Stopover ecology of autumn-migrating raptors in the Central Appalachians

Open Access
- Author:
- Goodrich, Laurie J
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 04, 2010
- Committee Members:
- Margaret Brittingham, Dissertation Advisor/Co-Advisor
Margaret Brittingham, Committee Chair/Co-Chair
Duane R Diefenbach, Committee Member
George Spencer Young, Committee Member
Richard Tucker Yahner, Committee Member
Christopher Barry Goguen, Committee Member - Keywords:
- stopover
raptors
behavior
habitat use
migration
Kittatinny Ridge
Appalachians - Abstract:
- Migration can be the most hazardous period of a bird’s life cycle. Conditions at stopover sites may influence individual condition, survival, and affect future reproductive potential and population numbers. Availability of suitable habitat for resting and feeding en route can be an important component determining the success of migration for many birds. In eastern North America, raptors follow two main corridors during autumn migration, the Atlantic coast and the Appalachian Mountains. In the Central Appalachians, the Kittatinny Ridge is a key autumn corridor for eastern raptors. However, little is known about how migrants use the ridge and associated habitats en route. To better understand migration behavior and habitat use by raptors along an inland flyway, I studied migrating raptors during autumn along the Kittatinny Ridge. My objectives included to assess the frequency of travel and stopover in individual raptors and document behavior during stopover to understand the role and importance of stopover in migration in raptors. I also studied which habitats were used by raptors during stopover and the frequency of use of the Kittatinny Ridge to determine the importance of habitat to raptors and whether a flyway corridor could be identified In one part of the study, I radio-tagged and followed 34 Sharp-shinned (Accipiter striatus) and 14 Cooper’s Hawks (A. cooperii) during autumn migration 2003 and 2004. Birds were trapped on the Kittatinny Ridge, and followed for one to 12 days each through Pennsylvania and neighboring states. Both species spent 1-5 days on stopover between travel periods (mean= 2 d.). Sharp-shinned Hawks spent an average of 33.6% of daylight foraging (+ 24.5 SD) and 32.6% of day roosting (+27.4), and Cooper’s Hawks spent 33.8% (+ 6.8) of day foraging and 47.2% (+26.2) roosting. Sharp-shinned Hawks spent less time roosting than Cooper’s Hawks; however, neither species differed in behavior by age, except that adults spent more time in non-migrating flight. On travel days, migrant Cooper’s and Sharp-shinned hawks spent an average of 45% and 35% of daylight in migratory flight respectively, reducing time spent roosting and foraging. Travel days were not strongly associated with cold front passage, but were associated with days of low cloud cover and for Sharp-shinned Hawks, northerly winds, and higher temperature. Travel time was longer on days with northerly (tail) winds for both species. The ratio of hours in travel to hours on stopover per bird during tracking was 1:7 (including night hours) suggesting that rest and foraging periods are an integral aspect of their migration journey. The two species traveled in different compass directions with Sharp-shinned Hawks flying to the southwest (mean= 216.5º +5.8 SE) and Cooper’s Hawks traveling more to the south (mean= 190.2º +8.3 SE). Direction did not vary by age, region, or date. Cooper’s Hawks spent more time per day traveling on migratory travel days than Sharp-shinned Hawks. Adult Cooper’s Hawks flew farther and faster on average than Sharp-shinned Hawks and hatch-year Cooper’s Hawks. Both species spent more time in active travel flight on the Kittatinny Ridge than in the adjacent valley or other regions, although the valley and northern ridges were equally available. When in the Piedmont and Coastal Plain region both displayed longer, faster flights than in Ridge and Valley region. Both species exhibited faster travel on days with tail winds, and Cooper’s Hawks also flew faster on days with strong thermals and light winds. Headwinds appeared to shorten the migration distance for hatch-year Sharp-shinned Hawks whereas adults compensated by flying longer to cover a similar distance. Cooper’s and Sharp-shinned hawks may use different strategies to migrate through the Central Appalachians. Cooper’s Hawks exhibited short travel periods with longer stopovers along the Kittatinny Ridge followed by long-distance ‘power’ flights with short stopovers when traveling south across the more open and developed Plain