It took almost one year to tabulate the results, but Merlin, the name given to an extensive mobile avian radar system that was placed in the lower half of Dave and Cathy Williams’ farm on May 8, 2018, has finally revealed the data gathered on bird and bat migration as the winged aviators flew across Lake Superior.
The first three nights of observation, U.S. Fish & Wildlife biologist Nathan Rathburn and Kevin Heist noted little migration.
“This could have been a lull in migration, or it could have represented late arrival of migrants to the area, especially due to the northern latitude,” noted the report.
“The highest night we recorded near Grand Marais was the night of May 18, 2018,” said Rathburn. “We recorded over 85,000 targets in one hour on the horizontal radar and over 4,000 targets on the vertical radar in one hour. Both of these high counts were right near midnight.
“These don’t directly represent the number of birds and bats that flew over, as there were some that were counted multiple times and others that were missed, due to various factors inherent to radar. While this was the highest activity night, there were many others from May 11 through May 28, 2018, that had high activity as well on both radar antennas.
“There may also have been high activity nights that occurred before we moved to Grand Marais from down in Duluth, but we just weren’t there to sample them.
“Once June arrived, activity had slowed down greatly. A few small movements of around 20,000 targets per hour occurred in the first week of June, but most nights after that had activity around 10,000 targets per hour, which likely represents birds and bats in their normal local activity patterns as well as a background level of false positives from clutter,” added Rathburn.
After May 12, the migration found at the Williams farm was similar to that found in Bad River, Wisconsin, and the Outfitter site in Duluth, with most of the movement tending to slow down or end around May 30.
The Williams farm is located about half mile above Lake Superior and about one mile west of Grand Marais. The farm is home to a lot of wildlife, song birds and visiting geese.
Merlin is one of two mobile avian radar units used by U.S. Fish and Wildlife to collect this type of data around the Great Lakes and has been in operation since 2011.
U.S. Fish & Wildlife biologist Nathan Rathburn and fellow co-worker, biologist Kevin Heist placed Merlin at Williams’ farm and spent about one month traveling from the Twin Cities to monitor the site and perform the routine maintenance.
Merlin’s ultrasonic devices pick up the songs of birds and ultrasonic sounds of bats. Each bird and bat makes distinctive sounds that can be distinguished between species.
In addition to radar, Merlin is also equipped with a weather station.
Merlin also has horizontal radar that extends two miles on each side of the unit. Additionally, it has a vertical detector that picks up movement three-quarters of a mile on each side of the mobile lab.
The computers, one with the vertical information and one with the horizontal data, give 15-minute pictures of the migration. The unit operates 24 hours of the day, seven days a week.
Peak bird and bat movement are typically from 11:45 p.m. to midnight. As depicted on the computer screens, the migration of birds, most of which are song birds, really picks up at dusk and slows to a crawl when the sun comes up.
Did you find anything unusual in the study done in Cook County? Rathburn was asked.
“Many of the patterns we observed in Grand Marais were what we expected and matched up with many of the other areas we have sampled around the rest of the Great Lakes.
“However, seeing the large numbers of migrants that regularly cross the large expanse of Lake Superior was somewhat surprising, as was the low altitude that these migrants were flying in after they crossed the lake. Our highest densities were all below 250 meters (825ft), with the densest band between 50-100 meters (165-330 ft.). So many of these birds and bats are flying lower than at our other sites.”
At other locations Merlin has recorded song birds flying, on average, about 1,000 to 2,000 feet in the sky as they come across the lake. Some soar as high as 8,000 feet, and probably higher, said Rathburn, but that’s the cut-off ceiling for the radar. Rathburn noted pilots flying jets have seen geese at extremely high altitudes.
Song birds, sparrow, thrushes, robins, wrens, etc., tend to fly about 20 miles per hour, so a flight from Ashland to Grand Marais, depending upon the direction of the wind, takes 3½ to 4 hours.
One thing peculiar about the migration pattern found in the Grand Marais area is that some birds were flying the opposite direction from most birds.
“There was some reverse migration,” Rathburn said. “There appears to be more reverse migration at Grand Marais than at the other sites we surveyed last season. Why this would be is unclear.”
Collecting information from Merlin and sorting it out is a lengthy process. The data from the Williams farm and two other locations was “scrubbed” of clutter (storms or bug swarms), compiled and chronicled in 57-page report, and will now be combined with current and historical bird surveys, banding data, weather data, N.E.X.R.A.D. radar, and information obtained from acoustic and ultrasonic monitors to give the U.S.F.W.S. a much clearer picture of bird and bat migration across the Great Lakes.
“This larger scale picture will help inform tools that land managers, policy makers, and conservationists can use to determine areas where the most value to migrant birds and bats can be found and the most effective use of conservation dollars and resources can be targeted,” Rathburn said.
Did you find anything that surprised you? Rathburn was asked.
“I am always surprised at the sheer volume of migrants that are flying overhead during migration, not just in the Great Lakes but all across the country. We can’t see them because it is dark out and most people don’t have a reason to look, but it always amazes me that we have this vast migration of animals that is hidden from most people who only think about what birds do during the daytime,” Rathburn concluded.
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