An Atlas of Cerulean Warbler Populations - Cornell Lab of Ornithology

DISCUSSION AND CONCLUSIONSThe Cerulean Warbler Atlas Project produced a listof several hundred sites that are important to this speciesin every state throughout its range. Although theserepresent a critical first step in conserving key populationsof Cerulean Warblers in each region, coverage andcompleteness of the atlas surveys was highly variablein different parts of the species’ range. In general, coveragenear the edge of the Cerulean’s range was probablymost complete and accurate. For example, it islikely that most of the existing populations and siteswere identified in New England, New Jersey, most ofNew York, Illinois, Alabama, Georgia, and Minnesota.In contrast, sites identified in West Virginia, Pennsylvania,Ohio, Kentucky, and much of Tennessee mostlikely represent only a small fraction of the populationsthat actually exist in these states. It is extremely difficultfor us to assess the completeness of CEWAP in moststates without much further fieldwork and consultationwith local experts.To compare with numbers of birds found by CEWAPparticipants, very crude estimates of Cerulean Warblertotal populations can be calculated from Breeding BirdSurvey relative abundances in each physiographic area.The BBS may provide landscape-level density estimatesthat can be converted into regional population estimatesif the following assumptions are made:1. BBS routes constitute a random sample of the landscape;2. habitats in question are fairly evenly distributed acrossthe region; and3. each bird species has a relatively fixed average detectiondistance at BBS stops, within which a reasonableestimate of the number of individuals presentmay be obtained.An entire BBS route composed of 50 stops, each consistingof a 0.25 mi. (400 m)-radius circular count, potentiallysurveys roughly 25 km 2 of heterogeneous landscape.Based on a study by Emlen and DeJong (1981),we may estimate the average maximum detection distancefor typical forest birds to be roughly 125 m—forthese species a BBS route samples an effective area of2.5 km 2 . If Cerulean Warblers are detected routinely outto 200 m at each stop, the effective area surveyed isincreased to 6.3 km 2 .Population estimates for a physiographic area are thencalculated as the average landscape-level density (numberof birds per route * effective area sampled by eachroute) multiplied by the size (km 2 ) of the physiographicarea. Note that landscape-level densities are not assumedto be similar to species densities in uniform optimumhabitats, but rather reflect habitat heterogeneity at largerscales as sampled by BBS routes. Because the greatmajority of detections on typical BBS routes are of singingor displaying males, the population estimate derivedfrom this method is assumed to represent numbers ofbreeding pairs.Applying this methodology produces a range of estimatesfor Cerulean Warblers throughout their range thatis usually much larger than that detected by CEWAP(Table 21). In fact a global population estimate of 85,000to 214,000 breeding pairs would indicate that CEWAPfound fewer than 10% of existing birds. As expected,the largest proportion of the total population occurs inthe Ohio Hills and Northern Cumberland Plateau, wherean average of 2 to 3 Cerulean Warblers are detectedannually on every BBS route in the last decade. In WestVirginia alone, the total population is almost certainlyin the 10,000s, and may be close to 100,000 pairs. Inphysiographic areas near the periphery of the Cerulean’srange; however, the number of birds found is not greatlydifferent from that estimated using BBS – for example,Southern New England, Lower Great Lakes Plain.Our atlas is therefore most valuable in areas awayfrom the center of the species’ distribution. Populationsand sites identified in most states may serve as thenucleus for a conservation strategy that should includecontinued monitoring, management, and possibleacquisition of currently unprotected sites. In the centerof the range, specific sites may also be important forlong-term monitoring and to provide a sample of therange of conditions required by this species. Becausemany sites identified are on public lands, these may alsoserve as core areas for sustaining regional populations.Where Cerulean Warblers are more continuouslydistributed and do not lend themselves to a “circles onmaps” atlas technique, a modeling approach, taking intoaccount different densities in different habitats, may benecessary to identify the most important areas forsustaining the bulk of the population.50