Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Photo of prothonotary warbler © Curtis W. Hart

The bird was named after officials in the Roman Catholic Church known as the protonotarii, who wore golden robes.
© Curtis W. Hart

August 2010

New kingdoms for little birds in golden robes

Biologists are enticing prothonotary warblers to Wisconsin's swampy forest lowlands. You can do it too.

Greg Matthews

Avon Bottoms, along the lower Sugar River in southwest Rock County is easier to appreciate on paper than in fact. Its lowland, hardwood forest in a meandering riverbottom floodplain offers an important corridor for biodiversity. It encompasses both a state wildlife area and a natural area. Rare tree and plant species thrive here with diverse wetland dependant wildlife. So do mosquitoes and poison ivy. Mosquitoes here seem so thick that no amount of netting or repellant can keep them at bay, and poison ivy covers the forest floor.

The bites and itching were just two of the challenges facing DNR biologists who have launched a pilot project to determine if artificial nest boxes can help augment breeding populations of the prothonotary warbler (Protonotarea citrea), a Species of Greatest Conservation Need for Wisconsin.

The small songbird is a mid-distance migrant that breeds in the eastern United States and spends the winter in mangrove swamps and wet forests of the lowland forests of southern Mexico, Central America, Columbia and Venezuela.

PROW, the nickname given by biologists for the prothonotary, has an olive back with blue-grey wings and tail, yellow underbody, a long pointed bill and black legs. Adult males have a bright orange-yellow head while females and immature birds are duller with yellow heads. The bird was named after officials in the Roman Catholic Church known as the protonotarii, who wore golden robes, according to the Cornell Lab of Ornithology.

Prothonotaries are the only eastern wood warblers that nest in cavities over water near or around mature trees. They breed in moist bottomland forests that are seasonally flooded or are permanent wetlands. They are sensitive to changes in water depth and flow as their nests suffer much higher rates of predation from raccoons, other predators and parasitism by brown-headed cowbirds when not placed over water, said Andy Paulios, Wisconsin Bird Conservation Initiative (WBCI) Coordinator with DNR's Bureau of Wildlife Management.

"Their nest success and life history are very closely tied to the Ďhealth' of the floodplain forest ecosystem," he added.

In Wisconsin, PROWs nest in floodplain forests along the Mississippi, Chippewa, Black, Wisconsin, Sugar, Wolf and other large river systems. Many of these systems no longer have intact floodplain forests, and warblers, red-shouldered hawks, yellow-crowned night herons and other important bird species have likely declined from historical numbers due to this habitat loss, according to Paulios.

That's why 13 DNR wildlife staffers and volunteers got together in the spring of 2009 and, in assembly-line fashion, constructed PROW nest boxes as part of a pilot project to see if the boxes could augment breeding success in floodplain forest areas that have lost a lot of tree cover.

Most of the boxes were placed in Avon Bottoms, but "we ended-up making so many houses that we also put some along the Sugar and Little Sugar rivers in Albany Wildlife Area in Green County and along the Bark River at Princess Point Wildlife Area in Jefferson County," noted DNR Wildlife Biologist Mike Foy, Fitchburg.

Much of the nest box is made from four-inch diameter PVC drain pipe adapted by Foy from a similar design developed by Steve Gilbertson for bluebird boxes. The nest boxes are sprayed with textured light-color plastic paint and insulated to help keep the boxes cool.

"Our goals were to come up with cheap, durable houses that would be acceptable to warblers, easy to construct and mount in the field. The nest boxes are built to withstand the elements, last for multiple seasons, mount on electrical conduit poles, allow for drainage and have entrance holes that are too small for cowbirds, a major warbler competitor. It took only a few minor changes to come up with the design we're building now," Foy pointed out.

The project's objectives are two-fold: determine if the nest boxes are effective in enhancing PROW populations, and assess whether warblers can be used as a "flagship" species to interest people in floodplain forest conservation, said Paulios.

Biologists are looking to find out if warblers will use the boxes and if so, increase both their productivity and the population in the study area. Secondly, researchers seek to answer questions such as:

  • Can volunteer nest box trails be maintained by citizens?
  • Will the PROW nest box trails build constituencies for local conservation?
  • Can warblers form a measure of success in efforts to restore floodplain forest ecosystems?

