Tuesday, July 17, 2012

The Suddenly Uncommon Common Grackle

(Or less common, anyhow.)

Phenomena are often detected by the presence of something. But sometimes they are inferred by the absence of something.

In May and June, I couldn't step foot in the Minneapolis Sculpture Garden without seeing and hearing Common Grackles. Usually dozens of them. But in the heat of July, they seem to have disappeared. I noticed this same phenomenon last year (post from July 1, 2011), too.

Curious about their peculiar absence, I surf over to E-Bird to explore the data. It looks as though my observations might fit the general pattern.
Here's a data visualization showing the frequency of Common Grackles on checklists submitted for Hennepin County, 2000-2012. The frequency of Common Grackles peak the week of May 1 at about 35%. Then frequency drops off and by mid-July it reaches lows close to 10%. I'm not sure how to explain this. But based on my observations of the Sculpture Garden, Common Grackles (adults as well as the new brood) vacate the nesting area once young reach the juvenile stage. It could be that during the juvenile stage of their development, Common Grackles redistribute & congregate in specific locations---locations where only 10% of checklists are generated. And that by contrast, during the nesting & fledgling phasea, Common Grackles are seen in 30% of locations where E-Bird checklists are generated.

To view a larger version of this interactive graph, courtesy E-Bird, click here.
What is "frequency"? Refer to the E-Bird glossary by clicking here.

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