An Atlas of Cerulean Warbler Populations - Cornell Lab of Ornithology

medium-sized waterways. Only a few pockets ofCeruleans persist in the Mississippi River Valley proper,but a number of tributaries support the bulk of the speciesin the Midwest Region. Other important riparianareas include the Delaware River Valley, Roanoke Riverin Virginia, middle Hudson River, and forested wetlandsof the Lake Ontario Plain in New York. A common featureof these riparian forests, nearly throughout the range,is the presence of mature stands of sycamores.Hamel (2000), as well as other authors, have struggledto find a common denominator among the varied descriptionsof Cerulean Warbler habitat structure and treespeciesuse. A tall, but broken, canopy seems to be themost frequently mentioned feature, along with large arearequirements. Indeed, a shared feature of the three verydifferent habitat types used by a majority of CeruleanWarblers may be the irregular canopy structure. On dryridges, tall oaks form a linear “internal edge,” wherewarbler territories may look out over the surroundingcanopy. This same linear canopy edge is a prominentfeature of mature riparian forests, especially where tallsycamores form an emergent layer above the other trees.On slopes with a diverse mixed mesophytic forest, thepresence of trees with a variety of canopy structures isprobably key to providing the same sort of canopy-edgeeffect desired by Cerulean Warblers. Melinda Welton’sobservation of Ceruleans inhabiting secondary forestpatches in Tennessee, where tulip trees form a brokenemergent canopy, suggests that this tree may be an importantstructural ingredient in otherwise closed-canopyoak forests.Landscape situation and context has a strong bearingon whether otherwise suitable breeding habitat will actuallycontain warblers (Hamel 1992). Cerulean Warblersare thought to prefer large, contiguous tracts ofdeciduous forests for breeding (Bond 1957, Hamel 1981,Robbins et al. 1992). Hamel (2000) notes the geographicvariation and inconsistency of published references toarea sensitivity, however. For example, this speciesseems to prefer large wooded tracts of at least 50-75acres, and typically avoids isolated woodlots less than20-25 acres in size in Ohio (Peterjohn and Rice 1991).In other areas, stands greater than 526 ha (1,300 acres)are considered optimal for Cerulean Warblers (Evansand Fischer 1997).CEWAP results, although providing only crude estimatesof habitat-patch sizes occupied by Cerulean Warblers,do suggest geographic variation in degree of areasensitivity. Whereas a large number of individuals occurredin extensive forest tracts in all regions, the proportionof birds in these large patches varied among theregions. In the Southeast, nearly all birds found were inforests ≥ 1,000 acres, suggesting strong area sensitivity,whereas in the Northeast, a substantial proportion ofpopulations were in much smaller forests. Further defyingthe conventional wisdom on Cerulean Warblers,a growing body of research in eastern Ontario suggeststhat birds there thrive in patches of secondary mapleforest as small as 25 acres (Jason Jones, pers. comm).Because quantitative studies of area requirements inCerulean Warbler come primarily from the Mid-Atlanticand southeastern states (Robbins et al. 1989, Hamel1992), rangewide assumptions of extreme area sensitivitymay be exaggerated.Monitoring and Research NeedsThis Atlas of Cerulean Warbler Populations may beconsidered a first step in identifying the key sites andhabitats required to protect this species into the future.For a successful conservation strategy that ensures themaintenance of healthy breeding Cerulean Warblerpopulations throughout the species’ range, werecommend the following monitoring and researchcomponents:• Repeat surveys of the 73 primary and secondary sitesidentified in Table 1 and 2, perhaps every five years,to monitor health of known, important populations.• Quantitative studies of reproductive success andpopulation turnover in upland vs. bottomland habitats,specific to each region.• Quantitative studies of regional area sensitivity, perhapsusing GIS analyses of habitat patches identifiedin CEWAP.• Habitat suitability modeling to determine new andpotential population sites, especially in areas whereCEWAP was less effective.• Quantitative studies of response to management options,such as canopy thinning, selective logging, orwilderness protection.• Determination of potential threats to important CEWApopulations, such as from mountaintop removal mining,residential development, or logging.• Determine pattern of land-ownerships at importantareas in each region: devise alternative strategies forconservation and management on public, vs industrial,vs private lands.52