Ensuring a Bright Future for the “Swamp Candle” at Hoover NP

Prothonotary Warbler right - Photo Earl Harrison

If you walk the wet woods around the Hoover Reservoir in Delaware County, above the din of swaying cottonwoods and box elder, and the passing conversations of leisuring anglers, it is hard to miss the forceful climbing shouts of “sweet, sweet, sweet, sweet”.  Seek out these mighty calls and you will find the enduring beacon of the flooded forest, the “swamp candle”, otherwise known as the Prothonotary Warbler. Platitudes aside, it is difficult to not be enamored with this striking yellow and blue bird from the time it arrives in our region in late April, until it returns to its tropical home in late August. Thus, bird lovers have sought to do all they can to ensure their numbers increase towards their previous heights, prior to the large scale loss of their preferred habitat, bottomland forest along our great rivers and streams.

Land preservation, like the establishment of the Hoover Nature Preserve around Hoover Reservoir, has been key to this effort, as it is for many declining migratory songbirds. But, thanks to their undeniable charm, Prothonotary Warblers additionally benefit from an especially dedicated fan club — and the fact that they will gladly nest in nest boxes.

The Prothonotary Warblers at Hoover have been championed for over 20 years by a local legend, Charlie Bombaci. Through his endless dedication, Charlie has “farmed” the Hoover Nature Preserve with an enormous network of nest boxes and seen the population grow to over 100 pairs, easily one of the largest populations in Ohio. In addition to Charlie’s efforts, Hoover has tree cavities (the natural nesting site for Prothonotary Warblers) available in spades, thanks to large populations of woodpeckers and a lot of natural tree decay that goes along with being in a wet forest. Thus, Hoover presents a very interesting opportunity to examine the benefits of nesting in boxes vs. nesting in natural cavities for this species of conservation concern.

With the support of Columbus Audubon’s Conservation Grant Program, my graduate student Elizabeth Ames and I initiated a project to explore the ecology of Prothonotary Warblers, including the relative value of nesting boxes and natural cavities to their breeding success. The CA funding has allowed us to accomplish an enormous amount of work by supporting a field technician to cover the numerous territories all over Hoover, and at some of the satellite breeding sites in central OH (e.g. collaborating with the Buckeye Lake Historical Society at Cranberry Bog).

For our work, we begin surveying for warblers before they arrive to determine the specific arrival dates of individuals, then we capture and band them with a unique combination of colored bands for identification in the field. We follow these individuals all throughout the breeding season, collecting data on what nest types they choose, and the success rate of every nest. We also band young, and use radio transmitters to determine how well they survive after leaving the nest. As of the beginning of this season, we have banded 90 adults and 60 nestlings, radio tracked 28 fledglings, and have monitored over 120 nests.

Our findings thus far have been quite surprising. For one thing, we expected nest boxes to be much better sites for nesting, as the hole size is more limited and we predicted this would exclude predators. Unfortunately, the hole size did not exclude the long reach of the masked bandits of the forest, raccoons. We have only had around 15% of our monitored nest boxes fledge, and in almost every case we have been able to implicate raccoons as the predators, based on tracks below and on the box. This compares to a higher, but still somewhat low in terms of what we look for in healthy bird populations, 37% fledging rate for birds in natural cavities. Thus, we have been able to identify a great need for updated box design at Hoover to avoid raccoon predation. Up until now, all boxes have been placed on trees, so in collaboration with Charlie and Darlene Sillick we are installing 40 new boxes to test out pole mounting and predator guards as a means to increase nesting success.  Our goal is to surpass even the fledging rate of the natural cavities to help ensure the persistence and, hopefully, continued growth of this population that Charlie has achieved through is endless dedication.

The support from Columbus Audubon has led to many other interesting findings about the Hoover Prothonotary Warblers. We were able to retrieve 3 geolocator tags we placed on warblers in 2015 and discover that all three spent their winter in Colombia, likely along the Magdalena River. Further, despite the low success rate in nests, through radio tracking we have found that those fledglings that make it survive at a rate of 53%, which is quite good. Those that did not survive succumbed to storms or were predated upon by Blue Jays, snakes, and even fish! Lastly, our work here in Ohio feeds in to a national network of Prothonotary Warbler researchers, the Prothonotary Warbler Working Group, who are collaborating to reveal all we can about their ecology and what action is needed to ensure their conservation. This includes work we at OSU are doing on the wintering ecology of this species in Panama.

Were it not for Charlie’s efforts we would not have such a large population of Prothonotary Warblers in our region, a true jewel of central Ohio’s birding. Through our collaboration with Charlie and the generous support of Columbus Audubon, we are making progress towards a healthy, vibrant warbler population for many generations to come. Through the combined efforts of passionate, dedicated citizen scientists and researchers we will ensure the swamp candle never dims at Hoover Nature Preserve.

Authors note: If you have any questions about our work, or are out and about at Hoover and see one of our banded birds, please let us know! You can find info on reading the color bands at our Prothonotary Warbler Web site or send me a message here.

Chris Tonra is an assistant professor in the School of Environment and Natural Resources at The Ohio State University