I just returned from a trip to north central Wisconsin. It is the beginning of May in the forest. Remnants of snow still cling to ditches on the shady side of the gravel road I am sitting on. It is, however, 70 degrees F and a sunny morning. Migratory birds have already returned (ducks, sandhill cranes, woodcock). I am primarily there to observe and photograph (as a way to preserve my chance encounters) four-legged wildlife, but for the few days I was there, birds dominated the scene. Among the gobbling of the turkeys and drumming of grouse, here were a pair of medium-sized (similar to robin or black bird) birds flying overhead making a loud sound most likely created by their wings. I unfortunately waited for another chance for them to fly directly overhead to get the best audio recording as a video on my phone because it was getting very low on storage. Big mistake since I never got a recording. I did have a few very good opportunities but the sound only lasts a few seconds and of course is unpredictable when they start it. I realize the audio would have been a huge benefit, but please post any and all guesses. I need a place to start. Searching for birds that make mating sounds with their wings has not been much help. Once I figure it out, I will report it here.
Here is specific information on the encounters. The birds were robin-sized with noticeably longer beaks. Their flight were mostly around 50 to 100 feet above the tree tops in pairs. They did not necessary descend while making the sound. Their flight when producing the sound appeared as if they were flapping their wings and almost looking like a woodcock in flight. The speed of the flight did not seem to slow or change direction because of the sound produced. The sound was loud enough to be heard probably from a quarter to half mile away. I'm assuming the sound is a mating call and only one of the birds in the pair are creating it. The actual audible call sounds like a baseball bat sized piece of flexible plastic quickly flicked back and forth in one's hand by only moving the wrist. The sound is mostly constant in volume but ascending in tone. It lasts only a few seconds but was repeated as quickly as eight to ten seconds. It was repeated several times up to ten minutes. This could be due to terminating their flight or just traveling out of ear shot. I do not know because they disappeared from sight over the tree line at most times.