Thinking like a warbler

"Our goal is to engage citizens in PROW nest box trails similar to the enthusiasm generated for bluebird projects," explained Paulios. "Our hope is that PROW can be a window into the larger world of floodplain forest ecosystems. Once folks engage in this (project), we think they will start thinking like warblers."

DNR staff and volunteers placed over 90 boxes in southern Wisconsin floodplain forests during April 2009 in appropriate habitats offering shade or moving water. All offered ready access by canoe, although checking them for activity during early June and early July was challenging, said Paulios. Besides the omnipresent mosquitoes and poison ivy, canoeing to the sites was at times difficult due to snags and fallen timber blocking the river channel that sometimes necessitated portaging around or cutting trails through the logs and branches using a chainsaw.

Monitoring the boxes entailed inspecting nest contents, finding evidence of male or female prothonotaries in the area, and taking notes on general habitat around the box, including water depths. All boxes and poles were removed in October to prevent damage from high water flow.


Almost 40 percent of nest boxes were used by prothonotary warblers in 2009 and another 50 percent were used by native birds including wrens, warblers and tree swallows. There was no evidence of nest parasitism in the artificial boxes compared to 27 percent losses in nests established in natural cavities along the Lower Wisconsin River, according to a 1996 study by Dave Flaspohler, said Paulios.

Photo of biologist inspecting a nest box © Greg Matthews
Wildlife Biologist Andy Paulios uses a mirror to inspect a nest box.
© Greg Matthews

"Personally, it was fun and gratifying to see warblers using nest boxes. Now we need to learn if boxes of this sort really have potential for helping warblers by providing more secure nesting sites. Our understanding from the literature is that prothonotary warblers using natural cavities for nesting are very vulnerable to both predation and cowbird parasitism," noted Foy.

The two biologists saw no evidence that either presented a problem for warblers using these boxes during the trial's first year. If these early observations hold, then the biologists believe that ramping up to place more PROW boxes throughout suitable habitat in southern Wisconsin could significantly benefit the species.

"Although monitoring was limited, we estimate (that) as many as 25 PROW nests could have produced 50 to 90 chicks. Previous research suggests that chicks that survive their first year will settle in the area where they are fledged. So we expect to see more birds using the boxes in 2010," Foy added.

And early observations indicate they are!

"Upon placing some nest boxes (in Avon Bottoms) during the first week of May 2010,Mike (Foy) and I saw a number of males and a couple pairs immediately fly to the box and start building nests. Instant gratification. One wonders if they were waiting for us to arrive," surmised Paulios.

Working with volunteers and DNR staff, nest boxes were also placed in April 2010, at River Falls, Hustisford, along the Lower Baraboo River, Outagamie County and in the Lake Koshkonong area.

Last May, during a field trip to Avon Bottoms, Paulios observed numerous active nests, including at least one nest with a complete clutch of eggs and a female brooding the nest.

"This is the earliest reported PROW nest for Wisconsin," he said. Field trip participants watched one female building a nest at a PROW box return every three to five minutes with a beak full of moss.

Volunteers sought

Citizen volunteers can get advice on setting up a prothonotary warbler nest box trail. Andy Paulios can offer more information about PROW habitat, box placement and box construction. E-mail Andy Paulios, or write to him at:

Wisconsin Department of Natural Resources
PO Box 7921
Madison WI 53707

Monitoring intensity is up to the volunteers, but Paulios encourages two to three visits a year during the June-July nesting season.

"This helps us understand when and how often the boxes are being used. Monitoring helps volunteers better gauge where to place boxes and also gives land managers an idea of the health of their floodplain forest ecosystems," said the biologist.

The WBCI Southern Forests Committee is working on a strategic plan that will set population and wildlife habitat goals for floodplain forests. The Committee will consider using prothonotary warblers and the bird's demographics as a performance measure for managing and restoring this lowland habitat, he added.

Greg Matthews is the public affairs manager for DNR's 14-county South Central Region based in Fitchburg.

