Each migratory bird species, including the raptors, has a specific period in which the bulk of specimens will migrate. Some species, like the Red-tailed Hawk, have protracted migration patterns, while others, such as the Broad-winged Hawk, migrate over Derby Hill in a much shorter time period. Among seasons, there is usually little variation in these patterns. Within a season, however, the migration of raptors is strongly dependent on weather conditions. One day can bring phenomenal numbers and the next day can be a disappointment.
At Derby Hill, regular visitors have come to anticipate big flights based on their understanding of the weather. Strong southerly winds tend to concentrate migrating raptors along the lake shore, and usually keeps them low as they avoid getting blown out over the lake. Light winds make them go higher, as do ‘tail winds’, i.e. west-southwest winds for birds that are going east-northeast over Derby Hill.
Another well-known concept in local hawk watching is the so-called ‘lake breeze’: a breeze that develops in the course of the day, coming off the lake, and usually not going very far inland. This lake breeze is strongest on sunny days, when the sun heats the earth and thermals develop, sending warm air high in the sky. Near the ground, cold air from the lake replaces the rising warm air.
Low-flying birds literally fly ‘under the radar’. Higher birds, however, can become visible as echoes on NEXRAD radar. At Derby, this generally happens on days with light winds or on days with west-southwest winds.
From years of observations, hawk watchers at Derby Hill have grown to understand that the lake breeze tends to push the flight of certain raptors more inland. They have also seen raptors go out high over the lake on light south winds or stronger west-southwest winds. Radar images illustrate these phenomena quite clearly.
Click on these Figures to animate the migration.
Figure 2 shows a line of raptors moving along the lake shore, flying over land until they get to Nine Mile Point (the ‘hump’ east of Oswego), where they ‘jump off’ and fly a little distance over the lake, just north of Derby Hill.
Compare Figure 3 with Figure 2; the same flight line one hour and a half later has shifted inland, probably as a result of the lake breeze. The flight line is now 5 or 6 miles south of Derby Hill, well beyond the vision of the hawk counter there.
The previous sequence already showed birds cutting the corner of the lake, and jumping off at Nine Mile Point. The following sequences, recorded on April 23, 2007, show this even more clearly. That day at Braddock Bay, a hawk watch site also on the Lake Ontario shoreline, approximately 70 miles west of Derby Hill, a massive raptor movement was observed. Almost 17,000 raptors were counted there, including 12,976 Broad-winged Hawks, 1684 Turkey Vultures and 1519 Sharp-shinned Hawks (source: http://www.hawkcount.org). All these birds at Braddock Bay fly east along the lake shore, and many of them should eventually show up at Derby Hill. However, at Derby Hill the count only got to 1660, including 951 broadwings, 419 Turkey Vultures, and 165 sharp-shins.
Some of the more extensive clusters of echoes show 1-2 miles north of Derby Hill, largely beyond the vision of the hawk counter on the ground. This phenomenon gets ‘worse’ as the day progresses and the intensity of the flight increases (Fig 5-7). Obviously a big flight occurred that day, but it was largely invisible from the count site at Derby Hill.