and Piedmont region. Sharp-shinned Hawks appeared less likely to undertake long flights and more consistently flew close to the Appalachians where more abundant forest habitat could be found. Both species seemed to use thermals and updrafts to aid their migration and save energy. Forest patch size was one of the most important factors in stopover site selection with both species choosing larger forests more than occurred at random at both the landscape scale and near scale. At a regional scale, both species selected more rural areas for stopover sites, selecting areas with more mixed forest and pasture. Sharp-shinned Hawks avoided suburban areas as well. Hatch-year Sharp-shinned Hawks used contiguous forest, wetlands, and evergreen forest less than adults. Nearly half of all roosts of both species were on the Kittatinny Ridge suggesting either an affinity to large forests or the flyway itself. Both species combined roosted at an average of 6.9 km from the ridge flyway when found within the Ridge and Valley region. Hatch-year birds roosted farther from the ridge than adults in both species and Sharp-shinned Hawks roosted farther from the ridge than Cooper’s Hawks. Cooper’s Hawks were more selective in their habitat choice during migration than during nesting, choosing large forests in more rural landscapes. The selection of roosts by Sharp-shinned Hawks near wetlands may indicate that the opportunity to drink is important during migration or that wetlands concentrate their songbird prey. Evergreen stands could provide important protective cover to roosting Sharp-shinned Hawks, as they may be more vulnerable to avian predation and more likely to seek out evergreen forest during stopover than the Cooper’s Hawk. During road surveys of all raptors on stopover, birds were most abundant within 1 to 6 km of the ridge and less numerous farther from the ridge or on the ridge itself. The lowest abundance was observed at the farthest distance from the ridge, 16 km. The pattern of higher abundance near the base of the ridge remained consistent regardless of changes in forest cover . The abundance near the Ridge indicates that distance to the flyway could be an important factor influencing migrant distribution. Habitat influenced distribution of migrants on stopover as well. Forest raptors were more numerous on the north side where forest cover was more abundant, and open habitat raptors, e.g. Red-tailed Hawk and Northern Harrier, were more numerous on the south slope where farmland predominates. Although age influences patterns of migration, no age difference was observed in habitat or distance of migrant raptors surveyed during road surveys. The road survey results together with the radio-tracking data on two accipiters suggests that migrant raptors may prefer to roost near the migration flyway during stopover, but will move away from the flyway to find appropriate habitat when necessary. Although songbirds demonstrate greater flexibility in habitat use on migration as compared the breeding season (Petit 2000, Rodewald and Brittingham 2004, Keller et al. 2009), raptors appear more conservative in their habitat selection during migration. Migrating raptors expend considerable energy to travel between breeding and wintering areas. As part of their migration strategy, many utilize updrafts along ridges, to save energy en route. In this study, accipiters relied extensively on thermal lift to aid their migration south but also used the Ridge for energy-savings and stopover. Migration travel occurred in a wide variety of weather conditions, excluding days of heavy cloud cover or rain. Migrant raptors appear to prioritize replenishing energy and finding safe areas to rest during their journey. Migrating accipiters regularly integrated stopover into their migration journey, perhaps following an energy-minimization strategy of migration (Newton 2008). Raptors settled preferentially near the migration route during stopover, a behavior that is also supportive of an energy minimization strategy of migration. Long-term conservation of North American raptor populations may rely on the protection of a rural mix of habitats along key migration corridors including large patches of contiguous forests, rural fields, and wetlands. Although more research is needed, raptors appear to show more conservative habitat selection patterns during migration than during breeding or wintering periods. The avoidance of suburban areas by migrant accipiters also suggests that conservation planning should limit suburban and urban development within important migration corridors.