Bird and bat activity differs greatly between day and night. During the day, bats are not active and birds are performing shorter, lower altitude flights looking for food and are not usually traveling in one specific direction. Many of the birds that are migrating are foraging within the woods and fields to replenish their energy or are resting before traveling the next night. Some birds such as hawks, cranes, vultures, and waterfowl do migrate during the day though. As dusk approaches, many birds start moving towards the tops of trees, readying to take off for migration. Once it is dark enough for them to travel without risk from predators, they take off and start their migration. Activity on the radar picks up quickly at dusk and increases until around midnight. Numbers then start to decrease as dawn approaches and some migrants start to land. At dawn, numbers drop off dramatically once it is light out as the migrants seek out shelter. The low daytime activity then begins the next day. A graph of typical daily activity is shown below for multiple sites. Data was averaged over the fall 2011 and 2012 seasons.
This graph of activity can also be examined by looking at the Trackplots during a 24-hour period at one of the sites. The animation below shows a set of hourly trackplots from noon on one day until noon on the next day. Each trackplot represents an hours activity. Activity is low during the day, and increases quickly as dusk arrives and peaks in the middle of the night, slowly decreasing until dawn when activity returns back to its low daily level. The vertical and horizontal trackplot animations are synced so that the same time shows on both animations. The time is displayed on the top of the horizontal trackplot in a 24 hour cycle.
If we display hourly activity out over the entire season, instead of summing it up by averages, we can see the graph shown below. Migrant activity is pulsed, often occurring for several nights in a row before having a small lull in movement and then picking up again for the next pulse. The peak activities on both the horizontal and vertical radars all occur around midnight. As the migration season ends, peaks become smaller and occur less frequently. By looking at the entire season like this, we can start to see when the most at risk times may be. Variation does occur between years as well and we are currently looking into how activity patterns relate to weather events (barometric pressure, wind direction, precipitation, etc.). With this analysis, in the future we may be able to predict when migrants may be at greatest risk from impacts by wind farms.
The graph above only examines the hourly counts from one of our antenna. If we plot the counts from the horizontal radar along with the vertical radar counts and zoom in to further examine the graphs we can get an even more detailed picture of what is occuring on the landscape.
The portion of the timeline above shows what migration numbers look like on both antennas. Peaks usually coincide between the horizontal radar antenna counts and vertical radar antenna counts and these peaks occur right around midnight. The graph below is from a low migration time period late in the fall season. A different pattern of peaks emerges later in the season when migration is dwindling, even though numbers on the horizontal antenna can remain high. These peaks occur right around dawn and dusk and are not accompanied by peaks on the vertical radar. This pattern is indicative of birds (in this case geese and gulls) moving between feeding and roosting grounds twice daily.
The differences between these graphs underline the importance of looking at the data on fine temporal scales (i.e. hourly) instead of lumping the numbers into the whole season. It also shows the importance of using data from both antenna to get a better picture of what is occuring on the landscape.
Migration and Non-migration
During the migration season, pulses of migrants move through an area. Each night however, may not have a large number of migrants moving. This may be due to unfavorable environmental conditions or a variety of other factors. Below are trackplots summarizing 1 hour of tracks from midnight to 1 AM on a night with migration and a night soon after that did not have migration.
Heavy and Light Migration
Even when migrants move through an area, they don't always move in the same magnitudes. Some nights have many more migrants traveling than others. We still call it migration because the targets are moving all in a concentrated direction, even though they may not be in large numbers. The differences in magnitude of targets moving each night may also be related to environmental factors such as temperature, wind speed, and wind direction. Below are 1 hour summary trackplots from a night with heavy migration and a night with much lighter migration at the same site on the eastern shore of Lake Michigan near Luddington, MI.
Migrants do not always move in the expected direction. Occasionally they will travel in the opposite direction than we normally associate with migration. This would mean moving north in the fall and south in the spring. Again, this phenomena is likely associated with environmental factors, such as an unexpected cold spell in the spring forcing targets to move south to find food. Below are example 1 hour summary trackplots from spring and fall sites on the western shore of Lake Michigan near Manitowoc, WI showing reverse migration. The color of the tracks indicates the direction the target was flying in according to the color wheel in the upper right corner. Orange and red tracks are moving in a southerly direction and blues and purples are moving in a northerly direction.
Migration Direction Variation
As with reverse migration, the direction that targets move in is not always north and south. Even at one site we can see that the major direction of travel for migrants varies. We can see targets moving at this location in the southerly direction as we would expect as well as north, southeast, and west. These are 1-hour summary trackplots from the western shore of Lake Michigan near Manitowoc, WI. Color of the tracks indicates the direction of movement according to the color wheel in the top right corner.
Movement in to Shore at Dawn
One consistent movement direction that we have observed at each of our locations is the movement of targets that are over the water in towards shore as dawn approaches. Migrants that are traveling during the night may fly out over the lake but cannot land in the water so they must return to land. Migrating passerines (songbirds) and bats tend not to fly during daylight because of the risk from predators such as hawks and falcons. To avoid this risk, they must be in cover onshore just after dawn. As a result, dawn may be one of the highest risk times for migrants since they are flying lower to land, may be exhausted from the nights migratory flight, and are concentrating along the lakeshore where many wind turbines have been proposed to be located. Below are trackplots from an hour during the night and an hour in the early morning of the same night showing the change in movement direction approaching dawn. These are 1-hour summaries of tracks and the color of the tracks indicate the direction of movement according to the color wheel in the top right corner.
This phenonena occurs on shorelines that are oriented in a north/south direction such as most shores of Lake Michigan and has been observed on both sides of Lake Michigan by our radar units. Movement is south (red-orange) at night and turns to move east (green-blue-yellow) at dawn.
Movement in to shore at dawn also occurs along shorelines that are oriented in an east/west direction such as the south shores of Lake Erie and Lake Ontario. Movement is to the north (blue) and east (green) at night and turns south (yellow-red) along the shoreline at dawn.