Post season update: The information below uses radar imagery from www.wunderground.com, but a potentially more useful site can be found here.

Since 1997, a network of 158 high-resolution Doppler radars, operated by the National Weather Service, has been providing weather data in the continental US. These radars are called WSR-88D (Weather Surveillance Radar, 1988, Doppler) or simply NEXRAD (Next generation Radar), and are mainly used to detect precipitation. Many weather sites, such as Weather Underground, provide free NEXRAD radar images, which can sometimes be used to study hawk migration.

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 right along the lake shore, and usually keeps them low, afraid as they are of 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. Low on the ground, cold air from the lake replaces the rising warm air.

Figure 1: lake breeze

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.

NEXRAD radar can be in two modes: the slow-scanning, sensitive “Clear Air Mode” or the faster-scanning, less sensitive “Precipitation Mode”, depending on whether the radar detects any precipitation in the area.

The radar does not make any distinction between smaller or bigger birds. A bigger bird does not return a bigger echo. A large, spread out group of birds (e.g. a group of broadwings streaming out of a thermal) will show up on radar as a cluster of echoes, and then ‘intensity’ (indicated by color) can be observed.

Since NEXRAD radar images are free, anyone with a computer and an internet connection can study hawk migration this way. Here’s how you do it:

1)Go to www.wunderground.com, and in the top left corner, type “Mexico, NY” for the location of Derby Hill, then click on the magnifying glass icon.

2)This will get you to a page with a small radar image under “Current Conditions”. You’ll recognize the eastern part of Lake Ontario. Click on the image.

3)A larger image will open in a new window. Now click and drag a rectangle over the southeastern corner of Lake Ontario (where Derby Hill is located).

4)This will zoom in on the selected area. In the left sidebar, you’ll see a clock, and readings for “Max. reflectivity” and “Vol. cov. pattern”. VCP (Volume Coverage Pattern) will tell you what mode the radar is in: Clear Air or Precipitation. VCP 21 means the radar is Precip Mode, while VCP 32 means it’s in Clear Air Mode. (Usually you can tell also from the bands of rain present or absent in the image.)

5)In the top left corner, click “Animate Map”. This will generate a series of 6 recent radar images, 10 minutes apart for Clear Air Mode, 6 minutes apart for Precip Mode.

6)You can save your images by clicking “View/Save this image”, which opens a new window. You can then save that window using the “save” function of your browser.

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.

“Lake breeze”

Figure 2: Raptor migration along the southeastern corner of Lake Ontario on May 1, 2006, 12:55 - 1:24 PM EDT

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. The winds were WSW when the sequence was recorded, turning W then WNW during the sequence (‘lake breeze’). Observe how the line of raptors gets closer to the lake shore (and to Derby Hill) in the course of the sequence.

Figure 3: Screen capture (not animated) of the flight line on May 1, 2006, 2:45 PM EDT

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.

“Jumping off at Nine Mile Point”

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: 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.

Figure 4: Raptor migration along the southeastern corner of Lake Ontario on April 23, 2007, between 11:04 - 11:33 AM EDT

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.

Figure 5: Raptor migration along the southeastern corner of Lake Ontario on April 23, 2007, between 12:02 - 12:31 PM EDT

Figure 6: Raptor migration along the southeastern corner of Lake Ontario on April 23, 2007, between 1:34 - 2:02 PM EDT

Figure 7: Raptor migration along the southeastern corner of Lake Ontario on April 23, 2007, between 2:25 - 2:54 PM EDT