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Cerulean Warbler Occurrence and Habitat Use in Oklahoma
Vincent S. Cavalieri, Timothy J. O’Connell, and David M. Leslie, Jr.

Southeastern Naturalist, Volume 10, Issue 1 (2011): 167–177

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2011 SOUTHEASTERN NATURALIST 10(1):167–177 Cerulean Warbler Occurrence and Habitat Use in Oklahoma Vincent S. Cavalieri1, Timothy J. O’Connell2,*, and David M. Leslie, Jr.1 Abstract - Dendroica cerulea (Cerulean Warbler) is a migrant songbird that has declined rangewide in recent decades. We surveyed 150 sites in 2006–2007 to determine if this species still occupied its former breeding range in Oklahoma. We located Cerulean Warblers at 5 sites and confirmed breeding on north slopes of two heavily forested ridges in the Ouachita Mountains. We did not encounter Cerulean Warblers in any bottomland hardwoods, despite the former widespread distribution and abundance of the species in such habitats. While habitat loss and degradation may limit occurrence of Cerulean Warbler in some areas, the pattern of decline for this species at the edge of its range in Oklahoma is also consistent with abandonment of peripheral range as the range-wide population declines. Introduction Dendroica cerulea Wilson (Cerulean Warbler) is a small, insectivorous wood warbler that breeds in deciduous forests of eastern North America. It is a Nearctic- Neotropical migrant that makes annual migrations between breeding areas concentrated in the east-central United States and wintering areas in northern South America (Hamel 2000a, 2000b). The species occupies a discontinuous breeding range from the eastern Great Plains to the Atlantic Coastal Plain, including areas from southern Arkansas to southern Quebec (Hamel 2000a). Population density varies greatly over the breeding range, with population centers in the central Appalachians of Ohio, West Virginia, and Kentucky, as well as in southern Wisconsin, southwestern Michigan, southern Missouri, and northwestern Arkansas (Hamel 2000b, Rosenberg et al. 2000). Cerulean Warblers winter primarily at mid-elevations in the Andes Mountains of Colombia, Venezuela, Ecuador, Peru, and Bolivia (Hamel 2000a). Based on North American Breeding Bird Survey data, the Cerulean Warbler has declined faster than almost any other species monitored (Hamel 2000a), suffering annual losses of ≥3% since 1966 (Link and Sauer 2002). The Cerulean Warbler was recently considered for listing under the Endangered Species Act (USFWS 2006). Investigative work for possible listing contributed greatly to knowledge of this species (Hamel et al. 2004, Jones et al. 2004, Rogers 2006), including a rangewide assessment of distribution and abundance (Rosenberg et al. 2000). Although listing was ultimately denied (USFWS 2006) because of the still relatively large population, the Cerulean Warbler is one of the highest conservation priority species for Partners in Flight (Jones et al. 2004, Rich et al. 2004). 1Department of Natural Resource Ecology and Management and Oklahoma Cooperative Fish and Wildlife Research Unit, Oklahoma State University, Stillwater, OK 74078. 2Department of Natural Resource Ecology and Management, Oklahoma State University, 240 Ag Hall, Stillwater, OK 74078. *Corresponding author - 168 Southeastern Naturalist Vol. 10, No. 1 Cerulean Warblers generally require large forested tracts for breeding (Robbins et al. 1992; Hamel 2000a, b). They typically occupy closed-canopy forest, but canopy gaps and patchy emergent canopies can be important habitat components (Hamel 2000a, Hamel and Rosenberg 2007, Hyde et al. 2000, Oliarnyk and Robinson 1996, Robbins et al. 1992, Wood et al. 2005). Habitat selection varies throughout the breeding range, with forested slopes at relatively high elevation (>1000 m) used in some areas (e.g., Hamel 2000a, Hyde et al. 2000) and bottomland hardwood forest used in others (e.g., Robbins et al. 1992). Wintering habitat in the Neotropics includes mature broad-leaf and second-growth forest and shade coffee plantations (Bakermans et al. 2009, DeGraff and Rappole 1995, Robbins et al. 1992, Terborgh 1989). Loss of mature riparian and bottomland deciduous forest in the United States, whether to urban development or to reservoirs, has been suggested as a major cause of Cerulean Warbler decline (Hamel 2000a). Significant areas of former breeding habitat also have been lost to surface-mining operations in the Appalachians. Habitat loss on the wintering grounds and mortality during migration also may contribute to the decline (Hamel 2000a). Despite the range-wide decline, the Cerulean Warbler has actually expanded its breeding range in recent years (Hamel 2000a, Oliarnyk and Robertson 1996), particularly in the Northeast. This apparent range expansion may be reclamation of former breeding range as forests continue to regenerate in the region (Steyaert and Knox 2008). This expansion of breeding range amid a