Despite an apparently rainy night, birds were moving in northeast Ohio on the night of April 23-24, 2013. My night flight call monitoring station detected 187 warbler/sparrow frequency calls between 1/2 hour after sunset and 1/2 hour before sunrise. Given the rain, I was somewhat surprised to detect so much activity, so I decided to analyze the timing of my detections and take a look at the radar for the night.
Here’s a chart showing the timing of the calls I detected and the approximate timing of significant rain.
For radar images, I went to Paul Hurtado’s excellent and useful compilation of nexrad radar (http://people.mbi.ohio-state.edu/hurtado.10/US_Composite_Radar/). The radar images clearly show the blue blobs of migrating birds being blotted out by the advancing storms.
The first image below is at 10:41 EDT, which corresponds approximately to the beginning of the 6th half hour after sunset in my graph above. The red “x” is the approximate location of the listening station. The rain has not yet arrived and the heaviest bird movement is underway.
The next image is at 01:41 EDT on 24 April, which corresponds approximately to the beginning of the 12th half hour after sunset. The early band of rain that was about to arrive in the first image has moved northeast and dissipated somewhat. A second, heavier and more sustained rain is about to begin.
The final image is at 04:41 EDT on 24 April, which corresponds approximately to the beginning of the 18th half hour after sunset. The rain has passed for the moment, and it looks like there is some light bird activity in the gap between the storms. You can find the full night’s radar in motion on Paul Hurtado’s page: http://people.mbi.ohio-state.edu/hurtado.10/US_Composite_Radar/2013-4-23/index.html
Looking at the periods of rain as shown by the radar and by my detection station, it looks like the periods of migratory movement identified by my listening station correspond well to the gaps between the rain. However, moderate or more intense rain makes it difficult or impossible to detect calls by either automated detection or hand browsing. I am therefore hesitant to draw too many conclusions about how much actual movement may be continuing as the rain advances.
Here are a couple of examples of what night flight calls look like when the birds are dodging rain drops.
The red squares above surround a call that was picked up by an automated detector in Cornell’s Raven sound analysis software. The strong vertical lines are raindrops hitting the microphone. The two short, slightly slanting lines inside the squares are the call of a probably Savannah Sparrow. Here’s what the the call looks like if you zoom in:
A second example shows a Chipping Sparrow call in heavier rain.