Building nest boxes

Prothonotary warblers are one of two species of wood warblers to nest in cavities, and the only one known from eastern states, including southern Wisconsin. They are most commonly found in floodplain forests and lowland hardwood swamps where they search for cavities near oxbow lakes, sloughs and slow moving rivers for their nest sites.

The nest box described below was modified from designs developed for other songbirds to be reasonably predator proof and resistant to parasitism by brown-headed cowbirds – two factors that often limit the nesting success of these rare warblers throughout their range. When placed in suitable habitat, these economical, durable nest boxes allow prothonotary warblers to raise their young with more security than is often available from nests in natural tree cavities.


  • 4-inch diameter thin-walled, nonperforated PVC drain pipe
  • Two 4-inch PVC end caps made for thin-walled pipe
  • ½-inch thick foam insulation board
  • 1 ½-inch x 1 ¼-inch PVC trap adapter
  • ½-inch conduit connector with set screw
  • 10-foot long, ½-inch diameter EMT metal electrical conduit pipe
  • Textured exterior spray paint for plastics in light (tan or gray) and dark (green, brown or black) colors


  • Hack or band saw
  • Drill or drill press
  • 1 5/8-inch drill bit or hole saw
  • 5/16-inch and 13/16-inch drill bits
  • Knife, shears or 4 ½-inch hole saw
  • Heavy duty PVC cement


Caution – PVC drain pipe is very hard and can be hazardous to work on as tools may bind when cutting through the pipe. Make a jig of scrap wood to hold the pipe securely on its side while you work, and wear both hand and eye protection to avoid injury.

Cut the PVC drain pipe into 8 inch lengths with a hack saw or band saw. A standard 10-foot pipe will make 14 boxes with only the flared end left as scrap. If using a band saw, a lot of saw blade will be exposed and special care should be taken. Securely support the free end of the pipe, and never reach across the blade to remove cut pieces.

Pick one end of an 8-inch piece for the top of the nest box. Cut a 1 5/8-inch entrance hole in the side of the pipe, centered 3 inches below the top edge. To the sides and back of the entrance hole, drill three equally spaced 5/16 inch ventilation holes 1 ½ inch below the top.

Attach the trap adapter to the entrance hole with the included plastic nut. This will serve as a hole guard to reduce the hole diameter to 1 ¼ inches and prevent access by cowbirds. Alternatively, trim all but ¼ inch of the threads from the adapter and glue it to the entrance hole with PVC cement, using a heavy rubber band to hold it in place until the glue sets.

Using a sharp knife, shears or a 4½-inch hole saw, cut a 4¼-inch diameter disk from the ½-inch foam insulation board and press it into one of the end caps to serve as the top of the nest box. This lid is held to the top of the box by friction and can be removed for inspection.

Drill a 13/16-inch hole in the center of the other end cap and secure the ½ inch conduit connector using the nut provided. Drill a few 5/16-inch holes around the bottom edge of the cap to provide drainage. Then glue the cap to the nest box assembly using PVC cement to form the bottom of the nest box.

Paint the outside of the finished box with light-colored, textured, exterior plastic spray paint to help keep the nest box cool and allow warblers to grip the edge of the hole guard. Leave the top of the box under the cap unpainted to allow easy cap removal for nest inspection. A bit of dark paint sprayed here and there will make the box look more like a dead tree, and a shot sprayed through the entrance hole against the back inside wall of the box will ensure the hole appears dark and inviting to a cavity-nesting warbler.

Locating the Boxes

Place nest boxes over or near slow moving rivers, lakes, backwaters, oxbows or sloughs in the shade under large tree canopies, preferably where you have seen or heard warblers in the past. Boxes placed by late April will have the best chance of being occupied.

Placing the conduit post in two feet of water minimizes the possibility of predation and allows for lower water levels later in the summer. Nest box orientation does not seem to matter as long as the box is shaded, but placing the box in open sloughs away from brush and small tree branches will discourage excessive competition by house wrens. Drive the conduit firmly into the slough bottom to resist current flow. If the conduit is still too tall to allow box inspection, consider cutting it a bit shorter. Place the box on top of the conduit and tighten the set screw. Thatís it, youíre done. Now itís up to the warblers!