Florida Field Naturalist PUBLISHED BY THE FLORIDA ORNITHOLOGICAL SOCIETY VOL. 44, NO. 1 MARCH 2016 PAGES 1-53 Florida Field Naturalist 44(1):1–13, 2016. FALL MIGRATION OF YELLOW WARBLERS (Setophaga petechia) IN WEST-CENTRAL FLORIDA KEN F. TRACEY 5662 Fieldspring Ave., New Port Richey, Florida 34655 E-mail: [email protected] Abstract.—A total of 5,993 Yellow Warblers (Setophaga petechia) were counted during fall migration at two study sites on and near the Gulf coast of Florida from 2006 through 2014. The total number of individuals counted per month for the nine years was 85 in July, 1,710 in August, 3,369 in September, and 829 in October. The migration pattern revealed three peaks in numbers: one in the middle of August, one at the end of the first week in September, and the highest peak in the third week of September. Photography was also used to classify 1,713 warblers by age and dorsal color. Of this sample, 807 warblers were “first- fall” birds and 906 were “after-hatch-year” adults. The high percentage of adult birds (53%) indicates that the west-central area of Florida is near the center of the main Yellow Warbler fall migration route, because first-fall birds are expected to predominate on the periphery. First-fall birds also appeared earliest in the migration and continued to be in the majority until the end of the second week in August. Birds with green and darker dorsal-plumage made up the majority of the migrants in the photo sample from September through October; this coloration, which is characteristic of more western and northern subspecies such as S. p. amnicola, suggests that the later arriving birds had relatively distant origins. INTRODUCTION Yellow Warblers (Setophaga petechia) are uncommon during spring migration in west-central Florida (Dunn and Garrett 1997), but are abundant during fall migration (Stevenson and Anderson 1994). A 10-year spring migration count at Green Key Peninsula, on the west-central coast of Florida, resulted in only six Yellow Warblers in a total of 12,885 warblers identified (Tracey and Greenlaw 2011), indicating that spring migration chiefly crosses the Gulf of Mexico west of Florida. Fall migration appears to begin along a broad front that corresponds to the width of the species’ breeding range in North 1 2 FLORIDA FIELD NatURALIST America (Lincoln et al. 1998), and continues across the lower United States with extralimital reports east to Bermuda (Lowther et al. 1999). Peak fall migration in Florida is believed to occur from mid-August to early September (Dunn and Garrett 1997, Floyd 2008). Some August migrants appear to be diurnal and use a circum-Gulf route (Duncan and Weber 1985).The Yellow Warbler has nine recognized subspecies (Browning 1994). The nesting areas for seven of these subspecies are in North America; the eighth is in Mexico, and the ninth is bisected by the border (Fig. 6). The fall migration routes of some subspecies have been confirmed by collected specimens. Confirmed fall subspecies passing through Florida include S. p. aestiva, which nests all over eastern/central U.S. and southern Canada; S. p. amnicola, which nests in northern Canada; and S. p. rubiginosa, which nests in the coastal areas of Alaska and British Columbia (Stevenson and Anderson 1994). Although western/Pacific subspecies generally migrate to Mexico and South America (Dunn and Garrett 1997), the fact that S. p. rubiginosa has been collected in eastern and southern states, ranging from Massachusetts to Louisiana (Chamberlin 1934, Griscom 1941, Mcatee et al. 1944, Parkes 1968), indicates that these subspecies can pass through Florida. The chief goal of this study was to document the timing and nature of the Yellow Warblers’ fall migration in west-central Florida. This goal was achieved through counting methods and through digital photography, which allowed me to assess characteristics of each bird. Characteristics included age, gender, and plumage color on the head and upperparts of adult males. These images were used to match colors and patterns of subspecies described in the literature (Browning 1994) known to pass through Florida (chieflyS. p. aestiva and S. p. amnicola), and to clarify the importance of the Florida gulf coast as a route of southward travel. METHODS Study areas This study covered nine fall seasons from 2006 through 2014 in Pasco County, Flor- ida. Data was collected from 21 July to 29 October of each year. The majority of Yellow Warblers counted in this study were at Robert K. Rees Park (28.25449 N, -82.75383 W), which is on the Gulf of Mexico at the Green Key peninsula, and at Lake Lisa Park (28.28756 N, -82.69742 W), which is two miles inland from the Gulf in Port Richey, FL. A 2014 point count survey was done along Green Key Road 145 m east of the entrance