American Kestrels are fairly common birds. You've probably seen one sitting on a power line near a farm field, even if you didn't realize what it was. But their populations are declining. Some areas have lost as much as half their kestrel population since the 1960's. The region including northern Wisconsin is estimated to have a population decline of 54%.
When researchers reviewed 50 years of data collected by kestrel banding, they found something even more surprising than a decrease in numbers. They found that kestrels are smaller than they used to be. On average, kestrels are 5 grams lighter than 50 years ago. The equivalent weight of 5 paper clips doesn't sound like a lot, but it is significant for a small bird.
The banding locations associated with the greatest size reductions were in the same areas with more population decline. Now the question is: are kestrels disappearing because they're getting smaller? Or are they getting smaller for the same unknown reasons they are disappearing?
Read more about kestrel populations and size declines from All About Birds.
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