long-term population decline illustrates that species conservation efforts directed toward the edges of ranges can be an important complement to efforts directed toward population centers. The Cerulean Warbler was formerly a widespread and locally abundant breeder at the southwestern edge of its breeding range in eastern Oklahoma (Sutton 1967, Tomer and Brodhead 1992). Carter (1967) estimated that there were 2.9 pairs/40 ha in appropriate habitat in the McCurtain County Wilderness Area. The population fell rapidly in Oklahoma following several reservoir construction projects that inundated former breeding areas in bottomland forests. Isolated encounters were reported in the 1990s (e.g., Couch 1996), all restricted to LeFlore County. Despite multiple years (1997–2001) of field effort for Oklahoma’s Breeding Bird Atlas (Reinking 2004), no Cerulean Warblers were reported. Because the range-wide Cerulean Warbler Atlas Project (Rosenberg et al. 2000) did not include Oklahoma, it was unknown if the lack of Ceruleans reported during Oklahoma’s Atlas indicated that the species had been extirpated in the state or not. Due to the incomplete knowledge about population size and distribution of the Cerulean Warbler in Oklahoma, we conducted targeted surveys to identify occupied localities in the state. We focused survey efforts on bottomland and upland hardwood forests in LeFlore, McCurtain, Delaware, and nearby counties in eastern Oklahoma that defined the most recent historical range in the state (Carter 1967, Couch 1996). Our objectives were to identify populations, confirm breeding, and characterize habitat use of Cerulean Warblers in Oklahoma. 2011 V.S. Cavalieri, T.J. O’Connell, and D.M. Leslie, Jr. 169 Methods Study area We focused field surveys on areas within the historical range of Cerulean Warbler that supported large tracts of upland and bottomland forests, primarily the Ouachita Mountains and the Ozark Highlands (Fig. 1). The Ouachita Mountains are characterized by a series of east–west ridges in western Arkansas and southeastern Oklahoma. The mountains occupy approximately 54,000 km2 in 26 counties in Arkansas and 10 counties in Oklahoma (Rafferty and Catau 1991). More than 3200 km2 in this region are managed for mature or old-growth forests (Chipley et al. 2003). Habitats in this area consist of upland mixed pine (Pinus echinata Mill [Shortleaf Pine] and Pinus taeda L. [Loblolly Pine])-hardwood and oak-hickory forests. Bottomland sites in the Ouachitas are dominated by Quercus (oak), Nyssa spp. (gum), Carya spp. (hickory), and Taxodium spp. (cypress), or Ulmus spp. (elm), Fraxinus spp. (ash), and Populus spp. (cottonwood) forests (Chipley et al. 2003). The Ozark Highlands occupy parts of southern Missouri, northern Arkansas, and northeastern Oklahoma and cover approximately 21,000 km2 (Brye et al. 2004). The region consists of low mountains that are dominated by oak-hickory forests (Brye et al. 2004). The Ozark Highlands and Ouachita Mountains contain some of the most extensive contiguous forested areas in the central United States (Chipley et al. 2003). Cerulean Warbler surveys We targeted survey areas across a gradient of forest types and ecosystem conditions. We specifically visited, or attempted to visit, historical locations Figure 1. Map of Oklahoma indicating general location of 150 sampling locations in the Ozark Highlands and Ouachita Mountain ecoregions, 2006–2007. 170 Southeastern Naturalist Vol. 10, No. 1 for the species, as identified in prior studies that had located Cerulean Warblers in Oklahoma, including Carter (1967) and Couch (1996). Prior to the start of field work, we used Terrain Navigator software (Maptech, Billings, MT) to construct maps of potential survey sites that included historical locations for Cerulean Warbler or likely supported appropriate habitat, i.e., both bottomland and upland hardwood forest (Hamel 2000a). We made final, non-random, survey-site selections in the field, often surveying multiple sites in a local area that presented an abundance of either mature bottomland or high-elevation, steeply sloped hardwood forest. To complement sampling in areas with potentially suitable habitat, we identified additional sites with varying slope, aspect, elevation, and extent of hardwood forest cover. We also conducted a limited number of surveys in habitats that are generally not used by Cerulean Warblers, e.g., pine plantations, agricultural areas, and urban-suburban areas. Thus, our field sampling included the full gradient of forest conditions in eastern Oklahoma that could potentially be used by Cerulean Warblers. We surveyed 75 independent sites in 2006 (13 May–27 June) and 75 different sites in 2007 (12 May–4 July), for a total of 150 sites (Fig. 1). Each survey site consisted of 4 plots spaced 250 m apart on a 1-km transect. We placed plots 250 m apart to avoid double counting (Hutto et al. 1986). We sampled birds at each of the plots using 6-min, 100-m, fixed-radius