gate at Robert K. Rees Park (28.254938 N, -82.749925 W). Because most migrants pass unseen overhead in both nocturnal and diurnal variations, I focused on two appealing habitats for stopover warblers: mangroves along the coast at Robert K. Rees Park, and Brazilian pepper (Schinus terebinthifolius) stands on the edge of an inland pond at Lake Lisa Park. These areas not only have suitable vegetation that provides food and shelter – they also resemble wooded and scrubby wetland areas that Yellow Warblers use for breeding (Lowther et al. 1999). TRACEY—YELLOW WARBLER MIGRATION 3 Counting methods Of the 909 possible count days during these fall seasons, I completed morning counts on 469. Bad weather and other commitments were limiting factors on some mornings, but the successful sample of count days represents more than half of the available days. An additional 28 evening counts were performed in 2014. All counts lasted approximate- ly one hour. Two counting methods were employed: standard count and point count. The standard count method was followed during each morning count. At Robert K. Rees Park, I walked the 0.58 km Green Key Road from the park gate to the beach searching for birds in the mangroves. Due to the very dense vegetation, I initially recognized the presence of warblers by the sound of their bills snapping on insects in the mangroves. I then played a Yellow Warbler recording to bring them into view. All identifiable warblers were counted. At Lake Lisa Park, which is an old lime-rock quarry, I walked the 0.65 km pedes- trian path along the east side of the quarry’s edge. Migrating warblers frequented a narrow strip of Brazilian Peppers between this path and the quarry’s edge. A Yellow Warbler recording was, again, used to lure the warblers out of the Brazilian peppers to photograph and count. Migrating Prairie Warblers (Setophaga discolor) were also counted at Robert K. Rees Park as a control measure. This data gave me the opportunity to conduct a com- parative analysis of fall migration patterns between the two species, in effort to look for potential environmental effects that are unique to this study area. The geography of the Green Key peninsula provides an excellent landmark for wind-displaced birds to find and reorient themselves (Tracey and Greenlaw 2011). If peaks in data are a result of wind-displaced birds finding their way back to land, those peaks would oc- cur simultaneously for both species. The data obtained by the standard count method provided an index of the total number of warblers involved in early morning migra- tion movements. Instead of comparing daily peaks during each of the nine seasons, I opted for three-day average counts. This approach smoothed the temporal pattern in the data by offering 34 data points in lieu of 101 one-day evaluations. In 2014, I initiated a stationary point-count (Hostetler and Main 2008) survey of Yel- low Warblers along Green Key Road, which is 145 meters east of the entrance gate at Robert K. Rees Park (28.254938 N, -82.749925 W). This count was utilized only during the last year to assist in validating the three possible migration peaks that appeared in the standard count data pattern. I chose this location after noticing a large evening flow of Yellow Warblers there in 2013. Although Yellow Warblers feed in the mangroves while moving south all day, I found that their numbers increased in the evening during their diurnal migration (Duncan and Weber 1985). I decided to count at that time to obtain a large sample size. The point-count was conducted from 18:30 to 19:30 for 28 count days: one day in July, 10 days in August, 13 days in September, and four days in October. At this location, most warblers were noted flying in from the north, with a few coming in from the west off the Gulf. All identifiable warblers were counted using wide-field binoculars (8X 32) while standing at the point-count center. The mangroves at this location were approximately 100 meters wide and followed the coast in a north to south direction. Photographic documentation Along with recording the total number of Yellow Warblers seen each day, I attempted to photograph each bird for later analysis. Since Yellow Warblers are usually in constant motion as they flit from branch to branch, three cameras that focus rapidly were used in this study: the Kodak Z990 and Fuji S8200, which allow a 30 or 40 optical zoom, respectively, and the Canon Rebel XT, which has a 300 mm lens. Photographs captured 4 FLORIDA FIELD NatURALIST plumage characteristics not seen in most binocular views. Each photograph was dated and assigned a unique code for categorization purposes. Evaluations of photographs focused on identifications of age, gender, and dorsal body plumage (hereafter called the “dorsum”).