Swainson's Hawks Buteo Swainsoni Research Articles

Response of migrating raptors to an increasing number of wind farms

Summary Flying birds have been documented to respond in different ways to the presence of wind farms. Such responses are species‐ and site‐specific, with wind farm design playing an important role. Between 2009 and 2014, the length of rows of wind turbines within our study area increased from 3·4 km to ∼7·5 km, and the total area occupied by wind farms increased from 6·47 km2 to 14·21 km2. This area is located on an important migratory corridor in southern Mexico. We used marine radar and hawk‐watch monitoring stations to collect data during six consecutive autumn seasons from a single wind farm. We analysed the response of migrating raptors to the presence of two new wind farms by comparing the mean bearing of flight trajectories and the number of intersections km−1 of trajectories with wind farm areas between 2009–2011 and 2012–2014, representing the pre‐ and post‐construction stages of the new wind farms. Mean raptor count was >600 000 individuals for the six seasons. The most abundant species were Turkey Vulture Cathartes aura, Swainson's Hawk Buteo swainsoni and Broad‐winged hawk Buteo platypterus. Between 79% and 97% of migration occurred in October. Radar monitoring overlapped with the peak migratory activity each season. We observed significant differences between periods, involving more scattering in flight bearings and less intersections km−1 of trajectory in the post‐ than in the pre‐construction period, implying an avoidance of the new wind farms. Synthesis and applications. We show that migrating raptors adjusted their flight trajectories to avoid new wind farms, but also discuss the extent and limitations of our findings. Our results from our hawk‐watch monitoring station, which represent the first published account about the seasonality and intensity of raptor migration in the area, could be used by decision‐makers for careful planning of future wind energy developments in the area. Our results might aid in the conservation of those species of raptors that migrate through the Isthmus of Tehuantepec.

Risk of feather damage explains fault bar occurrence in a migrant hawk, the Swainson's hawk Buteo swainsoni

Risk of feather damage explains fault bar occurrence in a migrant hawk, the Swainson's hawk Buteo swainsoni

Risk of feather damage explains fault bar occurrence in a migrant hawk, the Swainson's hawk Buteo swainsoni

Fault bars are common stress‐induced feather abnormalities that could produce feather damage thus reducing flight performance. For that reason, it has been hypothesized that birds may have evolved adaptive strategies that reduce the costs of fault bars (the ‘fault bar allocation hypothesis’). An untested prediction of this hypothesis is that fault bars in important feathers for flight (wing and tail) should be less abundant where they produce more feather damage. We tested such a prediction using moulted wing and tail feathers of the long‐distance migrant Swainson's hawk Buteo swainsoni in its Argentinean wintering quarters. We recorded the occurrence of fault bars of different strengths (light, medium and strong) and the damage (lost of a portion of the vane) produced by them. The occurrence of fault bars was very variable, with strong ones being rare throughout and light and medium fault bars being more frequent in the tail than in the wing. Risk of feather damage was similarly high and low across feather groups for strong and light fault bars, respectively, and higher in the wing than in the tail for medium strength. The occurrence of fault bars of different strengths on different feather groups was negatively correlated with their propensity to produce feather damage. At low damage risk (<5%), the occurrence of fault bars was highly variable depending on the feather group, but above 5% of feather damage the occurrence of fault bars was highly reduced throughout. Our results supports the ‘fault bar allocation hypothesis’ of natural selection reducing fault bar occurrence where fault bars are more risky, but further suggest that selection pressure could be relaxed in other instances, leaving the way free for other mechanisms to shape fault bar occurrence.

Routes and Travel Rates of Migrating Peregrine Falcons Falco peregrinus and Swainson's Hawks Buteo swainsoni in the Western Hemisphere

We describe and compare the migration routes, length of migration, and duration of migration of Peregrine Falcons Falco peregrinus tundrius and Swainson's Hawks Buteo swainsoni in the Western Hemisphere. We radio tracked migrants using the Argos satellite system. Our initial samples were 34 Swainson's Hawks from representative areas of their breeding range, and 61 Peregrine Falcons captured at nest sites across the North American boreal forest and low Arctic or on the migration routes along the mid-Atlantic and Gulf of Mexico coasts. The average distance of migration for Peregrines was 8,624 km southward, and 8,247 km northward. Peregrines travelled at an average rate of 172 km/d southward and 198 km/d going north. Peregrine Falcons used at least three broad, general routes south from the breeding areas, and individuals stopped migrating as far north as the U.S.A. mid-Atlantic coast and as far south as central Argentina. The radiomarked Peregrine Falcons used coastal routes, mid-continental routes, and water-crossing routes: the Davis Strait and Caribbean Sea. During northward migration, Peregrines migrating through at Padre Island, Texas diverged for destinations from central Alaska across the continent to central West Greenland. Swainson's Hawks migrated an average of about 13,504 km southward and 11,952 km northward, and travelled 188 km/d southward and 150 km/d northward. Swainson's Hawks converged in eastern Mexico on the Gulf of Mexico coast. Southward, these hawks followed a narrow, well-defined path through Central America, across the Andes Mountains in Columbia, and east of the Andes to central Argentina where they all spent the austral summer. Swainson's Hawks northward migration largely retraced their southward route.