point counts. We split 6-min counts into 3 equal intervals of 2 min to facilitate the application of post-hoc removal models to aid in the calculation of species-specific detection probabilities (Farnsworth et al. 2002). Counts took place from local sunrise to approximately 1030 h CDT (Hutto et al. 1986, Ralph et al. 1995). At the conclusion of each point count, we broadcast Cerulean Warbler song from a portable compact disk player and external speaker for 1 min and listened for a response for an additional minute (Rosenberg et al. 2000). If Cerulean Warblers were detected in response to the song broadcast, we invested 1 additional field day to determine if breeding could be confirmed based on observation of an occupied nest, fledglings, or an adult carrying food (Reinking 2004). Habitat sampling We quantified habitat and site characteristics from three, 5-m circular plots, 15 m from each of the 4 point-count centers at 0, 120, and 240 degrees. We used a modified version of the vegetation-sampling protocol described by Martin et al. (1997). Within each plot, we estimated percent slope using a clinometer and recorded elevation from a hand-held GPS unit. For overstory trees, we visually estimated percent cover and measured canopy height with a clinometer and basal area for all trees using an angle gauge from the center of each plot. We counted the number of stems >10 cm dbh for each tree species within each plot. We visually estimated the percent cover of low (<2 m) and high (>2 m) woody shrubs and trees <10 cm dbh and the relative percent ground cover of grasses, forbs, and 2011 V.S. Cavalieri, T.J. O’Connell, and D.M. Leslie, Jr. 171 leaves in each plot. All variables estimated at the plots were averaged for the site. We completed all vegetation surveys within 24 hrs of the point counts conducted at a site. To characterize general cover types in a larger area around each site, we first recorded the location of each point count in the field with a GPS unit and obtained coordinates for the midpoint of each sampling transect. From the midpoint, we created a 1-km buffer in ArcMap 9.2. We used a land-cover layer developed for the Oklahoma Gap Analysis Project (Fisher and Gregory 2001) to identify patches of forest and used Hawth’s tools in ArcMap to calculate percent cover of mature forest, regenerating forest and shrubland, pasture and cropland, and urban land use within the 1-km buffer. We compared 95% confidence intervals of data on vegetation, land cover, and site condition between sites where we did and did not detect Cerulean Warblers. For this analysis, we excluded sites with less than 50% forest cover within the 1-km buffer so we could focus on teasing out differences between well-forested sites only. This analysis included 123 of the 150 total survey sites. Results We detected Cerulean Warblers at 5 of 150 (3%) of the sites surveyed. All 5 sites were located in the Ouachita National Forest, on the north slopes of Lynn Mountain and Rich Mountain in southeastern Leflore County. We encountered Cerulean Warblers exclusively in forested stands near ridge tops at 641–721 m above mean sea level. The five sites where we encountered Cerulean Warblers were generally higher in elevation and slope, and supported a taller and more densely vegetated forest canopy than sites where we did not encounter this species (Table 1). All five sites that supported Cerulean Warblers were ≥99% forested in the 1-km buffer. These sites exhibited mean (± SD) canopy height of 18 ± 2 m and plot-level canopy cover of 70 ± 6%. Table 1. Confidence intervals (95%) for environmental variables summarized according to Cerulean Warbler occurrence at 123 sites with >50% forest cover in a 1-km buffer. All variables are summarized at the plot-level except % forest cover (1-km buffer level). Cerulean Warbler not detected Cerulean Warbler detected Environmental variable Mean SE 95% C.I. Mean SE 95% C.I. Basal area (m2/ha) 78 3.22 73–84 79 4.26 72–87 % canopy cover 58 1.67 55–61 70 3.06 64–75 Canopy height (m) 14 0.39 13–15 18 0.97 16–20 Elevation (m) 359 14.99 333–386 691 14.87 665–717 % forest cover 82 0.01 80–84 100 0.00 99–100 % grass cover 13 1.13 11–15 21 8.61 6–36 % leaf cover 61 1.65 58–64 56 9.55 39–73 % slope 11 0.88 9–13 30 5.77 20–40 % understory >2 m 26 1.45 24–29 28 8.43 13–43 % understory <2 m 39 1.43 36–41 60 3.96 53–67 172 Southeastern Naturalist Vol. 10, No. 1 We detected 8 males and 4 females at the five sites. In 2006, we observed one male and one female carrying food in the same territory on Lynn Mountain on 13 June, confirming breeding for this location. On 24 May 2006, we observed a pair of Cerulean Warblers feeding fledglings in this area. On 22 June 2007, we observed a pair of Cerulean Warblers feeding fledglings at a site on Rich Mountain, confirming breeding for that location. Although we had too few contacts to derive a robust estimate of breeding density incorporating probability of detection from removal