

Reese Partridge, one of the volunteers with Jackson Audubon Society’s Prothonotary Warbler Program, checks on the status of nest in a warbler nest box. The team does not check on nests when the chicks are close to fledging, to avoid frightening them into leaving early. Photo by Charles Pfeifer

The more easterly breeding warblers tend to migrate farther east through the Caribbean. This path still goes over bodies of water but not the marathon distance of the open water of the Gulf of Mexico. In the spring, the birds do it all over again with a journey back to their North American breeding territories.

Recent research shows how they may be accomplishing this. Another species of nocturnal migratory bird has been shown to have a specific protein in their eyes that may hold the key to understanding nocturnal migration. European Robins have such proteins, known as cryptochromes, in their retinas. Scientists suspect that a specific cryptochrome, Cry4, gives the robins magnetic field sensitivity. I believe that with more study, we will find more birds, such as the Prothonotary Warbler, that navigate in this way. It is a question that will have to be addressed scientifically.

Birds without borders

Prothonotary Warblers are literally birds without borders and are a shared responsibility with the Latin American countries that they migrate through and winter in. Through scientific research and the scientific method, we now know just how important Colombia in northwestern South America is to the conservation of the species. The migratory connectivity from breeding areas to stopover sites on the way to their wintering areas presents challenges to the bird’s conservation that must be understood to know where to concentrate habitat-preservation efforts.

Erik Johnson, director of conservation science for Audubon Delta, and others of the Prothonotary Warbler Working Group have deployed geolocators and nanotags, secured on birds captured by mist netting, on their breeding grounds in the U.S. The technology has developed to the point that devices light enough not to impair the flight of the small (14- to 16-gram) birds are possible. A geolocator is a 0.5-gram device that records sunrise and sunset information to estimate daily locations of birds throughout the year. This information is used to calculate the longitude and latitude of the bird each day. The catch is that the warblers must be recaptured when they return to the breeding grounds to access the data contained in the geolocator.

The other tracking device is the nanotag. A collection of antennas, known as the Motus network, automatically receives signals from radio-tagged birds. The birds do not have to be physically captured and handled. When this information is taken together, a revealing picture of where and how the Prothonotary Warbler migrates has emerged. What was discovered shows something profoundly important. Most Prothonotary Warblers, about 88 percent, winter in Colombia. This means that most birds converge on their wintering grounds in an area about 20 percent the size of the area used for breeding in North America.

No matter where a Prothonotary Warbler was hatched in North America, most converge on that relatively small area, unlike many Neotropical migrants. The consequences are that an acre of habitat destruction in Colombia is five times more damaging than an acre lost in North America. With this knowledge, we can concentrate conservation efforts in the areas that will do the most good. (Of course, Colombia is of extreme importance to birds. It has the highest number of bird species in the world at around 1,900.)

Prothonotary Warblers need more habitat

For more than 60 years, the Colombian government was locked in a civil war with a rebel group known as FARC. In 2016, a negotiated end to this tragic conflict earned President Juan Manuel Santos a Nobel Peace Prize. Unfortunately, there was one undesirable side effect. The armed conflict had been keeping out loggers, miners, and farmers who now entered areas that had previously been too dangerous. Now, one of our favorite birds is wintering in an area with Colombia’s least remaining forest cover and its second-highest rate of deforestation.

The key to conserving the Prothonotary Warblers will be protecting as much habitat as possible and showing the value of the birds to local economies. Birders who travel to Colombia and other bird hotspots can make real contributions to bird conservation by spending money to bird in these places and giving local communities a reason to want to save their birds. Our current COVID pandemic has put a damper on international ecotourism that we hope will end sooner rather than later.

A Prothonotary rests is a bander’s grip while receiving a nanotag at Jean Lafitte National Park and Preserve in Louisiana. The device will track the bird on its migratory cycle. Researchers have learned a lot about the species in recent years thanks to nanotags. Photo by Charles Pfeifer

Well-protected national parks and nature reserves will be vital. Conservation organizations can play a vital role in this. In a development reported previously in BirdWatching, American Bird Conservancy, Rainforest Trust, World Land Trust, and Fundación Biodiversa Colombia collaborated to double the size of the El Silencio Nature Reserve. The reserve is in the Magdalena Valley, a recognized Prothonotary Warbler hotspot. Over 98 percent of the lowland rainforest in the Magdalena Valley has been destroyed for cattle ranching, illegal coca production, and illegal logging. Like LeFleur’s Bluff State Park, El Silencio is not just about the Prothonotary Warblers. The critically endangered Blue-billed Curassow and brown spider monkey, jaguars, nearly 300 bird species, and much more live there. It’s an amazing biodiversity hotspot.

By no means are all the Prothonotary Warbler’s conservation problems located in Latin America. Destruction of prime wildlife habitat is always a looming threat in the U.S. as well. We need to look no further than our very own project site, LeFleur’s Bluff State Park here in Mississippi. A development has been proposed for over 20 years known as One Lake, previously Two Lakes, on the Pearl River, which borders the park. It involves dredging and widening nearly 10 miles of the river, building a dam to form a 1,900-acre lake, and using the dredged material to build up land for real estate development in the Pearl River floodplain.

This ecologically destructive project would cause unavoidable damage to park land and the quality habitat that supports thriving bird populations. Some of this quality habitat is 80-year-old bottomland hardwood swamp and sloughs that support Prothonotary Warblers. At the end of the day, One Lake would create new developable waterfront property along with questionable flood-control benefits for the Jackson metropolitan area. A diverse coalition that includes conservation organizations, outdoor businesses, and political leaders has emerged to stop One Lake from moving forward.

The Prothonotary Warbler is a fascinating and charismatic bird that deserves our stewardship. All birders should be bird conservationists. I encourage all birders and birding organizations to choose your own “Prothonotary Warbler” — a species or habitat that you can champion. The Cornell Lab of Ornithology and several conservation and research groups have assessed that nearly 3 billion North American birds have been lost since 1970. The stakes are just too high for every birder to not become engaged on behalf of the birds that we enjoy.

Read about 10 places to find Prothonotary Warblers

Geolocators reveal varied migration routes of Prothonotary Warblers

Listen to author Charles Pfeifer discuss the Prothonotary Warbler on the “Creature Comforts” podcast

Build your own

If you are lucky enough to live or bird in locations that have Prothonotary Warblers, place some nest boxes, stand back, and enjoy the birds! If you’re not trained and experienced with this kind of project, please leave the boxes untouched during the breeding season while the birds are using them. This eliminates any chance of nest disturbance or forced fledging. Before the birds arrive in the spring and after they leave in the fall, the nest box should be cleaned out and any repairs made.

To learn how to place nest boxes for the species, see this excellent resource from the Cornell Lab of Ornithology. For more information about the bird’s nesting behaviors, nest box requirements, and much more, see www.sialis.org/prow.htm.

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