Warblers & Restoration

Decision Rules (published in Ecology in 2003)

Overview: Using some novel experiments, we were able to demonstrate that individual male and female Prothonotary Warblers decide whether or not to return to sites within the Cache River watershed based on their reproductive performance. Individuals producing 2 batches of offspring (broods) in a breeding season returned to the same habitat patch the following year at a rate of 80%. Individuals producing 1 batch of offspring returned at a rate of 50% and those producing no offspring at 25%. The warblers use their own nesting success as a cue to return to good sites but to avoid returning to bad ones. These “decision rules” lead to the build-up (increased densities) of warblers on good sites because many of the breeding adults return year after year, and also because the presence of these returning adults is attractive to other warblers looking for a good place to breed (including older birds who were on a bad site the previous year and 1-year-olds who are looking for a place to breed for the first time).

Neighborhood and Patch Effects (dissertation research - Schelsky)

Overview:We have discovered that the “decision rules” of these warblers are quite refined. Some unsuccessful males and females (those attempting but failing to produce any offspring for an entire breeding season) do return to patches of habitat between years. Each patch of habitat may be viewed as subdivision with many neighbors. Unsuccessful males seem to pay attention to what their neighbors are doing and are more likely to return if some of their neighbors are successful. Returns of unsuccessful males increase with an increase in the number of successful neighbors that they have. These males return relatively close to where they were the previous year. Unsuccessful females seem to pay more attention to what is happening in the subdivision as a whole and are more likely to return to habitat patches (subdivisions) that, on average, produce more offspring. These females (compared to males) return farther from their breeding site of the previous year. Paternity analyses determined that “unsuccessful”male warblers that returned were more likely to have fathered some offspring with neighboring females than those “unsuccessful” males that did not return.

Water Depth and Nest Predation (published in 2006 in Biological Conservation)

Overview: This study documented that nest predation by raccoons was the primary factor limiting reproductive success of Prothonotary Warblers breeding in the Cache watershed and that rates of nest predation decrease with an increase of the depth of water beneath nests. This is not surprising given that raccoons thrive in fragmented landscapes, particularly in bottomland forests fragmented by agriculture. Prothonotary Warblers prefer to nest over water (in cavities; or in nest boxes) and therefore require forested wetlands. Nests that were over water deeper than 60 cm (2 feet) were particularly successful because raccoons appear to not like foraging in water that they cannot walk in without having to swim. Forested wetlands and swamps that have deep water in them for a long (1-3 month) duration during the warblers’ breeding season (May-July) are critical to the nesting success and maintenance of healthy populations of Prothonotary Warblers.

Natal Philopatry (published in 2013 in PloS ONE)

General Overview: Since 1994, we have banded >10,000 Prothonotary Warbler nestlings in the Cache River watershed, and searched for them in subsequent years across southern Illinois and northern Kentucky. We have found that the vast majority (80%) of offspring that return to breed, do so to within 2 km (roughly a mile) of where they were produced. We looked at 2,500 breeding adults in the 20-40 km (12-25 mile) range (away from our core study area) and did not find a single banded bird. The implication of this result is that offspring recruit into the population near where they were produced. The great management implication of this result is that local conservation efforts to improve nesting success will benefit local population dynamics. Simply put, it seems that birds produced in the Cache return to the Cache or very close to it, and local habitat management efforts that improve nesting success (land acquisition, restoration, consolidation of forests, managing water levels, etc.) will now provide an even greater benefit to the local bird community.

Cowbird Mafia and Farming (Published in 2007 in PNAS)

Overview: Why do hosts accept parasitism when reproductive costs are high and parasitic eggs and nestlings differ dramatically in appearance from their own? Using a unique set of experiments studying the interactions of Prothonotary Warblers (the host) with Brown-headed Cowbirds (the brood parasite) we were able to document that female cowbirds use “mafia-like” tactics (destroying a parasitized warbler nest if the cowbird egg is removed) and also “farming” tactics (destroying a non-parasitized nest that is too far along for the cowbird to successfully put an egg into it). This demonstrated that the cowbird’s behavior is very sophisticated and that they pay attention to what happens to the eggs that they put in other bird’s nests.

Local Edge Effects in the Shawnee National Forest (Published in 2006 in the Journal of Field Ornithology)

Overview: We investigated local edge effects for birds breeding in a nearly contiguous forest divisively fragmented by relatively narrow agricultural corridors in Illinois. We measured rates of nest predation and brood parasitism for Acadian Flycatchers over a continuum of distances from the edge of an agricultural inholding. Nest predation and brood parasitism both were highest near the edge and decreased with increasing distance from the edge. Given the cumulative effects of nest predation and brood parasitism on reproductive success, we determined that the forest within 600 m of the inholding was currently acting as sink habitat. Our results suggest that the deeper forest interior areas currently serve as source habitat, and that the conversion of the entire 205 ha agricultural corridor to forest would add an additional 1350 ha of source habitat for Acadian Flycatchers. The results of this study provide site- and species-specific data that support a local conservation strategy of forest consolidation and establish baseline measures necessary to determine the relative effectiveness of any subsequent reforestation efforts.

Reforestation in the Cache River Watershed Improves Nesting Success of Forest Songbirds (Data analyses nearly complete)

Overview: Reforestation and “unfragmenting” forests in the Cache has significantly reduced the problem of cowbird parasitism and has also reduced nest predation. We found and monitored thousands of bird nests in several forests within the Cache River watershed during 1993-1995 (beginning of land acquisition) and again during 2010-2012 (many areas newly forested). Cowbird parasitism has dropped markedly in the study sites since the initial period 1993-1995. For Acadian Flycatchers, the species with the most nesting data, overall rates of cowbird parasitism are now half what they were (38% during 1993-1995 vs. 19% during 2010-2012). Averaging across the other 6 species of hosts we have data for, overall rates of cowbird parasitism are less than half what they once were (53% during 1993-1995 vs. 22% during 2010-2012) The likely reasons for this decrease in cowbird parasitism in association with bottomland forest restoration (acquiring agricultural land and converting in back to bottomland forest) include the potential for 1) the restoration to increase the commuting distance of female cowbirds that now have to fly farther away from their forest breeding areas to find suitable feeding areas and 2) the absorbing of some cowbird eggs by the many cowbird hosts now occupying the early successional stages of restored bottomlands adjacent to the original mature bottomland forest. Rates of nest predation (different types of predators eating eggs or chicks in nests) dropped about 12% between the two time periods across the 7 species (62% during 1993-1995 vs. 50% during 2010-2012). This trend for lower rates of nest predation overall is promising and suggests that the decrease in the amount of agricultural matrix in the watershed may be playing a role.