models, all 8 male Cerulean Warblers were detected within the initial minutes of the point-count intervals. This pattern of detection during point counts indicates a high probability of detection where the species occurred and, conversely, a low probability that it occurred at sites where it was not detected. Discussion Although the Oklahoma Breeding Bird Atlas included 14 survey blocks in LeFlore County (Reinking 2004), Cerulean Warblers were not identified in any of the blocks during Atlas survey efforts in 1997–2001. Kuhnert (2004) mentioned that “a few” territories were located on the north slope of Lynn Mountain in 1998; this is apparently the same occurrence we documented there in 2006, although it is unclear if the location was continuously occupied during the intervening years. The Rich Mountain occurrence we detected is not specifically described in historical accounts of Cerulean Warblers in Oklahoma. Taken together, the two occurrences supported 8 males and 4 females at five 1-km sites. While all 8 males encountered reacted aggressively to broadcast Cerulean Warbler song, we only observed a social mate for 4 of those males. We confirmed breeding on both Rich and Lynn mountains through observations of adults feeding fledglings. We conclude that a small breeding population of Cerulean Warblers occupied at least two ridge tops in the Ouachita Mountains of Oklahoma in 2006 and 2007 (Fig. 2). Although we were generally successful in obtaining access to high-elevation forests with the potential to support Cerulean Warblers, there are areas that warrant further study which we were unable to survey. These include Blue Bouncer and Black Fork mountains in Leflore County; both contain highelevation forest with extensive north-facing slopes. Continuous sampling over the entire strata on Lynn and Rich mountains also could reveal more individuals outside of our specific survey sites. Farther north, Cerulean Warblers breed at relatively high density in upland forests on the Ozark Plateau in adjacent Arkansas and Missouri (Hamel 2000a). Our surveys of 41 Ozark forests on the Oklahoma side of the Plateau did not reveal any breeding Ceruleans. However, due to the potential for additional, unsurveyed areas in both the Ozarks and Ouachitas to support localized occurrences of Cerulean Warbler, we consider the 12 individuals we encountered to represent a conservative, minimum population estimate for the state. 2011 V.S. Cavalieri, T.J. O’Connell, and D.M. Leslie, Jr. 173 High-elevation ridge tops where we found Cerulean Warblers in the Ouachitas were structurally similar to forests used by the species in the Appalachian Mountains (Hamel 2000a, b; Robbins et al. 1992; Roth and Islam 2008; Weakland and Wood 2005). Sites in our study area that supported breeding Cerulean Warblers had greater canopy cover, canopy height, and slope than found in the surrounding landscape matrix (Table 1). Dominant overstory trees at these sites included Juglans nigra L. (Black Walnut), Quercus alba L. (White Oak), Carya tomentosa (Lam. ex Poir.) Nutt. (Mockernut Hickory), and Acer rubrum L. (Red Maple). Of the 150 total sites surveyed, 23 supported the combination of environmental variables within the 95% confidence interval of sites where we encountered Cerulean Warblers (Fig. 2). In addition to upland forests, several sources (Carter 1967, Sutton 1967, Tomer and Brodhead 1992) indicate that Cerulean Warblers were once widespread in bottomland forests in Oklahoma. Sutton (1967) reported that the species was abundant along the Arkansas River in the 1800s. An important historical location along the Mountain Fork River (Carter 1967) was inundated by construction of the Broken Bow Reservoir. We intentionally surveyed current Figure 2. Locations (filled circles) where we encountered Cerulean Warbler territories on Rich and Lynn mountains in LeFlore County, OK, 2006–2007. Shading represents the total extent of forested, north-facing slopes >500 m in elevation. Open circles indicate additional sampling locations in forested areas on north slopes >500 m in elevation where we did not detect Cerulean Warblers. 174 Southeastern Naturalist Vol. 10, No. 1 and historical bottomland forests, including the Little River National Wildlife Refuge (NWR), the Arkansas River in the Sequoyah NWR, and Spavinaw Creek. We did not detect Cerulean Warblers at any of those locations. Based on our surveys, we have no evidence that Cerulean Warblers nested in any Oklahoma bottomlands in 2006–2007. Our study suggests that the species has experienced a general decline statewide, and is now restricted to a few highelevation forests in the Ouachita Mountains. Reasons for the statewide decline are complex, and this renders recommendations for management speculative. We know, for example, that habitat has been lost and degraded. Reservoir construction in recent decades has inundated historical occurrences in bottomland forests in the Midwest and Mississippi Alluvial Valley (Robbins et al. 1992, Roth and Islam 2008) as well as in Oklahoma (Carter 1967). Forest fragmentation in uplands has been implicated in increased rates of nest predation (Hoover et al. 1995, Robinson et al. 1995) and brood parasitism by Molothrus ater Boddaert (Brown-headed Cowbird) (Darr et al. 1998, Ortega 1998, Rogers 2006) on forest songbirds. Buehler et al. (2008) found that approximately 20–33% of the variability in Cerulean Warbler population growth rate range-wide was associated with fecundity, with nest predation as the most common cause of nest failure. In Oklahoma, broad-scale forest cover is fragmented in the Ozark Mountains (where we did not detect the species), but less so in the Ouachita Mountains (where we did) (National Atlas 2009). The Arkansas portion of the Ozark Mountains is also dominated by comparatively unfragmented forest (National Atlas 2009) and supports breeding Cerulean Warblers (Hamel 2000a). The loss of bottomland forest, fragmentation of upland forest, and occurrence of Cerulean Warblers at a few unusual habitats in the state suggests that habitat loss and degradation has contributed to the species’ decline in Oklahoma. Cerulean Warbler populations are not productive, however, anywhere within the species’ range. Buehler et al. (2008) found that none of five populations studied from across the species’ range during 1992–2006 were reproducing at a stable rate. All populations, especially those in the Midwest and Mississippi Alluvial Valley, were supported to some extent by immigrants from other areas. Under a broad demographic decline, perhaps largely influenced by survivorship of adults during migration and in winter (Buehler et al. 2008, Hamel 2000a), Cerulean Warblers decline throughout their range from all habitats in which they might otherwise occur. Those populations occupying the smallest or most isolated fragments could be the first to suffer local extinctions due to mechanisms such as skewed sex ratios that rapidly reduce overall fitness in small populations (Stephens et al. 1999). A range-wide population decline could be most pronounced at the edge of the species’ range, which would help explain the absence of Cerulean Warblers from many areas, especially remaining bottomlands, where apparently suitable habitat remains. We detected Cerulean Warblers at 5 of 23 (22%) of the sites on Lynn and Rich mountains and at none of the bottomland sites surveyed. 2011 V.S. Cavalieri, T.J. O’Connell, and D.M. Leslie, Jr. 175 Given uncertainty on the mechanism of Cerulean Warbler decline in Oklahoma, management could have little effect on the population trajectory. A continued range-wide decline will likely result in further contraction of the species’ range away from the edges, and possible extirpation from Oklahoma. Cerulean Warblers have, however, demonstrated the ability to recolonize former portions of the range (Oliarnyk and Robertson 1996), and both land-use and climatic changes in the future could again make the Oklahoma portion of the range productive. Thus, a long-term plan for conservation should include the provisioning of large blocks of closed-canopy, broadleaf forest, especially in conjunction with strategic canopy gaps and emergent canopies (Hamel and Rosenberg 2007) in both upland and bottomland sites. Because the only sites we found to support breeding Cerulean Warblers were high-elevation forests on northern Ouachita slopes with tall canopies, we consider such forests to be both the highest priorities on which to focus management in the near term as well as the most likely sites to experience local population growth or host recolonization. Because all five locations occurred within large tracts of mature forest in the Ouachita National Forest, coordination with the US Forest Service is likely to be important in provisioning desirable breeding habitat for Cerulean Warblers at the edge of their range in Oklahoma. Acknowledgments Financial support for this project was provided from State Wildlife Grants under Project T-31-P-1 of the Oklahoma Department of Wildlife Conservation and Oklahoma State University and administered through the Oklahoma Cooperative Fish and Wildlife Research Unit (Oklahoma Department of Wildlife Conservation, Oklahoma State University, United States Geological Survey, United Fish and Wildlife Service, and Wildlife Management Institute cooperating). Additional support was provided by the Payne County Audubon Society. We thank field assistants Jason Heinen, Jim Taulman, and Allysa Lapine. This manuscript was significantly improved thanks to the comments provided by S. Riffell and two anonymous reviewers. Literature Cited Bakermans, M.H., A.C. Vitz, A.D. Rodewald, and C.G. Rengifo. 2009. Migratory songbird use of shade coffee in the Venezuelan Andes with implications for conservation of Cerulean Warbler. Biological Conservation 142:2476–2483. Brye, K.R., C.P. West, and E.E. 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